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Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
Open AccessResearch Article
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ISSN: 2157-7633
Hernández-Bule et al., J Stem Cell Res Ther 2017, 7:12
DOI: 10.4172/2157-7633.1000407 Chondrogenic Differentiation of Adipose-Derived Stem Cells by
Radiofrequency Electric Stimulation
María Luisa Hernández-Bule
1*
, María Ángeles Trillo
1
, María Ángeles Martínez-García
2
, Carlos Abilahoud
3
and Alejandro Úbeda
1*
1
BEM-Research Service, Ramón y Cajal University Hospital - IRYCIS, Madrid, Spain
2
Department of Endocrinology and Nutrition, Ramón y Cajal University Hospital - IRYCIS and CIBERDEM Biomedical Research Center, Madrid, Spain
3
Department of Electrical Engineering, E.T.S. of Engineering and Industrial Design, Polytechnic University of Madrid, Madrid, Spain
*Corresponding author: M Luisa Hernández-Bule, Servicio BEM-Investigación
Hospital Universitario Ramón y Cajal – IRYCIS, 28034 Madrid, Spain, Tel: +34-
913581365; +34-913368699; E-mail: mluisa.hernandez@hrc.es
Received December 19, 2017; Accepted December 22, 2017; Published
December 29, 2017
Citation: Hernández-Bule ML, María Trillo Á, Martínez-García MÁ, Abilahoud
C, Úbeda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi:
10.4172/2157-7633.1000407
Copyright: © 2017 Hernández-Bule ML, et al. This is an open-access article
distributed under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Keywords: Electrotherapy; Radiofrequency; Chondrogenesis;
Extracellular matrix; Sox5, Sox6; Collagen type II
Introduction
There is ample evidence that stimulation with electric currents
and electric and/or magnetic fields can induce a variety of cellular
and molecular responses, including microfilament reorganization
[1], redistribution of cell surface receptors [2] or cell migration [3],
as well as changes in intracellular calcium dynamics [4] and in stem
cell proliferation or differentiation [5-7]. This body of evidence has
provided indications that some electric and/or magnetic stimuli may
exert favourable effects in the control of cell and tissue homeostasis,
thus intervening in tissue repair and regeneration processes. Indeed,
electrotherapy has been successfully applied to bone fracture
consolidation [8], soft tissue regeneration [9], nerve fibre repair [10]
or treatment of cancerous lesions [11]. Electric and electromagnetic
therapies have proven also effective in the treatment of osteoarticular
lesions such as osteoarthritis [12,13] or degenerative disc disease [14,15].
Similarly, capacitive-resistive electric transfer (CRET) electrothermal
therapies, based on non-invasive application of radiofrequency (RF)
electric currents, have been used successfully in regeneration of muscle
[16-19], tendons [16,19-21] and ligaments [22].
As for cartilage and other tissues having poor capacity for regeneration
and cellular self-renewal [23], although the potential repairing effects
of electrotherapies remain a matter of debate, it has been reported that
exposure to electric or electromagnetic stimulation can induce in articular
chondrocytes cellular responses involved in prevention of degenerative
damage [24]. Evidence of this kind has served as a basis for proposing
that stimulation with specific electric and/or magnetic parameters
could favour cartilage regeneration through promotion of extracellular
matrix protein synthesis and/or of chondrocyte or prechondrocyte
proliferation [25,26]. The mechanisms underlying these effects would
involve electrical stimulation of cell membrane receptors which, through
activation of signalling molecules, would trigger a cascade of effects
resulting in cellular migration, proliferation or differentiation [27,28].
In fact, evidence exists that stem cells present in the cartilaginous tissue
could be a plausible target for treatment with electric fields or currents.
Indeed, stimulation with 500 mV/mm, direct current electric field has
been reported to promote survival of grafted neural stem cells, guiding
Abstract
Objective: Although capacitive-resistive electric transfer (CRET) therapies, based on transdermal application of
electrothermal radiofrequency currents, have shown promising therapeutic effectiveness in regeneration of traumatic
or degenerative tissue lesions, their potential effects on tissues like cartilage, having poor regenerative capabilities,
have not been studied sufficiently. Here we investigate the effects of the exposure to a 448 kHz current typically used
in CRET therapy, on the early chondrogenic differentiation of human, adipose-derived stem cells (ADSC).
Materials and methods: Stem cells obtained from healthy donors were differentiated in chondrogenic medium
for 16 days. During the last 2 days of incubation the cultures were intermittently exposed or sham-exposed to a 448-
kHz, sine wave current, administered at a 50 µA/mm
2
subthermal density. The cellular response was assessed by:
XTT proliferation assay, glycosaminoglycans (GAG) and collagen quantification (image analysis, Blyscan assay and
immunoblot) and analysis of the expression of chondrogenic factors Sox5 and Sox6, and of the transcription factor
ERK1/2 and its active form p-ERK1/2 (immunoflorescence, immunoblot and RT-PCR).
Results: The electric stimulus significantly increased the levels of both, cartilage-specific collagen type II and
GAG in the extracellular matrix of the differentiating cultures. Although no changes were observed in the expression of
the SOX genes at the end of the 48-hour treatment, the stimulus did induce significant overexpression of transcription
factors L-Sox5, Sox6 and p-ERK1/2. Since these proteins are crucial regulators of the synthesis of the extracellular
matrix during chondrogenic differentiation, it is likely that their overexpression is involved in the observed increases in
the content of extracellular collagen and GAG.
Conclusion: The present data set provides support to the hypothesis that the electric component of the
electrothermal treatment applied in CRET therapies could stimulate cartilage repair by promoting chondrogenic
differentiation. These data, coupled with previously reported results that in vitro treatment with the same type of
subthermal electric signal promotes proliferation of undifferentiated ADSC, identify molecular phenomena underlying
the potential repairing and regenerative effects of such radiofrequency currents.

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 2 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
their migration and stimulating their differentiation and functioning
within the lesion [29]. Also, time-varying electric fields (60 kHz, 20 mV/
cm) and pulsed electromagnetic fields (27.1 MHz) have been reported to
promote osteogenic differentiation of mesenchymal stem cells [30,31].
Previous results by our group have shown that intermittent exposure
to subthermal densities of RF (448 kHz) currents of the type applied
in CRET therapies promotes proliferation in undifferentiated cultures
of adipose-derived stem cells (ADSC) obtained from healthy donors
[6]. Such proliferative response did not affect the subsequent ability of
the ADSC to normally differentiate towards chondrocyte, adipocyte
or osteocyte lineages when the cultures were supplemented with the
corresponding differentiating factors. Our studies also revealed that in
vitro exposure to the above subthermal RF currents can modulate the
expression of genes controlling the synthesis and expression of proteins
intervening in early stages of the adipogenic differentiation of ADSC [7].
Based on the above experimental evidence, the possibility can
be posed that CRET currents could also be effective in modulating
processes intervening in cartilage regeneration, through stimulation
of stem cells present in the damaged or degenerating tissue. Thus, the
aim of the present study was to investigate the potential action of the
in vitro electrostimulation with subthermal pulses of CRET current on
early chondrogenic differentiation of ADSC. The cellular response was
assessed by analysis of cell proliferation, quantification of extracellular
matrix components synthesized during chondrogenesis, analysis of
gene and protein expression of the chondrogenic markers L-Sox5
and Sox6, and assessment of the activation of the Mitogen-Activated
Protein Kinase Extracellular Signal-Regulated Kinases 1 and 2 (MAPK
ERK1/2) signaling pathway, which has proven an important regulator
of cartilage-specific gene expression in a variety of chondroprogenitors
and chondrogenic cell types [32].
Material and Methods
Cell culture
ADSC were isolated from subcutaneous adipose tissue surgically
obtained from 4 healthy donors: two men, 65 and 69 years old, and
two women of 28 and 35. This protocol, which has been described in
detail in previous studies [6], met the ethical standards applicable in
the European Union, and was approved by the ethics committee for
clinical trials of Hospital Universitario Ramón y Cajal. Briefly, ADSC
were isolated from 0.5-1 cm
3
pieces of fat and sliced into 1-2 mm
3

fragments which were subsequently digested with 1 mg/ml collagenase
A (Roche Applied Science, Basel, Switzerland) and centrifuged to isolate
the vascular-stromal fraction. The resulting pellet was resuspended in
culture medium (MesenPro-RSTM, Gibco, Invitrogen, Camarillo, CA,
USA) supplemented with 1% glutamine (Gibco) and 1% penicillin-
streptomycin (Gibco), and the cells were seeded in a 75 cm
2
T-flask
(Falcon, Corning incorporated, Life Sciences, Durham, NC, USA).
After 4 days the culture medium was renewed, and 3 days after, when
confluent, the cells were subcultured. Flow cytometry analysis of
expression of characteristic markers of multipotential mesenchimal
cells, CD29, CD44, CD73, CD90 and CD105 was conducted. The
results confirmed that the ADSC were positive for all these markers (see
supplementary information).
Chondrogenic differentiation
Preliminary tests revealed that in our model of electrical stimulation,
the RF current distribution within the Petri dish and in the plated cells
is influenced by the culture type. Namely, cells forming multi-cellular,
spheroidal structures or micromasses were found to be less sensitive to
the electrical treatment than those adopting a monolayer distribution
on the dish surface (data not shown). This would be attributable to the
fact that monolayer configuration allows homogenous exposure of all
cells in the culture to the electrical stimulus, whereas when grouped into
three-dimensional micromasses with relatively high electrical resistivity,
the stimulus would reach only those cells located at the outermost layer
of the spheroid. This methodological requirement, together with the fact
that monolayer culture has been reported advantageous to chondrocyte
differentiation within the first three weeks of incubation [33,34] led us
to adopt monolayer culture as a suitable model for studying the early
chondrogenic response to RF electrostimulation.
ADSC in passages 3 to 6 were seeded in 60 mm Petri dishes (Nunc,
Roskilde, Denmark) at a density of 2270 cells/cm
2
. The cells were plated
directly on the bottom of the dish, except for immunofluorescence
assays and Alcian blue staining, in which the cells were seeded on
glass coverslips placed on the bottom of the plate. A total of 12 Petri
dishes were used in each experimental run. At day four after platting, 8
of the dishes were incubated in chondrogenic differentiating medium,
composed of high-glucose D-MEM (Biowhittaker, Lonza, Verviers,
Belgium) supplemented with 10% inactivated foetal bovine serum
(Gibco), 1% glutamine and 1% penicillin-streptomycin (Gibco), 37.5
μg/ml ascorbic acid-2 phosphate (Sigma), 10 ng/ml TGF-β1 (Peprotech,
Rocky Hill, NJ, USA), 10 μg/ml insulin and 39.25 μg/ml dexamethasone
(Sigma). During the last 48 h of incubation in this chondrogenic
medium, the cultures were RF- or sham-exposed. The remaining 4
dishes were incubated in basal medium composed of high-glucose
D-MEM (Biowhittaker, Lonza, Verviers, Belgium) supplemented with
10% inactivated foetal bovine serum (Gibco), 1% glutamine and 1%
penicillin-streptomycin (Gibco).
Electric treatment
The radiofrequency exposure procedure has been described in
detail elsewhere [6]. Briefly, the exposure was carried out by means
of pairs of sterile stainless steel electrodes designed ad hoc for in vitro
stimulation. All electrode pairs were connected in series to a signal
generator (model Indiba Activ HCR 902, INDIBA®, Barcelona, Spain),
though only those inserted in the RF-exposed samples were energized.
The stimulation pattern consisted of 5-minute pulses of 448 kHz, sine
wave current at a subthermal density of 50 µA/mm
2
, separated by 4-h
interpulse lapses, for a total period of 48 h. Such exposure parameters
have shown to affect human ADSC proliferation and differentiation in
previous studies by our group [6,7]. In the present study, after 14 days
of incubation in chondrogenic differentiating medium (CD, 8 dishes
per experimental run) or in non-differentiating basal medium (ND, 4
dishes per run), electrode pairs were fitted inside all 12 Petri dishes. The
electrodes were energized during 48 hours with CRET current in 4 of
the samples incubated in differentiating medium (CD+CRET), but not
in the remaining 8 dishes, CD and ND sham-exposed controls.
XTT proliferation assay
Cell proliferation of cultures in passages 3 to 7 was determined by
XTT assay (Roche). After 48 h of CRET- or sham- treatment, the cells
were incubated for 3 hours with the tetrazolium salt XTT in a 37ºC and
6.5% CO
2
atmosphere. The metabolically active cells reduced XTT into
coloured formazan compounds that were quantified with a microplate
reader (TECAN, Männedorf, Switzerland) at a 492 nm wavelength. The
obtained colorimetric values correlated directly with the number of
active cells. A total of 3 experimental replicates were conducted.

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 3 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
allowing accurate collagen segmentation (Figure 1d). Once collagen
was segmented (Figure 1e), the method determined the number of
pixels and converted the green colour tonalities in greyscale according
to their intensity, which is directly proportional to the collagen amount.
Finally, to optimize the analysis, a negative image was obtained showing
the collagen in greyscale on a black background (Figure 1f). From this
segmentation, the amount and intensity of pixels in each image was
computer quantified, and a comparative statistical analysis of the data
corresponding to the different experimental groups was carried out.
Alcian blue quantification for assessment of
glycosaminoglycan content
RF- and sham-exposed samples grown on coverslips were fixed in
paraformaldehyde and stained with a stock solution of 1% Alcian blue
(BDH, Poole, UK) in 3% acetic acid. Photomicrographs were taken
of the cultures and the images were computer analyzed as described
above. The glycosaminoglycan (GAG) content was determined by
quantification of Alcian blue positive staining using AnalySIS 3.1
software. Four replicates were carried out per experiment, and a total
of 280 micrographs of cell cultures grown in chondrogenic medium
were processed: 140 images of sham-stimulated controls and 140 of RF-
exposed samples.
Blyscan assay for glycosaminoglycan quantification
The RF- or sham-exposed cells were resuspended in a digestion
solution containing 0.2 M sodium phosphate buffer (pH=6.4), 0.1
M sodium acetate, 0.01 M EDTA, 5 mM cysteine-HCl (Sigma) and
0.2 mg/ml Papain (Roche), and were incubated overnight at 65°C.
The GAG content was quantified by Blyscan assay following the
manufacturer’s instructions (Biocolor, UK). Absorbance was read in a
spectrophotometer (Cecil CE 2021; UK) at a 656 nm wavelenght. GAG
concentration in the samples was assessed through the GAG standard
curve and total GAG was normalized over DNA content. The DNA
Image segmentation for quantitative assessment of collagen
content
The total collagen content in the cultures was evaluated by
Light green staining. Cells grown on coverslips were fixed with 4%
paraformaldehyde, and their nuclei and collagen matrix were stained
with Harris’ hematoxylin (Merk) and 0.2% Light green (Sigma-
Aldrich), respectively. Bright-field micrographs were taken with a
1280 × 1024 × 24 Nikon DS-Ri2 digital camera attached to a Nikon
Eclipse TE300 microscope. AnalySIS 3.1 software (Soft Imaging
Systems GmbH, Münster, Germany) was used for data acquisition.
Three replicates were conducted of each experiment and a total of 150
images were processed: 60 images of the non-differentiated group, 43
of the control group incubated in chondrogenic medium and sham-
stimulated, and 47 of the group incubated in chondrogenic medium
and RF-stimulated. The chromatic information in the images allows
segmenting collagen with respect to other photographed structures.
Figure 1a shows a representative micrograph acquired according
the described procedure, portraying ADSC in an early stage of
chondrogenic differentiation and the extracellular matrix synthesized
by these cells. Collagen is green stained, while cytoplasms and cell
nuclei are purple stained, being the nuclei paler. The original images
were processed through a collagen segmentation method which uses
the Matlab R2013a platform. This method reduces the image resolution
by 60% in order to optimize the segmentation time. Next, the image
noise was filtered to maintain the average intensity of the images within
a 130-160 pixel range. The RGB colour obtained image was converted
to the LAB system (Figure 1b) by using the RGB2LAB algorithm [35],
which applies the “decorrstretch” function to exclude pixels within
the red-purple colour spectrum and generates a negative greyscale
image. This image was binarized assigning a zero (0) value to those
pixels with intensities below 0.29 and discarding them (Figure 1c).
Then, the remaining pixels, with value 1, were converted back to
their original colour and the “decorrstretch" function was reapplied,
Figure 1: Computational method for quantification of collagen content in micrographic images. (a) Original image of a monolayer culture showing green-stained
collagen fibers. (b)The RGB color image is converted to LAB color system. (c) Binarization. (d) Decorrstretch. (e) Segmented collagen. (f) Negative of the previous
image, displaying collagen in greyscale.

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 4 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
concentration was determined with bisBenzimide (33258 dye Sigma),
using salmon sperm DNA solution to generate the standard curve.
Excitation (at 360 nm) and emission (at 465 nm) levels were quantified
using a microplate reader (TECAN, Männedorf, Switzerland).
Immunofluorescence for L-Sox5 and Sox6
Transcription factors Sox5 and Sox6 act as regulators of cartilage
development by boosting the activation of genes such as Col11a2,
Col2a1 and Agc1 which control the synthesis of chondrocyte-specific
extracellular matrix proteins [36]. To evaluate the effects of the electric
stimulation on the expression of L-Sox5 (the long isoform of protein
Sox5) and Sox6, the cultures were seeded on coverslips and RF- or sham-
exposed as described above. Subsequently, the samples were fixed with
4% paraformaldehyde and incubated overnight at 4ºC with anti-L-Sox5
polyclonal antibody (1:50, Santa Cruz Biotechnologies, Texas, USA)
and anti-Sox6 polyclonal antibody (1:50, Santa Cruz Biotechnologies).
Afterwards, the samples were fluorescence stained with Alexa Fluor®
488 goat anti-rabbit IgG (Molecular Probes, Eugene, Oregon, USA)
for 1 h at room temperature. The cell nuclei were counterstained with
bisBenzimide H 33258. Photomicrographs were taken and analyzed as
described above. Three replicates of each experiment were performed
and a total of 540 images were processed: 180 images of the samples
incubated in the absence of differentiating medium (ND) and 180 of
each of the two groups grown in chondrogenic medium: electrically
stimulated samples (CD+CRET) and sham-exposed controls (CD).
Immunoblot for L-Sox5, Sox6, ERK, p-ERK1/2 and collagen
type II
The immunoblot procedure has been described in detail elsewhere
[37]. Briefly, the blots were incubated overnight in anti-L-Sox5
polyclonal antibody (1:1200, Santa Cruz Biotechnologies), anti-Sox6
polyclonal antibody (1:100, Santa Cruz Biotechnologies), anti-p-
ERK1/2 polyclonal antibody (1:500, Cell Signalling, Danvers, MA,
USA), anti-ERK1/2 polyclonal antibody (1:1000, Invitrogen, USA) and
anti-collagen II polyclonal antibody (1:1000, Sigma-Aldrich, China).
Anti-β-actin monoclonal antibody (1:5000, Sigma-Aldrich, Israel) was
used as loading control. The membranes were incubated for one hour
at room temperature with anti-rabbit IgG conjugated to IRdye 800 CW
(1:10000, LI-COR Biosciences, Nebraska, USA) and with anti-mouse
IgG conjugated to IRdye 680 LT (1:15000, LI-COR Biosciences). Then,
the membranes were scanned with a LI-COR Odyssey scanner (LI-
COR Biosciences). When ECL-chemiluminescence was required, the
membranes were incubated with ECL-anti-mouse IgG horseradish
peroxidase-linked antibody (GE Healthcare, Little Chalfont,
Buckinghamshire, UK) or with ECL-anti-rabbit IgG horseradish
peroxidase-linked antibody (GE Healthcare). The obtained bands were
densitometry evaluated (PDI Quantity One 4.5.2 software, BioRad). At
least five experimental replicates were conducted for each protein. All
values were normalized over the loading control.
RT-PCR for the SOX5 and SOX6
RT-PCR assay was applied for studying the potential influence
of the RF stimulus on SOX5 and SOX6 gene expression. Total
RNA was isolated using an RNeasy Lipid Tissue Mini Kit (Qiagen,
Germantown, MD, USA) according to the manufacturer’s instructions.
The concentration and quality of the extracted RNA were assessed
by measuring the 260/280 and 260/230 absorbance ratios with a
NanoDrop 1000 spectrophotometer (Thermo Scientific, Wilmington,
DE, USA). First-strand cDNA was synthesized with SuperScript
VILOMasterMix (Invitrogen by Life Technologies, USA) using an equal
amount of total RNA and following the manufacturer’s instructions.
Prevalidated TaqMan Gene Expression assays were applied for relative
quantification of SOX5 (Assay ID Hs00753050_s1, Applied Biosystems)
and SOX6 (Assay ID Hs00264525_m1, Applied Biosystems), using a
StepOnePlus Real-Time PCR System (Applied Biosystems). Human
GAPD (GAPDH) Endogenous control (Applied Biosystems) was used
to normalize the expression levels of the target genes in each sample. A
threshold cycle (Ct) was obtained for each amplification curve, and a
ΔCt value was first calculated by subtracting the Ct value for GAPDH
cDNA from the Ct value of the specific transcript. Data were expressed
in arbitrary units and normalized over the differentiated, sham-exposed
controls, using the following transformation: ΔCtx – ΔCtCD=2-ΔΔCt.
RNA from mature chondrocytes (DV Biologics, CA, USA) was used
as positive control of expression. A total of six experimental replicates,
with four dishes per experimental group, were processed. Each sample
from four pooled dishes per experimental condition was processed in
triplicate. Negative controls with no RT enzyme were included in all
reactions.
Statistical analysis
All procedures and analyses were conducted in blind conditions
for RF- or sham-exposure and for presence or absence of chondrogenic
medium. Three or more independent replicates were conducted per
experiment, the results being expressed as means ± standard deviation
(SD) or means ± standard error of the mean (SEM). The data obtained by
digital segmentation of images were analyzed using the StatGragphics
XVII.1 software, providing the average values of IPixeles (number
of pixels multiplied by their average intensity) corresponding to the
segmented collagen. An ANOVA one-way test was applied, followed by
Fisher's Least Significant Difference (LSD) test. The statistical method
was validated by confirming the homoscedasticity of the residuals,
which follow a normal distribution. For the rest of the experiments
the one-way ANOVA test was applied, followed by two-tailed unpaired
Student's t-test and/or Bonferroni post-test, using Graph-Pad Prism
software (GraphPad Software, San Diego, CA, USA). Differences
p<0.05 were considered significant statistically.
Results
RF effects on cell proliferation
The results of the XTT assay summarized in Figure 2 show that
compared to samples incubated for 16 days in basal medium (ND),
incubation in chondrogenic medium (CD) induced a statistically
significant decrease in cell proliferation. Such antiproliferative response
was slightly, but significantly increased by exposure to RF during the last
48 hours of incubation in differentiating medium (CD vs. CD+CRET).
RF effects on collagen content
The image segmentation analysis of Light green stained cultures
revealed that, compared with undifferentiated cultures (ND),
incubation in chondrogenic medium induced a 26%, statistically
significant increase in the average collagen content (CD; Figure 3a
and 3b). As for samples exposed to RF during the last two days of
chondrogenic differentiation (CD+CRET), they showed a 23% average
increase, statistically significant, in collagen content over their sham-
exposed controls, CD. The results of the immunoblot densitometry
(Figure 3c and 3d) revealed that a significant proportion of the collagen
synthesized in response to the electrical stimulus corresponded to
cartilage-specific type II collagen (51% over differentiated controls,
CD).

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 5 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
that incubation in the presence of differentiating medium (CD)
increased the amount and condensation of GAG with respect to that
in undifferentiated cultures kept in basal medium (ND; Figure 4a).
Computerized Image analysis also showed that, compared to sham-
exposed differentiated controls (CD), stimulation with CRET during the
last 48 h of incubation in chondrogenic medium significantly increased
(10%) the average levels of GAG labeling (Figure 4b). These results
were consistent with those from the Blyscan assay, which revealed a
20%, statistically significant increase with respect to the sham-exposed
controls, in the average GAG content after electrical stimulation of
differentiating samples (Figure 4b).
RF effects on L-Sox5 and Sox6: immunofluorescence assay
Presence of nuclear labelling for the proteins L-Sox5 and Sox6,
involved in early chondrogenic differentiation, was observed in all
experimental groups (Figure 5a). Indeed, even the control cultures
maintained in standard, non-differentiating medium showed traces of
this type of labeling, since typically a small proportion of ADSC can
spontaneously pre-differentiate into chondrocytes and other cell types
[7]. Compared with sham-exposed cultures grown in chondrogenic
medium (CD) the RF stimulated samples (CD+CRET) showed
significantly increased nuclear labeling for L-Sox5 and Sox6 (16% and
44%, respectively; Figure 5b). By contrast, incubation of sham-exposed
controls in chondrogenic medium did not induce significant changes in
the labeling of these transcription factors (ND vs. CD).
XTT assay
CD CD+CRET
80
90
100
110
*
**
Absorbance
% over ND
Figure 2: XTT proliferation assay. CD: differentiated cells, grown in chondrogenic
differentiating medium for 16 days post-seeding and sham-exposed to CRET
during the last 48 h. CD + CRET: cells incubated in chrondrogenic differentiating
medium for 16 days and exposed to CRET during the last 48 h. Data are means
± SEM normalized over those of samples grown in basal medium for 16 days
and sham-exposed to CRET during the last 48 h (ND). Three repeats per
experimental group *:0.01 ≤ p<0.05; **:0.001 ≤ p<0.01; Student's t test.

a b







c







d










CD+CRET

CD

ND

β-actin
Col II
CD CD+CRE T ND
Collagen
CD CD+CRET
0
50
100
150
200
***
***
IPixels (x10
6
)
% over ND
Collagen type II
CD CD+CRET
0
50
100
150
200
250
*
*
Densitometry (INT/mm
2
)
Col II/<
-act ratio
% over ND
Figure 3: Quantification of collagen content. (a) Collagen staining. Representative micrographs of cultures stained with Light green and Harris' hematoxylin. ND,
CD and CD+CRET: same notations as in Figure 2; Bar: 100 μm. (b) Computerized quantification of collagen content. IPixels: number of pixels multiplied by their
intensity. Data are means ± SD of N=40 to 60 images per experimental group, out of a total of 150 images obtained from 3 experimental replicates of each group. Data
normalized over sham-exposed, non-differentiated controls ND. ***: p<0.001; ANOVA and Fisher’s LSD tests. (c) Representative blots for collagen type II expression;
100 μg protein per lane. (d) Collagen type II expression in CRET-exposed samples, normalized over sham-exposed, non-differentiated controls ND. Data are means
± SD of 5 repeats per experimental group. Collagen type II/β-actin ratio; INT/mm
2
: Intensity per mm
2
. *: 0.01 ≤ p<0.05; Student’s t test.
RF effects on glycosaminoglycan content
The effects of CRET exposure on GAG content was estimated
through image analysis quantification of Alcian blue staining and
by Blyscan assay. The analysis of the micrographic images revealed

Citation: Hernández-Bule ML, Trillo MÁ, Martínez-García MÁ, Abilahoud C, Úbeda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 6 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633

a








b









ND CD CD+C RET
Glycosaminoglycans
AB Blyscan
100
110
120
130
140
*
*
ni tnetnoc GAG
CD+CRET
% over CD
Figure 4: Quantification of GAG content. (a) GAG staining in ADSC cultures grown for 16 days post-plating and Alcian blue stained for GAG labeling. ND, CD and
CD+CRET: same notation as in previous figures. Bar: 100 μm. (b) GAG quantification. AB: image analysis of the Alcian blue intensity incorporated by the cultures.
Blyscan: quantification of GAG/DNA ratio by Blyscan assay. Data are mean ± SEM values in CRET-exposed samples, normalized over those in the corresponding
sham-exposed, differentiated controls (CD). Between 4 and 6 experimental replicates per assay. *0.01 ≤ p<0.05; Student's t test.

a b

























ND CD CD+C RET
Sox6+

L
-
Sox5+

Nuclei

Nuclei

L-Sox5
ND CD+CRE T
0
50
100
150
*sllec +5xoS-L
% over CD
Sox6
ND CD+CRE T
0
50
100
150 ***
sllec +6xoS
% over CD
Figure 5: Immunofluorescence for L-Sox5 and Sox6. (a) Representative immunofluorescence images for L-Sox5 and Sox6. ND, CD and CD+CRET: same notation
as in previous figures. Bar: 100 μm. (b) Image analysis quantification of L-Sox5+ and Sox6+ labeled cells. Data are mean ± SEM values, normalized over those in the
corresponding sham-exposed differentiated controls (CD). Between 4 and 6 experimental replicates per assay. *0.01 ≤ p<0.05; ***p<0.001; Student's t test.

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 7 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
RF effects on L-Sox5, Sox6, ERK1/2 and p-ERK1/2 expression:
immunoblot assay
Electric stimulation significantly increased the expression
of factors L-Sox5 (25%) and Sox6 (20%) with respect to that in
sham-exposed controls ND and CD (Figure 6a and 6b). As for
factor ERK1/2, its expression was not significantly affected, either
by the chondrogenic medium or by electrostimulation, whereas
its active form, factor p-ERK1/2, was underexpressed by the
chondrogenic medium in the sham-stimulated samples (89% of that
in ND controls) and overexpressed by CRET stimulation (33% over
differentiated controls CD).
RF effects on SOX5 and SOX6 gene expression: RT-PCR assay
Data in Figure 7 indicate that the chondrogenic medium can induce
overexpression of genes SOX5 and SOX6, although only for SOX6
the differences over undifferentiated controls (ND) were statistically
significant. On the other hand, the analysis of gene expression at the
end of the 48 hours of RF exposure showed no significant differences
with respect to the sham-exposed controls for either of the two genes
studied. Inclusion in these experiments of RNA samples from human
mature chondrocytes allowed validation of the above results by
verifying that, as expected, these positive controls showed significantly
high expression levels of genes SOX5 and SOX6 (2 and 3 orders of
magnitude, respectively, over the CD differentiated samples; data no
shown).
a b
















L-Sox5
ND CD+CRET
0
50
100
150
200
*
Densitometry (INT/mm
2
)
L-Sox5/β-act ratio
% over CD
Sox6
ND CD+CRET
0
50
100
150
200
*
Densitometry (INT/mm
2
)
Sox6/
β
-act ratio
% over CD
p-ERK1/2
ND CD+C RET
0
50
100
150
200
*
***
Densitometry (INT/mm
2
)
p-ERK1/2/
β
-act ratio
% over CD
ERK1/2
ND CD+CRET
0
50
100
150
200
Densitometry (INT/mm
2
)
ERK1/2/-act ratio
% over CD
ββ-actin
β-actin
β-actin
β-actin
p-ERK1/2
ERK1/2
Sox6
L-Sox5
CD CD+CRE T ND
Figure 6: Western blot for L-Sox5, Sox6, ERK1/2 and p-ERK1/2 expression. 100 μg protein per lane. ND, CD and CD+CRET: same notation as in previous figures.
(a) Representative blots. (b) Means ± SD of protein expression (Protein/β-actin ratio; INT/mm
2
: Intensity per mm
2
) in non-differentiated and in CRET-exposed,
differentiated samples, normalized over sham-exposed, differentiated controls CD. Between 5 and 10 repeats per experimental group. *0.01 ≤ p< 0.05; ***p<0.001;
Student’s t test.
SOX6
ND CD+CRET
0
50
100
150
*
Relative mRNA expression
(SOX6/GADPH)
% over CD
SOX5
ND CD+CRET
0
50
100
150noisserpxe ANRm evitaleR
(SOX5/GADPH)
% over CD
Figure 7: RT-PCR analysis of CRET effects on the expression of genes SOX5 and SOX6 at the end of the 48 hours of RF- or sham-exposure. ND, CD and CD+CRET:
same notations as in previous figures. Data are means ± SD of 6 replicates, normalized over CD samples differentiated for 16 days and sham-exposed to CRET.
*0.01 ≤ p<0.05. Student’s t test.

Citation: Hern�ndez-Bule ML, Trillo M�, Mart�nez-Garc�a M�, Abilahoud C, �beda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 8 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
Discussion
Previous studies by our group have shown that cyclic exposure to
a subthermal dose of 448-kHz, sine wave electric current of the type
applied in CRET therapy, can stimulate proliferation of ADSC cultures
obtained from healthy human donors and grown in proliferating
medium [6]. These data suggested that the beneficial effects on tissue
repair and regeneration attributed to CRET therapies could be mediated
in part by electrically-induced stimulation of the proliferation of stem
cells present in damaged tissues.
The histological and physiological characteristics of the cartilage,
with very low cell proliferation rate and a dense and fibrous
extracellular matrix that hinders cell migration, restrict the repairing
and regenerative capacities of this tissue in osteoarticular lesions or
pathologies [23]. Several electrical therapies have been used as adjuvants
in the treatment of joint cartilage injuries, and indications exist that
electrical stimulation could potentiate cellular phenomena involved in
chondrogenesis through promotion of stem cell differentiation [38].
Within this context, the present study investigates whether the electric
component of the electrothermal stimulus applied in CRET therapies,
besides of promoting proliferation of ADSC, could influence the early
chondrogenic differentiation of these cells.
The XTT proliferation assay (Figure 2) revealed that chemical
induction of chondrocytic differentiation also comports inhibition of
cell proliferation (CD vs. ND). And, in contrast to the aforementioned
proliferative effect of CRET in undifferentiated ADSC grown in
proliferating medium [6], the same RF treatment exerts antiproliferative
effects on ADSC undergoing chondrocytic differentiation (CD+CRET
vs. CD). This could represent a basic indication that CRET may promote
early chondrogenic differentiation.
The potential effects of the electrostimulation on chondrogenic
differentiation were further assessed through quantification of the
content of collagen and GAG, the major structural constituents of the
cartilage extracellular matrix. As shown in Figures 3 and 4, the obtained
results revealed that, compared with their sham-stimulated controls,
the matrix of the samples intermittently exposed to the electric
stimulus for 48 hours showed significantly increased levels of both, the
cartilage-specific collagen type II, and GAG. These results add to the
body of evidence that exposure to electric and/or magnetic signals at
a variety of frequencies and durations, can induce significant changes
in the synthesis and content of extracellular GAG and collagen during
chondrogenic differentiation, both in vivo and in vitro [39]. In what
regards specifically to the effects of electrostimulation at the intermediate
frequency and radiofrequency ranges, our results are consistent in part
with those reported by Wang et al. [40] showing that in vitro electrical
stimulation at 60 kHz and 20 mV/cm promotes extracellular matrix
synthesis in bovine articular chondrocytes. These authors based their
study on previous data showing that in vivo treatment with capacitive
electric signals can promote nonunion fracture healing and spinal
fusion [41].
During chondrogenic differentiation, the synthesis of extracellular
matrix is regulated by the expression of transcription factors Sox5 and
Sox6 [42]. In humans, these proteins are encoded by the 20-membered
SOX gene family (Sry-type HMG box). Sox5 and Sox6 regulate cartilage
and cartilage-specific matrix formation, and maintain the chondrocyte
phenotype in mature cartilage by activating the expression of several
cartilage-specific genes, including those involved in collagen synthesis
[43]. The extracellular matrix, in turn, regulates the signaling pathways
to co-ordinate cartilage and bone formation [44]. Therefore, here we
investigated the potential involvement of the RF effects on L-Sox5
and Sox6 protein expression in the above described electroinduced
promotion of extracellular matrix synthesis. Both of the complementary
techniques applied, immunofluorescence and immunoblot, revealed
that, compared to sham-exposure, the electric treatment significantly
increased L-Sox5 and Sox6 expression (Figures 5 and 6).
As describe above, the results of the XTT assay (Figure 2) show
that incubation in chondrogenic differentiating medium significantly
inhibits cell proliferation (CD vs. ND) and that this antiproliferative
effect is reinforced by RF electric stimulation (CD+CRET vs. CD).
In relation to this, the immunoblot data in Figure 6 had shown that
while the differentiating medium inhibits p-ERK1/2 expression, CRET
induces overexpression of this activated form of MAPK, and that
neither of both treatments affects the expression of total ERK1/2. These
two data groups are coherent with each other, and are indicative of an
inhibition of the ADSC cytoproliferative pathway by the differentiating
medium, as well as of an electroinduced activation of ERK1/2, which
is essential for progression of the chondrogenic differentiation.
Indeed, the MAPK-ERK1/2 signaling pathway is a crucial regulator
of cartilage-specific gene expression in a variety of chondroprogenitor
and chondrogenic cell types [32]. It has also been reported that in bone
marrow mesenchymal stem cells, chondrogenesis is accompanied with
p-ERK1/2 overexpression and increased contents of collagen type II
and glycosaminoglycans, and that underexpression of the ERK pathway
members MEK1 and ERK1 induces underexpression of a variety of
chondrogenic markers, including Sox5 and Sox6 [45,46]. Within this
context, the set of results reported here are indicative that the RF effects
in promoting synthesis of chondrogenic extracellular matrix would be
mediated, at least in part, by electro-induced overexpression of proteins
L-Sox5 and Sox6, and activation of the MAPK-ERK1/2 pathway.
With regard to gene expression, the RT-PCR analysis revealed that
incubation in chondrogenic medium increased the expression of genes
SOX5 and SOX6 with respect to undifferentiated samples, though only
for SOX6 the differences reached statistical significance. Although, as
described above, the expression of transcription factors Sox5 and Sox6
was significantly increased at the end of 48 hours of CRET treatment, the
expression of the corresponding genes, SOX5 and SOX6, did not show
significant differences with respect to their controls, sham-exposed
during the same time lapse. The possibility cannot be ruled out that
an activation of genes SOX5 and SOX6 that had occurred during the
early phases of the electric treatment, being therefore undetectable at
the end of the exposure interval, was responsible for the increase in the
expression of the corresponding transcription factors, observed after
48 hours of CRET treatment. However, since the understanding of the
way in which the expression of those transcription factors is regulated
during chondrogenesis remains incomplete [47,48], the potential
effects of CRET on gene expression cannot be sufficiently elucidated
on the basis of the present results and need to be further investigated.
On the other hand, CRET stimulation might also influence epigenetic
mechanisms that regulate gene expression during chondrogenic
differentiation. Such mechanisms include, among others, methylation
of cartilage-specific promoter genes, or acetylation, methylation,
phosphorylation and SUMOylation of histones, which depending
on the case, are able of promoting or repressing the transcriptional
activation of chondrogenic genes such as SOX9 (see [49] for a review).
In fact, in contrast to the observations in this paper, ERK1/2 signaling
pathway has shown able to facilitate chondrogenic differentiation in the
dental pulp stem cells (DPSCs) [50], what indicates that MAPK pathway
activation might play distinct roles in the differentiation of stem cells
from different lineages. As epigenetic mechanisms play a pivotal role in

Citation: Hernández-Bule ML, Trillo MÁ, Martínez-García MÁ, Abilahoud C, Úbeda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 9 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
the stem cell fate commitment, it is possible that epigenetic factors are
involved in the herein reported effects of CRET on the chondrogenic
differentiation of ADSC. For instance, histone methyltransferase G9a
is able to inactivate MAPK signaling via negative regulation of the dual
specificity phosphatase-4 (DUSP4) in head and neck squamous cell
carcinoma [51]. In addition, pharmacological inhibition of G9a has also
been found to inactivate MAPK signaling in non-small-cell lung cancer
(NSCLC) [52]. At the molecular level, the main function of G9a and its
partner protein GLP is depositing H3K9me2/3 at the euchromatin loci
[53]. Apart from being a histone modifier, G9a recently has also been
found to play a role in the maintenance of imprinted DNA methylation
in embryonic stem cells [54]. Therefore, the study of the electrical
stimulation effects on the patterns of G9a associated H3K9me2/3 as well
as on DNA methylation, might elucidate the epigenetic mechanisms
potentially involved in the lineage commitment of CRET-exposed
ADSC during the differentiation process.
In sum, together with previously reported results [6], the
present dataset provides support to the hypothesis that the electric
component of the electrothermal stimulus applied in CRET therapy
could promote tissue repair and regeneration. Concerning cartilage
tissue, at initial repair stages the electric stimulation could promote
stem cell proliferation in the injured tissues [6], whereas later, at early
or intermediate differentiational stages, the electric treatment may
stimulate chondrogenesis through overexpression of transcription
factors such as Sox5 and Sox6, and activation of p-ERK1/2. These
effects would result in the observed increases in the synthesis and
content of type II collagen and GAG in the extracellular matrix of the
CRET-exposed cultures. This opens the possibility that radiofrequency
currents can be successfully applied in early/intermediate phases
of chondrogenic repair and regeneration. Nevertheless, important
aspects of the in vitro response to electrical stimulation are still to
be investigated, especially at advanced stages of the chondrogenic
differentiation. Furthermore, the effects on the cartilaginous tissue
of the joint action of the electric stimulation in combination with the
concomitant hyperthermia, as applied to the patient in electrothermal
CRET therapies, is yet to be elucidated.
Author’s Contribution
Conceived and designed the experiments: MLHB, AU. Performed the
experiments:MLHB. Analyzed the data: MLHB, CA, MAMG, MAT, AU. Designed
and developed the algorithms for image-based quantification of collagen: CA.
Performed the RT-PCR analysis: MAMG. Wrote the paper: MLHB, MAT, AU.
Funding
This work was financially supported by Fundación para la Investigación
Biomédica del Hospital Ramón y Cajal (Project FiBio-HRC No. 2012/0032). The
founder had no role in the study design, data collection and analysis, decision to
publish or preparation of the manuscript.
Acknowledgments
The authors thank Ms. Silvia Sacristán and Ms. Elena Toledano-Macías (both
Hospital Universitario Ramón y Cajal-IRYCIS) for helpful technical advice and
assistance.
Declaration
The authors declare no conflict of interest.
References
1. Li X, Kolega J (2002) Effects of direct current electric fields on cell migration
and actin filament distribution in bovine vascular endothelial cells. J Vasc Res
39: 391-404. [PubMed]
2. Cho MR, Thatte HS, Lee RC, Golan DE (1994) Induced redistribution of cell
surface receptors by alternating current electric fields. FASEB J 8: 771-776.
[PubMed]
3. Ross CL (2017) The use of electric, magnetic, and electromagnetic field for
directed cell migration and adhesion in regenerative medicine. Biotechnol
Prog 33: 5-16. [PubMed]
4. Titushkin IA, Rao VS, Cho MR (2004) Mode- and cell-type dependent
calcium responses induced by electrical stimulus. IEEE Trans Plasma Sci
32: 1614-1619.
5. Clark CC, Wang W, Brighton CT (2014) Up-regulation of expression of
selected genes in human bone cells with specific capacitively coupled
electric fields. J Orthop Res 32: 894-903. [PubMed]
6. Hernández-Bule ML, Paíno CL, Trillo MA, Úbeda A (2014) Electric stimulation
at 448 kHz promotes proliferation of human mesenchymal stem cells. Cell
Physiol Biochem 34: 1741-1755. [PubMed]
7. Hernández-Bule ML, Martínez-Botas J, Trillo MA, Paíno CL, Úbeda A (2016)
Antiadipogenic effects of subthermal electric stimulation at 448 kHz on
differentiating human mesenchymal stem cells. Mol Med Rep 13: 3895-3903.
[PubMed]
8. Ciombor DM, Aaron RK (2005) The role of electrical stimulation in bone
repair. Foot Ankle Clin 10: 579-593. [PubMed]
9. Ashrafi M, Alonso-Rasgado T, Baguneid M, Bayat A (2017) The efficacy
of electrical stimulation in lower extremity cutaneous wound healing: a
systematic review. Exp Dermatol 26: 171-178. [PubMed]
10. Willand MP, Nguyen MA, Borschel GH, Gordon T (2016) Electrical
stimulation to promote peripheral nerve regeneration. Neurorehabil
Neural Repair 30: 490-496.
11. Tuszynski JA, Wenger C, Friesen DE, Preto J (2016) An overview of sub-
cellular mechanisms involved in the action of TTFields. Int J Environ Res
Public Health 12: E1128. [PubMed]
12. Garland D, Holt P, Harrington JT, Caldwell J, Zizic T, et al. (2007) A 3-month,
randomized, double-blind, placebo-controlled study to evaluate the safety
and efficacy of a highly optimized, capacitively coupled, pulsed electrical
stimulator in patients with osteoarthritis of the knee. Osteoarthritis Cartilage
15: 630-637. [PubMed]
13. Fary RE, Carroll GJ, Briffa TG, Gupta R, Briffa NK (2008) The effectiveness
of pulsed electrical stimulation (E-PES) in the management of osteoarthritis
of the knee: a protocol for a randomised controlled trial. BMC Musculoskelet
Disord 9: 18. [PubMed]
14. Maurer P, Block JE, Squillante D (2008) Intradiscal electrothermal therapy
(IDET) provides effective symptom relief in patients with discogenic low back
pain. J Spinal Disord Tech 21: 55-62. [PubMed]
15. Rohof O (2012) Intradiscal pulsed radiofrequency application following
provocative discography for the management of degenerative disc disease
and concordant pain: a pilot study. Pain Pract 12: 342-349.
16. Parolo E, Honesta MP (1998) HCR 900: hyperthermia by capacitive and
resistive energy transfer in the treatment of acute and chronic muscular-
skeletal injuries. La Riabilitazione 31 :81-83.
17. Takahashi K, Suyama T, Onodera M, Hirabayashi S, Tsuzuki N, et al. (1999)
Clinical effects of capacitive electric transfer hyperthermia for lumbago. J
Phys Ther Sci 11: 45-51.
18. Mondardini P, Tanzi R, Verardi L, Briglia S, Maione A, et al. (1999) New
methods for the treatment of traumatic muscle pathology in athletes: C.R.E.T
therapy. Excerpt from medicina dello sport 52: 201-213.
19. Ganzit GP (2000) New methods in the treatment of joint-muscular pathologies
in athletes: CRET therapy. Excerpt from Medicina dello sport 53: 361-367.
20. Melegati G, Volpi P, Tornese D, Mele G (1999) Rehabilitation in tendinopathies.
Sports Traumatol Rel Res 21: 66-83.
21. Ganzit CP, Gabriele G (2001) CRET therapy in treatment of tendinopathies.
Il Medico Sportivo Suppl Nº.1.
22. Melegati G, Tornese D, Bindi M (2000) The use of CRET therapy ankle
sprains. La Riabilitazione 33: 163-167.
23. Mauck RL, Martínez-Díaz GJ, Yuan X, Tuan RS (2007) Regional multilineage
differentiation potential of meniscal fibrochondrocytes: implications for
meniscus repair. Anat Rec (Hoboken) 290: 48-58. [PubMed]

Citation: Hernández-Bule ML, Trillo MÁ, Martínez-García MÁ, Abilahoud C, Úbeda A (2017) Chondrogenic Differentiation of Adipose-Derived Stem
Cells by Radiofrequency Electric Stimulation. J Stem Cell Res Ther 7: 407. doi: 10.4172/2157-7633.1000407 Page 10 of 10Volume 7 � Issue 12 � 1000407
J Stem Cell Res Ther, an open access journal
ISSN: 2157-7633
24. Tan L, Ren Y, van Kooten TG, Grijpma DW, Kuijer R (2015) Low-intensity
pulsed ultrasound (LIPUS) and pulsed electromagnetic field (PEMF)
treatments affect degeneration of cultured articular cartilage explants. Int
Orthop 39: 549-557. [PubMed]
25. Aaron RK, Ciombor DMK (1996) Acceleration of experimental endochondral
ossification by biophysical stimulation of the progenitor cell pool. J Orth Res
14: 582-589. [PubMed]
26. Creecy CM, O'Neill CF, Arulanandam BP, Sylvia VL, Navara CS, et al. (2013)
Mesenchymal stem cell osteodifferentiation in response to alternating electric
current. Tissue Eng Part A 19: 467-474. [PubMed]
27. Taghian T, Narmoneva DA, Kogan AB (2015) Modulation of cell function
by electric field: a high-resolution analysis. J R Soc Interface 6: 20150153.
[PubMed]
28. Aaron RK, Boyan BD, Ciombor DM, Schwartz Z, Simon BJ (2004) Stimulation
of growth factor synthesis by electric and electromagnetic fields. Clin Orthop
Relat Res 419: 30-37. [PubMed]
29. Meng X, Arocena M, Penninger J, Gage FH, Zhao M, et al. (2011) PI3K
mediated electrotaxis of embryonic and adult neural progenitor cells in the
presence of growth factors. Exp Neurol 227: 210-217. [PubMed]
30. Hronik-Tupaj M, Rice WL, Cronin-Golomb M, Kaplan DL, Georgakoudi I (2011)
Osteoblastic differentiation and stress response of human mesenchymal
stem cells exposed to alternating current electric fields. Biomed Eng Online
26: 9. [PubMed]
31. Teven CM, Greives M, Natale RB, Su Y, Luo Q, et al. (2012) Differentiation
of osteoprogenitor cells is induced by high-frequency pulsed electromagnetic
fields. J Craniofac Surg 23: 586-593. [PubMed]
32. Bobick BE, Kulyk WM (2008) Regulation of cartilage formation and maturation
by mitogen-activated protein kinase signaling. Birth Defects Res C Embryo
Today 84: 131-154. [PubMed]
33. Ikeda T, Kawaguchi H, Kamekura S, Ogata N, Mori Y, et al. (2005)
Distinct roles of Sox5, Sox6, and Sox9 in different stages of chondrogenic
differentiation. J Bone Miner Metab 23: 337-340. [PubMed]
34. Hamid AA, Idrus RB, Saim AB, Sathappan S, Chua KH (2012) Characterization
of human adipose-derived stem cells and expression of chondrogenic genes
during induction of cartilage differentiation. Clinics (Sao Paulo) 67: 99-106.
[PubMed]
35. Ruzon M, Rubner Y (1997) https://www2.eecs.berkeley.edu/Research/
Projects/CS/vision/bsds/code/Util/Lab2RGB.m Accessed 11 July 2017.
36. Hernández-Bule ML, Trillo MA, Cid MA, Leal J, Ubeda A (2007) In vitro
exposure to 0.57-MHz electric currents exerts cytostatic effects in HepG2
human hepatocarcinoma cells. Int J Oncol 30: 583-592. [PubMed]
37. Yamashita S, Miyaki S, Kato Y, Yokoyama S, Sato T, et al. (2012) L-Sox5
and Sox6 proteins enhance chondrogenic miR-140 microRNA expression
by strengthening dimeric Sox9 activity. J Biol Chem 287: 22206-22215.
[PubMed]
38. Chen CH, Lin YS, Fu YC, Wang CK, Wu SC, et al. (1985) Electromagnetic
fields enhance chondrogenesis of human adipose-derived stem cells in
a chondrogenic microenvironment in vitro. J Appl Physiol 114: 647-655.
[PubMed]
39. Ciombor DM, Lester G, Aaron RK, Neame P, Caterson B (2002) Low
frequency EMF regulates chondrocyte differentiation and expression of
matrix proteins. J Orthop Res 20: 40-50. [PubMed]
40. Wang W, Wang Z, Zhang G, Clark CC, Brighton CT (2004) Up-regulation of
chondrocyte matrix genes and products by electric fields. Clin Orthop Relat
Res 427 Suppl: S163-173. [PubMed]
41. Goodwin CB, Brighton CT, Guyer RD, Johnson JR, Light KI, et al. (1999) A
double-blind study of capacitively coupled electrical stimulation as an adjunct
to lumbar spinal fusions. Spine 24: 1349-1357. [PubMed]
42. Lefebvre V, Li P, de Crombrugghe B (1998) A new long form of Sox5 (L-Sox5),
Sox6 and Sox9 are coexpressed in chondrogenesis and cooperatively
activate the type II collagen gene. EMBO J 17:.5718-33. [PubMed]
43. Liu CF, Lefebvre V (2015) The transcription factors SOX9 and SOX5/SOX6
cooperate genome-wide through super-enhancers to drive chondrogenesis.
Nucleic Acids Res 43: 8183-8203. [PubMed]
44. Gao Y, Liu S, Huang J, Guo W, Chen J, et al. (2014) The ECM-cell interaction
of cartilage extracellular matrix on chondrocytes. Biomed Res Int 2014:
648459. [PubMed]
45. Smits P, Li P, Mandel J, Zhang Z, Deng JM, et al. (2001) The transcription
factors L-Sox5 and Sox6 are essential for cartilage formation. Dev Cell 1:
277-290. [PubMed]
46. Bobick BE, Matsche AI, Chen FH, Tuan RS (2010) The ERK5 and ERK1/2
signaling pathways play opposing regulatory roles during chondrogenesis of
adult human bone marrow-derived multipotent progenitor cells. J Cell Physiol
224: 178-186. [PubMed]
47. Akiyama H, Chaboissier MC, Martin JF, Schedl A, de Crombrugghe B (2002)
The transcription factor Sox9 has essential roles in successive steps of the
chondrocyte differentiation pathway and is required for expression of Sox5
and Sox6. Genes Dev 16: 2813-2828. [PubMed]
48. Hagiwara N (2011) Sox6, Jack of all trades: A versatile regulatory protein in
vertebrate development. Dev Dyn 240: 1311-1321. [PubMed]
49. Hata K (2015) Epigenetic regulation of chondrocyte differentiation. Japanese
Dental Science Review 51: 105-113.
50. Ba P, Duan X, Fu G, Lv S, Yang P, Sun Q (2017) Differential effects of p38
and Erk1/2 on the chondrogenic and osteogenic differentiation of dental pulp
stem cells. Mol Med Rep 16: 63-68. [PubMed]
51. Li KC, Hua KT, Lin YS, Su CY, Ko JY, et al. (2014) Inhibition of G9a induces
DUSP4-dependent autophagic cell death in head and neck squamous cell
carcinoma. Mol Cancer 13: 172. [PubMed]
52. Huang T, Zhang P, Li W, Zhao T, Zhang Z, et al. (2017) G9A promotes tumor
cell growth and invasion by silencing CASP1 in non-small-cell lung cancer
cells. Cell Death Dis 6: e2726. [PubMed]
53. Tachibana M, Sugimoto K, Fukushima T, Shinkai Y (2001) Set domain-
containing protein, G9a, is a novel lysine-preferring mammalian histone
methyltransferase with hyperactivity and specific selectivity to lysines 9 and
27 of histone H3. J Biol Chem 276: 25309-17. [PubMed]
54. Zhang T, Termanis A, Özkan B, Bao XX, Culley J, et al. (2016) G9a/GLP
complex maintains imprinted DNA methylation in embryonic stem cells. Cell
Rep 15: 77-85. [PubMed]