The TRPV4-TAZ Mechanotransduction Signaling Axis in Matrix Stiffness- and TGFβ1-Induced Epithelial-Mesenchymal Transition

Abstract

Introduction

The implantation of biomaterials into soft tissue leads to the development of foreign body response, a non-specific inflammatory condition that is characterized by the presence of fibrotic tissue. Epithelial–mesenchymal transition (EMT) is a key event in development, fibrosis, and oncogenesis. Emerging data support a role for both a mechanical signal and a biochemical signal in EMT. We hypothesized that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive channel, is a mediator of EMT.

Methods

Normal human primary epidermal keratinocytes (NHEKs) were seeded on collagen-coated plastic plates or varied stiffness polyacrylamide gels in the presence or absence of TGFβ1. Immunofluorescence, immunoblot, and polymerase chain reaction analysis were performed to determine expression level of EMT markers and signaling proteins. Knock-down of TRPV4 function was achieved by siRNA transfection or by GSK2193874 treatment.

Results

We found that knock-down of TRPV4 blocked both matrix stiffness- and TGFβ1-induced EMT in NHEKs. In a murine skin fibrosis model, TRPV4 deletion resulted in decreased expression of the mesenchymal marker, α-SMA, and increased expression of epithelial marker, E-cadherin. Mechanistically, our data showed that: (i) TRPV4 was essential for the nuclear translocation of TAZ in response to matrix stiffness and TGFβ1; (ii) Antagonism of TRPV4 inhibited both matrix stiffness-induced and TGFβ1-induced expression of TAZ proteins; and (iii) TRPV4 antagonism suppressed both matrix stiffness-induced and TGFβ1-induced activation of Smad2/3, but not of AKT.

Conclusions

These data identify a novel role for TRPV4-TAZ mechanotransduction signaling axis in regulating EMT in NHEKs in response to both matrix stiffness and TGFβ1.

Electronic supplementary material

The online version of this article (10.1007/s12195-018-00565-w) contains supplementary material, which is available to authorized users.

Introduction

Emerging data support a critical role of matrix stiffness (or rigidity) in numerous pathophysiological and cellular processes including embryonic development, wound healing, fibrosis, oncogenesis, differentiation, migration and proliferation.15,33,35,68,76,88 Tissue stiffness is not static; it changes during injury, aging, and disease.15,32,68,75 Cells sense and respond to the changing stiffness of their surroundings through the mechanotransduction pathway. Alterations in the matrix stiffness promote transdifferentiation of epithelial to mesenchymal (EMT) phenotype, and thereby, play a key role in the development of fibrosis and tumor.5,7,8,37,46,55,61,80 EMT is a cell differentiation process promoted by numerous biochemical/molecular changes in which epithelial cells lose their polarity, lose cell–cell adhesion, and acquire motile mesenchymal cell properties.5,7,72,80 Fibrotic diseases including skin fibrosis are characterized by an increase in the invasion and migration of mesenchymal cells across a stiffened ECM, which is associated with induction of expression of TGFβ1, differentiation of fibroblasts, and hallmarks of EMT.50,54 However, it is not fully understood how mechanical and biochemical signals are transduced and propagated to drive EMT.

Ca²⁺ influx occurring through plasma membrane channels is involved in regulation of various cellular events/pathways, including muscle contraction, gene expression, neurotransmitter release, cell proliferation, differentiation, and migration.6,28,29,49 EMT induction in cancer cells is associated with altered cytosolic calcium levels.3,13 Specific Ca²⁺-permeable channels have been identified that play key roles in cancer cell proliferation, migration, and EMT induction.3,13,62 Recent studies have reported that TRPV4 regulates both biochemical stimulus- and mechanical stimulus-induced lung myofibroblast differentiation, numerous functions of epithelial cells, and contributes to the development of in vivo pulmonary and dermal fibrosis in murine models.10,17,18,20,22,45,58,60,66,69 Further, we reported that crosstalk between TRPV4 and TGFβ1 signals was essential for optimal induction of myofibroblast differentiation.60 Previous findings that TRPV4 was linked to epidermal barrier maintenance, contributes to the development of in vivo pulmonary and skin fibrosis in murine models, associated with scleroderma, and that TGFβ1 signals were associated with EMT4,16,19,30,31,65,85,87,89 hinted at this crosstalk. TRPV4 is associated with numerous physiological and pathological processes.10,18,45,60,69 Mutations in TRPV4 channels have been associated with human diseases.17,20 TRPV4 is abundantly present in skin, and has been shown to be activated by both biochemical and physical stimuli in numerous cell types.58,66 However, the specific role of TRPV4 in TGFβ1/matrix stiffness-induced EMT has not been determined.

EMT is associated with changes in levels of expression and activation of numerous proteins including ECAD (E-cadherin), NCAD (E-cadherin), α-SMA, vimentin, YAP, and TAZ.61,89 Both YAP and TAZ have been reported to regulate various cell processes including EMT in response to stiffness.16 Crosstalk between YAP/TAZ signaling and TGFβ1 signaling has been suggested as an underlying mechanism during EMT.16 TGFβ1 is known to induce EMT via canonical (Smad-dependent) or non-canonical (Smad-independent) pathways.4,19,31,54,85,87 TGFβ1 binds to its receptor complex and activates Smad2/3 or non-Smad pathways (such as PI3 K, p38, JNK, or ERK), which transmit signals to the nucleus.4,19,31,54,85,87 However, the role of TRPV4 in regulation of YAP/TAZ activity and its role in EMT have not been determined.

In the present study, we found that TRPV4 was required for both TGFβ1- and matrix stiffness-induced EMT-like events in human keratinocytes. Additionally, we found that TRPV4 antagonism abrogated phosphorylation of Smad2/3, and abrogated nuclear accumulation and expression of TAZ in keratinocytes. We also acquired in vivo evidence that TRPV4 associates with EMT in a murine skin fibrosis model. Altogether, these results identify a novel regulatory role for TRPV4-TAZ signaling axis in EMT induced by both TGFβ1 and matrix stiffness.

Materials and Methods

Reagents

Antibodies against phospho-Smad2 (p-Smad2), p-Smad3, Smad2, Smad3, AKT, p-AKT, p-p38, p38, p-Erk1/2, Erk1/2, ECAD, NCAD, and TAZ were purchased from Cell Signaling Technology (Beverly, MA, USA). Anti-TRPV4 primary antibody was purchased from Alomone Labs (Jerusalem, Israel). Antibodies against β-Actin and GAPDH were purchased from Santa Cruz Biotechnology (Dallas, TX, USA). Anti-α-SMA, GSK2193874 (GSK219), GSK1016790A (GSK101), SD208, and A23187 (A23) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Mouse and rabbit anti-goat IgG were purchased from Jackson ImmunoResearch (West Grove, PA, USA). TGFβ1 was purchased from R&D Systems (Minneapolis, MN, USA). Alexa Fluor 488/594 conjugated IgG and Prolong diamond DAPI were purchased from Thermo Fisher Scientific (Waltham, MA, USA). FLIPR Calcium 5 assay kit was purchased from Molecular devices (Sunnyvale, CA, USA). Easy coat hydrogels of various degrees of stiffnesses (0.5, 1, 8, 12, 25, and 50 kPa) were purchased from Matrigen Life Technologies (Brea, CA, USA). Catalog number of all reagents and antibodies are included in the supplementary Table 1.

Cell Culture

Normal human primary epidermal keratinocytes (NHEKs) were purchased from ATCC (PCS-200-011). To assess morphological changes in NHEKs, cells were seeded on collagen-coated hydrogels (10 µg/mL) with compliant (0.5) or stiff (12 kPa) matrices. Cells were incubated with or without GSK219 and TGFβ1 (5 ng/mL) in complete keratinocyte media, for 48 h. Treated cells were compared by phase contrast microscopy (Carl Zeiss, Germany) with vehicle controls for any EMT-like morphological changes. Images were captured, and percent EMT-like morphological changes was calculated by counting total number of cuboidal and spindle shaped cells. We have used varied stiffness hydrogels for practical reasons. For immunoblot analysis where we needed more number of cells we used 1 and 25 kPa hydrogels. On contrary, for immunofluorescence assay where we needed few cells we used 0.5 and 8 or 12 kPa hydrogels. We did not observe a significant difference in EMT induction when NHEKs were grown on 8 kPa, 12 or 25 kPa. However, we found significant differences in EMT when NHEKs were grown on soft (0.5 or 1 kPa) vs. stiff matrices (8 kPa, 12 kPa, or 25 kPa). Cells were plated on collagen-coated polyacrylamide gels with pathophysiologically relevant stiffness, which enabled us to decipher the effect of either normal (soft; 0.5–1 kPa) or fibrotic (stiff; 8–50 kPa) skin tissue matrix on TRPV4-dependent EMT-like processes.

Immunofluorescence Staining

Cells were grown on collagen coated coverslips or polyacrylamide hydrogels (10 µg/mL), treated with or without TGFβ1 (5 ng/mL) for 48 h. Samples were immunostained for ECAD, α-SMA and TAZ, followed by incubation with Alexa Fluor 488 or 594 conjugated secondary antibody (1:300; Thermo Fisher Scientific). Immunofluorescence intensity was quantified using ImageJ software (NIH), and the results are presented as Integrated Density or Int. Density (the product of Area and Mean Gray Value). For quantifications of TAZ subcellular localization, TAZ immunofluorescence signal was scored as predominantly nuclear (overlaps with DAPI signal) vs. predominantly cytoplasmic and expressed as a fraction of total cell number. If the ratio of nuclear to cytoplasmic fluorescence exceeds 1, then the cell is positive for nuclear localization.

Western Blot Analysis

NHEKs were seeded on collagen-coated plates or varied stiffness hydrogels (10 µg/mL). At designated times, cells were harvested by digesting in RIPA buffer. Whole cell lysates were separated on 10% SDS-polyacrylamide gels. GAPDH and Actin band densities were used as a loading control.

Murine Skin Fibrosis Model

Congenic wild type C57BL/6 mice (WT) were purchased from Charles River Laboratories (Wilmington, MA). Trpv4 knock out (TRPV4 KO) mice originally generated by Dr. Suzuki (Jichi Medical University, Japan)70 were acquired from Dr. Zhang (Medical College of Wisconsin, Milwaukee, WI).90 The study protocol was approved by the University of Maryland Review Committee, and all experiments were performed in accordance with the IACUC guidelines. Skin fibrosis was induced by bleomycin as described previously.22,84 Briefly, WT and TRPV4 KO mice (n=5 per group) were injected subcutaneously with equal volumes (0.1 mL) of bleomycin (10 mg/kg) or PBS (control) every alternate day for 28 days.

Immunohistochemistry

Skin tissue samples of bleomycin or PBS treated mice were embedded in OCT (Sakura Finetek, USA), and stored at − 80 °C. Cryostat sections (7 µm) were mounted on slides. Skin sections were immunostained for ECAD and α-SMA.

Intracellular Calcium Assay

Ca²⁺ influx in NHEKs was measured by FlexStation3 system using FLIPR calcium 5 Assay Kit (Molecular Devices, Sunnyvale, CA) as previously described.1,60,74 Briefly, NHEKs (1.5 × 10⁴ cells/well) were treated with or without TGFβ1 at 37 °C, 5% CO₂. Cells were incubated with FLIPR kit reagents (Calcium 5 dye in 1X HBSS) for 45 min at 37 °C, followed by incubation with vehicle or TRPV4 antagonist GSK2193874 (GSK219) for 45 min at 37 °C.74 Ca²⁺ influx was induced by the TRPV4 agonist GSK1016790A (GSK101) in vehicle- or GSK219-pretreated NHEKs, and recorded by measuring ΔF/F (Max-Min) as described previously.60 Data are shown as relative fluorescence units (RFU).

Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR)

Total RNA was extracted from NHEKs pretreated with GSK219 and TGFβ1 or vehicle using RNeasy Micro kit (Qiagen) according to the manufacturer’s instructions. qRT-PCR was carried out per the manufacture’s instructions using TRPV4, ECAD, NCAD, Vimentin and GAPDH primers (SYBR Green gene expression Assay, Bio-Rad). Expression of a gene was determined as the amount of the gene relative to mRNA for GAPDH using the comparative C_T method described in the Bio-Rad qRT-PCR system user bulletin.

Knockdown of TRPV4 Expression

Knock-down of TRPV4 expression was achieved by siRNA transfection of cells. Cells were transfected with 50 nM scrambled siRNA, 20 or 50 nM TRPV4 siRNA (Origene), using siLentFect lipid reagent (Bio-Rad), according to the manufacturer’s protocol. Briefly, lipid reagent and siRNAs were diluted into serum free medium. Diluted lipids were mixed with diluted siRNAs and incubated for 20 min at room temperature for complex formation. The complexes were then added to 35 mm dishes. After 4 h, cells were treated with TGFβ1. Cells were harvested and assayed 48 h post-transfection.

Statistical Analysis

All data are expressed as mean ± SEM. Statistical comparisons were performed with Student’s t test or One-way analysis of variance; p < 0.05 was considered significant.

Results

TRPV4 Deletion Blocks EMT Marker Expression in an Experimental Murine Model of Skin Fibrosis

Bleomycin has been shown to induce EMT in skin fibrosis.92 To assess whether TRPV4 deficiency abrogated EMT in a murine model of skin fibrosis, we employed the bleomycin-induced fibrosis model, and analyzed expression of EMT markers between bleomycin- or PBS-treated TRPV4 KO and WT mice. We found a decrease in ECAD and an increase in α-SMA expression in bleomycin-treated WT mice compared to PBS-treated WT mice, as expected (Figs. 1a and 1b). However, we found a significant reduction in the expression of α-SMA and an increase in expression of ECAD in skin sections from bleomycin-treated TRPV4 KO mice compared to skin of WT mice (Figs. 1a and 1b). These results suggest that TRPV4 may play a critical role in skin fibrosis in vivo by facilitating EMT.

Figure 1.

TRPV4 deletion suppresses EMT marker expression in an experimental murine skin fibrosis model. WT and TRPV4 KO mice were injected subcutaneously with bleomycin (Bleo) or PBS (control) 28 days prior to analysis. Skin tissue sections were immunostained for ECAD and α-SMA proteins. (a) Representative immunofluorescence images showing ECAD (green) and α-SMA (red) expression in the epidermis of Bleo-treated WT mice and TRPV4 KO mice. Nuclei were stained with DAPI (blue). Scale bars: 50 µm. (b) Quantitation of results shown in (a). Data are expressed as mean ± SEM, n = 5 mice/group; **p < 0.01, ***p < 0.001, 1-way ANOVA.

TRPV4 Regulates Both TGFβ1- and Matrix Stiffness-Induced EMT-Associated Changes in Primary Human Epidermal Keratinocytes

To assess the role of TRPV4 mechanosensing on EMT in response to increasing matrix stiffness alone or in combination with TGFβ1, we seeded NHEKs on soft (0.5 kPa) or stiff (12 kPa) polyacrylamide hydrogels treated with or without TGFβ1, and examined the occurrence of EMT-like changes. To specifically ascertain whether TRPV4 is involved in matrix stiffness and TGFβ1-induced EMT, we blocked TRPV4 channel activity (Ca²⁺ influx) using the selective inhibitor GSK219.1,60,74 Immunofluorescence analysis showed that TGFβ1 was unable to drive EMT-associated morphological changes in NHEKs under soft (0.5 kPa) conditions (normal skin tissue stiffness) (Figs. 2a and 2b). Under conditions of stiff (12 kPa) matrix (fibrotic skin tissue stiffness) with or without TGFβ1 treatment, the normal epithelial morphology of NHEKs changed to the elongated and spindle-like mesenchymal morphology (Figs. 2a and 2b). However, stiff matrix and TGFβ1 did not induce EMT-like morphological changes in NHEKs pretreated with TRPV4 antagonist, GSK219, and under these conditions cells retained epithelial morphology (Figs. 2a and 2b). Furthermore, immunofluorescent analysis showed that NHEKs grown on soft matrix expressed more ECAD and less α-SMA than cells grown on stiff matrix (Figs. 2a and 2b). Intriguingly, despite the fact that soft matrix did not drive EMT, the addition of TGFβ1 augmented the EMT-like phenotypic and biochemical (expression of α-SMA) changes in NHEKs under soft matrix conditions (Figs. 2a and 2b). We found that NHEKs treated with GSK219 showed increased ECAD and reduced α-SMA staining under stiffness and TGFβ1-treated conditions, indicating loss of mesenchymal properties (Figs. 2a and 2b).

Figure 2.

TRPV4 inhibition prevents matrix stiffness-induced and TGFβ1-induced EMT-like phenotypic changes in primary normal human epidermal keratinocytes (NHEKs). NHEKs were plated on soft (0.5 or 1 kPa) or stiff (12 or 25 kPa) polyacrylamide hydrogels, and were incubated with or without TGFβ1 (5 ng/mL). (a) Cells were incubated with or without TGFβ1 (5 ng/mL) for 72 h. The data shown is one of the representative immunofluorescence images from five different fields per condition. ECAD (green), α-SMA (red), DAPI (blue). Scale bars: 10 µm. (b) Quantitation of results shown in (a). Data are expressed as mean ± SEM, n = 40 cells/condition. ns = non-significant; *p < 0.05, **p < 0.01, ***p < 0.001; 1-way ANOVA. For quantification data are derived from three independent experiments per condition. hpf: high power field. (c) Cells were incubated with or without GSK219 for 48 h. Representative immunoblots from three independent experiments per condition to assess the capacity of TRPV4 inhibition (by GSK219) to inhibit matrix stiffness-induced increases in the expression of NCAD and α-SMA. (d) Quantitation of results shown in (c) using GAPDH as an internal reference. Data are expressed as mean ± SEM of three independent experiments. **p < 0.01; ***p < 0.001; Student’s t test. (e) Cells were incubated with or without TGFβ1 for 48 h. Representative immunoblots from three independent experiments per condition to assess the capacity of TGFβ1 to induce expression of ECAD, NCAD, and α-SMA in NHEKs with or without GSK219. GAPDH is included as a control. (f) Quantitation of results shown in (e). Data are expressed as mean ± SEM, *p < 0.05; ***p < 0.001; Student’s t test. (g) Representative images from five different fields per condition assessing the effect of stiff matrix and inhibition of TRPV4 by GSK219 pretreatment on TGFβ1-induced EMT-like morphological changes in NHEKs. Scale bars: 10 µm. (h) Quantitation of results in (a) by examining mesenchymal phenotype. Data are expressed as mean ± SEM of three independent experiments. n = 40 cells/condition. **p < 0.01, ***p < 0.001; 1-way ANOVA. (i) qRT-PCR analysis was performed to determine TRPV4, ECAD, NCAD, GAPDH, and Vimentin mRNA levels using SYBR Green gene Expression Assay. Ct values were normalized to GAPDH levels. **p < 0.01, ***p < 0.001; 1-way ANOVA.

We further evaluated the status of expression of EMT markers between untreated and GSK219-treated NHEKs in response to increasing matrix stiffnesses by immunoblot analysis. We found that in the absence of GSK219-treatment cells grown on stiff matrix (25 kPa) displayed upregulated α-SMA and NCAD expression and unchanged ECAD compared to soft matrix (1 kPa) (Figs. 2c and 2d). GSK219 treatment reduced matrix stiffness-induced increase in expression level of mesenchymal proteins (NCAD and α-SMA) (Figs. 2c and 2d). As expected, we found that in the absence of GSK219-treatment cells grown on plastic (infinite stiffness) displayed upregulated α-SMA and NCAD expression and suppressed ECAD compared to without TGFβ1 (Figs. 2e and 2f). GSK219 treatment reduced TGFβ1-induced increase in expression level of mesenchymal proteins (NCAD and α-SMA) (Figs. 2e and 2f). However, GSK219 treatment did not show any impact on TGFβ1-induced ECAD level (Figs. 2e and 2f). We inhibited TRPV4 channels in NHEKs, and assessed morphological changes associated with EMT in response to stiffness and TGFβ1. We found that on soft matrix (0.5 kPa) with or without TGFβ1 NHEKs retained their cobblestone epithelial morphology (Figs. 2g and 2h). However, on stiff matrix NHEKs showed significant induction of EMT as evidenced by a change in morphology of numerous NHEKs from cuboidal to spindle shaped (Figs. 2g and 2h). In addition, our real-time quantitative PCR (qRT-PCR) data of EMT markers (ECAD, NCAD, and Vimentin) in TRPV4 siRNA and scramble control transfected NHEKs further support our results (Fig. 2i). Altogether, these results indicate that TRPV4 is required for induction of EMT by either matrix stiffness or TGFβ1.

We have tested the specificity of TRPV4 inhibitor, GSK219, by three different experiments. We have determined Ca²⁺ influx in NHEKs where GSK219 at 500 and 1000 fold higher concentration (2.5 and 5 μM) than IC₅₀ (5 nM) did not inhibit Ca²⁺ influx induced by calcium ionophore A23187 (a non selective inducer of calcium influx) (Supplementary Fig. 1A). We have performed qRT-PCR analysis of EMT markers to check the efficacy of GSK219 at lower concentration (25 and 100 nM). We found that short-term (24 h) GSK219 treatment upregulated expression of ECAD while downregulated vimentin (Supplementary Figs. 1B and 1C). In addition, we tested concentration of GSK219 at levels similar to its IC₅₀ value (5 nM) to assess the capacity of GSK219 to inhibit matrix stiffness and TGFβ1-induced changes in the expression of ECAD and α-SMA. Immunofluorescence analysis revealed that NHEKs treated with GSK219 showed increased ECAD and reduced α-SMA staining under stiffness and TGFβ1-treated conditions, indicating loss of mesenchymal properties (Supplementary Figs. 1D and 1E). We further confirmed the results by blocking expression of TRPV4 with TRPV4 specific siRNA. Our immunofluorescence data of EMT markers (ECAD and α-SMA) in TRPV4 siRNA and scramble control transfected NHEKs showed that NHEKs treated with TRPV4 siRNA contained increased ECAD and reduced α-SMA staining under stiffness and TGFβ1-treated conditions (Supplementary Figs. 2A and 2B).

TRPV4 Channel is Functional in Primary Human Epidermal Keratinocytes

TRPV4 channels are expressed in NHEKs.30 To determine whether TRPV4 channels are required in EMT, we determined if functional TRPV4 channels were present in NHEKs by measuring Ca²⁺ influx in response to increasing concentrations of the TRPV4 specific agonist, GSK101 (1–1000 nM).18 We found that GSK101 induced an increase in Ca²⁺ influx in NHEKs (EC₅₀ = 30 nM) (Figs. 3a and 3b), which was inhibited (IC₅₀ = 5 nM) when cells were pretreated with TRPV4 selective antagonist, GSK219 74 (Figs. 3c and 3d). These results indicate that functional Ca²⁺-permeable TRPV4 channels are expressed in NHEKs.

Figure 3.

TRPV4 calcium-permeable channels are functional in NHEKs. NHEKs (15,000 cells per well) were seeded in collagen-coated (10 μg/mL) 96-well plastic plates. Ca²⁺ influx is shown by relative fluorescence units (RFU) measuring ΔF/F (Max-Min). A23 (2 μM), a calcium ionophore, was used as a positive control. (a) FlexStation 3 recording of Calcium 5 dye-loaded NHEK monolayers assessing concentration-dependent effects of TRPV4 selective agonist, GSK101, on Ca²⁺ influx. (b) Quantitation of results (mean ± SEM) shown in (a). All experiments were performed 3 times in quadruplicate. (c) Effect of TRPV4 selective antagonist, GSK219, on TRPV4 elicited Ca²⁺ influx by its agonist GSK101 (20 nM). (d) Quantitation of results (mean ± SEM) shown in (c). All experiments were performed 3 times in quadruplicate.

TRPV4 Activity Mediates Matrix Stiffness- and TGFβ1-Induced TAZ Expression and Nuclear Accumulation

TAZ is reported to be a critical regulator of TGFβ1-induced EMT.38 It has also been reported that increasing matrix stiffness controls EMT via promoting localization of YAP and TAZ in epithelial cells.2,16,67,71 To assess a possible association between TRPV4 activity and YAP/TAZ signaling in response to TGFβ1, we treated NHEKs with TRPV4 antagonist GSK219 and then stimulated the cells with TGFβ1 for 48 h. Treatment with TGFβ1 induced upregulation of TAZ protein expression, but no change was observed in YAP levels compared to NHEKs not stimulated with TGFβ1 (Figs. 4a and 4b). We found that stiff matrix augmented the TAZ protein expression level compared to soft matrix (Figs. 4c and 4d). Pre-treatment with TRPV4 antagonist GSK219 significantly attenuated both TGFβ1- and matrix stiffness-induced increases in expression of TAZ proteins (Figs. 4a–4d). These data suggest that TRPV4 may play a critical role in expression of TAZ proteins in response to both matrix stiffness and TGFβ1.

Figure 4.

TRPV4 antagonism suppresses expression and nuclear accumulation of TGFβ1-induced and matrix stiffness-induced TAZ protein. Total whole cell protein lysates from NHEKs were resolved on 10% SDS-PAGE and immunoblotted for YAP, TAZ, and GAPDH proteins. (a) Representative immunoblots from three independent experiments per condition assessing effects of TGFβ1 and TRPV4 inhibitor GSK219 on TAZ and YAP protein expression. (b) Densitometric analysis of TAZ in immunoblots shown in (a) using GAPDH as the internal reference. Data are expressed as mean ± SEM of three independent experiments. ***p < 0.001, t test. (c) Representative immunoblots assessing effects of matrix stiffness (1 and 25 kPa) with or without TRPV4 inhibition (by GSK219) on expression of TAZ and YAP in NHEKs. (d) Quantitation of results shown in (c). Data are expressed as mean ± SEM of three independent experiments. ***p < 0.001, t test. (e) NHEKs were seeded on collagen-coated soft (0.5 kPa) or stiff (8 kPa) polyacrylamide hydrogels and incubated with TGFβ1 for 48 h. Representative fluorescent micrographs of cells stained for TAZ (green); nuclei were stained with DAPI (blue). Scale bars: 50 µm. (f) Quantitation of nuclear staining results shown in (e). Data are expressed as mean ± SEM of three independent experiments. ***p < 0.001, n = 20 cells/condition, 1-way ANOVA. (g) Quantitation of TAZ protein expression levels shown in (e). Data are expressed as mean ± SEM of three independent experiments. ***p < 0.001, n = 20 cells/condition, 1-way ANOVA. hpf: high power field.

To assess nuclear accumulation of TAZ, NHEKs were grown on soft (0.5 kPa) or stiff (8 kPa) matrices, with or without GSK219 and TGFβ1. Immunofluorescence staining analysis showed cytoplasmic localization of TAZ in cells grown on soft matrix, as expected (Figs. 4e and 4f). We observed predominant nuclear staining of TAZ in NHEKs grown on stiff matrix with or without TGFβ1, which confirmed that TAZ nuclear accumulation was sensitive to degrees of matrix stiffness (Figs. 4e and 4f). Further, we observed a decrease in TAZ protein in cells grown on soft matrix compared to stiff (Figs. 4e and 4g). TRPV4 antagonism by GSK219 significantly inhibited nuclear localization and expression of TAZ in NHEKs grown on stiff matrix with or without TGFβ1 (Figs. 4e–4g). These results suggest that signals mediated by matrix stiffness and TGFβ1 regulate TAZ activity in a TRPV4-dependent manner.

TRPV4 Inhibition Abrogates Smad2/3 Phosphorylation Required for EMT

We found that compared to untreated NHEKs, TGFβ1 treatment promoted phosphorylation of Smad2 and Smad3 (p-Smad2/3), but there was no change in AKT phosphorylation (Figs. 5a–5d). The absence of TRPV4 activity significantly inhibited p-Smad2/3 levels compared to intact NHEKs with or without TGFβ1 (Figs. 5a and 5b). We also found that stiff (25 kPa) matrix caused a significant increase in p-Smad2/3 levels compared soft matrix (1 kPa) (Figs. 5c and 5d). Blocking TRPV4 channels with GSK219 significantly inhibited matrix stiffness-induced p-Smad2/3 levels (Figs. 5c and 5d). In contrast, TRPV4 antagonism did not cause any significant decrease in phosphorylation levels of AKT, p38, or Erk1/2 under any of the tested conditions (Figs. 5c–5e). These results suggest that TRPV4 is essential in both TGFβ1- and matrix stiffness-induced phosphorylation of Smad2/3 in HNEKs. We used TGFβ receptor inhibitor (SD208) to test whether stiffness induced activation of Smad2/3 and expression of TAZ depends on TGFβ1 pathway. We found that SD208 pretreatment inhibited both p-Smad2/3 and TAZ protein levels in response to stiff matrix in NHEK (Figs. 5f and 5g), suggesting that stiffness results depend upon the TGFβ pathway.

Figure 5.

TRPV4 antagonism blocks TGFβ1-induced and matrix stiffness-induced Smad2/3 phosphorylation. (a) Representative immunoblots assessing TGFβ1-induced phosphorylation of Smad2/3 in the presence or absence of TRPV4 antagonist, GSK219. (b) Quantitation of results shown in (a). Data are expressed as mean ± SEM of three independent experiments. **p < 0.01; ***p < 0.001, t test. (c) Immunoblots showing effect of TRPV4 inhibition on matrix stiffness-induced phosphorylation of Smad2/3 and AKT. (d) Quantitation of results shown in (c). Data are expressed as mean ± SEM of three independent experiments. ns = non-significant; **p < 0.01, ***p < 0.001; t test. (e) Representative immunoblots from three independent experiments per condition assessing TGFβ1-induced phosphorylation of AKT, p38, and Erk1/2 proteins with or without GSK219. (f) Representative immunoblots from three independent experiments per condition assessing TGFβ1-induced p-Smad2/3, total Smad2/3, and TAZ with or without SD208. NHEKs were grown on 50 kDa polyacrylamide hydrogels. (g) Quantitation of results shown in (f). Data are expressed as mean ± SEM of three independent experiments. ***p < 0.001, t test.

Discussion

The importance of EMT in numerous pathophysiological processes including development, organogenesis, oncogenesis, tissue repair, and fibrosis is well recognized.32,35,68,72,75 Emerging data have indicated that augmented matrix stiffness promotes transdifferentiation of epithelial cells to a mesenchymal phenotype, and plays a critical role in the development of fibrosis and oncogenesis.5,7,8,27,37,46,61,80 However, the precise molecular pathway by which a mechanical signal is transduced and maintained intracellularly to drive EMT is not well understood. Previous studies have documented the importance of EMT in lung fibrosis induced by bleomycin, a well-recognized model of fibrotic disease.23,68,72 Our results are in line with previous studies that have identified cells demonstrating features of EMT in fibrotic disease models.14,23,73 Bleomycin administration increases TGFβ1 expression64 and causes decreased expression of epithelial markers and increased expression of mesenchymal markers an in vivo fibrosis model.23,92 A recent report suggests that active TGFβ1 signaling is accompanied by EMT-like changes with an increase in Snail1 and no loss of ECAD in the fibrotic skin of scleroderma patients.50 Scleroderma epithelial cells were not completely transformed, but did adopt some features of mesenchymal cells resembling EMT-like changes that might contribute to fibrosis. In the present study, we found that epidermis of TRPV4 KO mice did not exhibit any EMT-like features, and the mice were protected from bleomycin-induced skin fibrosis, confirming our previous findings.22

It has been shown that the extent and duration of fibrosis-associated EMT can be regulated by both physical and biochemical cues.2,8,16,57 Cells respond phenotypically to changes in the mechanical properties of the surrounding environment. It is known that tissues become stiffer in fibrosis and cancer.9,21 It has been suggested that mechanical stimuli (matrix stiffness) activate latent-TGFβ1 and contribute to its bioavailability.15,82 Recently, we reported that lung and dermal fibroblasts were mechanosensitive, and were induced by stiff matrix to undergo myofibroblast differentiation, a critical process in wound healing and fibrogenesis.60,65 Cells grown on a soft matrix retained epithelial morphology, and they did not transform into elongated mesenchymal cells in response to reduced α-SMA and cytoskeletal tension; however, treatment with TGFβ1 promoted transition to a mesenchymal-like morphology, which was associated with reduced ECAD at the cell periphery.8 This is in agreement with recent reports showing EMT in scleroderma keratinocytes and in airway epithelial cells from asthma patients in response to TGFβ1.24,50 During EMT, loss of ECAD at the cell border facilitates disruption of cell-cell contacts, and synthesis and upregulation of mesenchymal cytoskeletal proteins, such as α-SMA, vimentin, and NCAD, which may exert large contractile forces on the cell and promote activation of latent TGFβ1.8,53,91 In our present study, we did not observe a robust effect of exogenous TGFβ1 on cells plated on stiff matrix, possibly because endogenous latent-TGFβ1 was activated in response to increased matrix stiffness.8,53,91 Increased matrix rigidity is known to regulate TGFβ1-induced EMT through multiple pathways, and is known to contribute to the development of fibrosis.8,21,37,53 Our data here showed that TRPV4 mechanosensing regulated EMT in response to both matrix stiffness and TGFβ1 signals in NHEKs, suggesting an important role for TRPV4 in EMT.

It has been well established that TGFβ1-induced EMT occurs via both Smad and non-Smad pathways.54,87,89 TGFβ1 can induce transcription of ECAD-repressors, Snail1 and Slug through Smad, PI3 K, and ERK pathways, and can promote a reduction in ECAD and an increase in NCAD and α-SMA expression.11,12,47,56,59,83,85 Mechanical forces including matrix stiffness can act directly on latent-TGFβ1 to release soluble active-TGFβ1.15,82 It is also known that stiffness-induced signals can be converted to biochemical/soluble signals that intersect with other signaling pathways.26,43,77 For example, the transcription cofactors YAP and TAZ respond to changes in matrix stiffness,16,52 and are known to activate TGFβ1 signaling in the context of oncogenesis and fibrosis.2,16,39,57,63,71 Though YAP and TAZ share some common functions, in comparison to YAP, TAZ binds to more DNA transcription factors and participates in various cellular processes such as proliferation, EMT, and migration.36,79,86 Of relevance, in fibrotic lung, TAZ mediates stiffness signals independent of TGFβ1 signaling, driving fibroblast activation and fibrosis, which is associated with prominent nuclear expression of TAZ.39,51 Similarly, in our studies, we noted that both stiffness and TGFβ1 induced upregulation of TAZ but not YAP in NHEKs. Our results are in line with a recent report showing that TGFβ1 induced activation of TAZ in mesenchymal and epithelial cells, and induced upregulation of TAZ in a kidney fibrosis model.44 Our data also showed increased nuclear accumulation of TAZ in cells grown on stiff matrix compared to cells grown on soft matrix or treated with TGFβ1. These observed responses may be related to increased cytoskeletal tension in the cells grown on stiff matrices compared to soft, as published.2,16,51 These results highlight a possible regulatory role of TRPV4 mechanosensing in TAZ transcriptional activity.

Crosstalk has been observed between YAP and TAZ signaling and TGFβ1 signaling.39,42,63,71 Binding of TGFβ1 to its receptors triggers EMT through Smad-dependent and Smad-independent pathways.4,19,31,54,85,87 We found that TRPV4 inhibition suppressed Smad2/3 phosphorylation in response to both TGFβ1 and increased matrix stiffness. The decrease in stiffness-induced Smad2/3 phosphorylation and a reduction in expression of TAZ proteins by TGFβ receptor inhibitor, SD208, suggest that stiffness results depend upon the TGFβ pathway.71,78 Recently, it has been reported that stiff matrix potentiates TGFβ1-induced renal fibrosis in a YAP/TAZ- and Smad2/3-dependent manner.71 Our findings suggest an association between TRPV4, TAZ, and Smad-dependent pathway in response to both TGFβ1- and matrix stiffness-induced EMT-like changes.

It is known that TGFβ1-induced EMT can be regulated by activation of the PI3 K/AKT pathway.54,87,89 It has also been shown that increasing stiffness regulates TGFβ1-induced EMT through PI3 K/AKT signaling.37 In our present study, we found that in NHEKs neither TGFβ1- nor matrix stiffness-induced upregulation of p-AKT were sensitive to TRPV4 antagonism. MAPK pathway components such as p38 and ERK are reported to be downstream activators of TRPV4.25,48 We found that TRPV4 inhibition did not induce significant changes in the phosphorylation levels of p38 and ERK1/2 proteins after TGFβ1 stimulation. However, a recent report suggested that mechanical stretch-mediated activation of ERK and p38 pathways is partially mediated via TRPV4 in fetal lung epithelial cells.48 Further investigations will be required to determine the role of MAPK or AKT signaling pathways in TGFβ1- and stiffness-induced EMT.

The cytosolic calcium level is critical for mediating numerous cellular signaling pathways including EMT.1,22,30,43,60,65,83 For example, removal of intracellular Ca²⁺ blocks induction of proteins associated with EMT in epithelial cells and breast cancer cells.13,41 Recent studies identified a critical role of calcium-permeable channels in EMT induction in the context of fibrosis and oncogenesis.34,40,81 Here, we found that pharmacologic inhibition of TRPV4-induced Ca²⁺ influx in NHEKs prevented both matrix stiffness- and TGFβ1-induced loss of ECAD, and this was accompanied by increases in NCAD and α-SMA expression along with EMT-like morphological changes. We recently reported that TRPV4-induced Ca²⁺ influx was essential for TGFβ1-induced dermal myofibroblast differentiation in response to TGFβ1 and matrix stiffness.22,60,65 We found that genetic deletion of TRPV4 protected mice from bleomycin-induced lung and skin fibrosis.22,60,65 Although TGFβ1 and matrix stiffness was shown to play an important role in EMT and fibrosis, a mechanosensing role of TRPV4 in regulating EMT has not been reported.

In summary, our data identify a novel role of TRPV4 in regulating matrix stiffness- and TGFβ1-induced EMT-like behavior in normal human epidermal keratinocytes. Our data may suggest a plausible mechanism whereby mechanotransduction via TRPV4 channels induce protein expression and nuclear accumulation of TAZ, which is followed by Smad2/3 activation to promote EMT, and may contribute to the progression of fibrosis/oncogenesis. Overall, our findings suggest that TRPV4 blockade by small selective inhibitors could serve as a targeted therapeutic approach to prevent EMT-associated diseases.

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