A novel mechanoeffector role of fibroblast S100A4 in myofibroblast transdifferentiation and fibrosis

Fibroblast to myofibroblast transdifferentiation mediates numerous fibrotic disorders, such as idiopathic pulmonary fibrosis (IPF). We have previously demonstrated that non-muscle myosin II (NMII) is activated in response to fibrotic lung extracellular matrix, thereby mediating myofibroblast transdifferentiation. NMII-A is known to interact with the calcium-binding protein S100A4, but the mechanism by which S100A4 regulates fibrotic disorders is unclear. In this study, we show that fibroblast S100A4 is a calcium-dependent, mechanoeffector protein that is uniquely sensitive to pathophysiologic-range lung stiffness (8-25 kPa) and thereby mediates myofibroblast transdifferentiation. Re-expression of endogenous fibroblast S100A4 rescues the myofibroblastic phenotype in S100A4 KO fibroblasts. Analysis of NMII-A/actin dynamics reveals that S100A4 mediates the unraveling and redistribution of peripheral actomyosin to a central location, resulting in a contractile myofibroblast. Furthermore, S100A4 loss protects against murine in vivo pulmonary fibrosis, and S100A4 expression is dysregulated in IPF. Our data reveal a novel mechanosensor/effector role for endogenous fibroblast S100A4 in inducing cytoskeletal redistribution in fibrotic disorders such as IPF.

Stretching the Function of Innate Immune Cells

The importance of innate immune cells to sense and respond to their physical environment is becoming increasingly recognized. Innate immune cells (e.g. macrophages and neutrophils) are able to receive mechanical signals through several mechanisms. In this review, we discuss the role of mechanosensitive ion channels, such as Piezo1 and transient receptor potential vanilloid 4 (TRPV4), and cell adhesion molecules, such as integrins, selectins, and cadherins in biology and human disease. Furthermore, we explain that these mechanical stimuli activate intracellular signaling pathways, such as MAPK (p38, JNK), YAP/TAZ, EDN1, NF-kB, and HIF-1α, to induce protein conformation changes and modulate gene expression to drive cellular function. Understanding the mechanisms by which immune cells interpret mechanosensitive information presents potential targets to treat human disease. Important areas of future study in this area include autoimmune, allergic, infectious, and malignant conditions.

The Role of TRPV4 in Regulating Innate Immune Cell Function in Lung Inflammation

Ion channels/pumps are essential regulators of innate immune cell function. Macrophages have been increasingly recognized to have phenotypic plasticity and location-specific functions in the lung. Transient receptor potential vanilloid 4 (TRPV4) function in lung injury has been shown to be stimulus- and cell-type specific. In the current review, we discuss the importance of TRPV4 in macrophages and its role in phagocytosis and cytokine secretion in acute lung injury/acute respiratory distress syndrome (ARDS). Furthermore, TRPV4 controls a MAPK molecular switch from predominately c-Jun N-terminal kinase, JNK activation, to that of p38 activation, that mediates phagocytosis and cytokine secretion in a matrix stiffness-dependent manner. Expanding knowledge regarding the downstream mechanisms by which TRPV4 acts to tailor macrophage function in pulmonary inflammatory diseases will allow for formulation of novel therapeutics.

Transient Receptor Potential Vanilloid 4 (TRPV4) Protects the Lung from Bacterial Pneumonia via MAPK Molecular Switching

Mechanical cell-matrix interactions can drive the innate immune responses to infection, however the molecular underpinnings of these responses remain elusive. We have discovered that the biophysical properties of the matrix in the range of injured fibrotic lung (≥ 25 kPa) conditions the macrophage response to LPS and infection respectively through the mechanosensitive cation channel, TRPV4 in vitro and in vivo. Studies suggest that LPS-induced macrophage activation is controlled in part by the MAPK pathway (i.e. p38, ERK, JNK). Thus, we investigated if TRPV4 plays a role in the macrophage activation response after LPS through alteration of the MAPK pathway. TRPV4 KO mice exhibited reduced lung bacterial clearance by macrophages (6-fold, p=0.012) after intratracheal P. aeruginosa administration and increased lung injury as measured by inflammatory cell infiltration (≥80±3%, p<0.05), vascular permeability (BAL total protein ≥63±6%, p<0.05), and pro-inflammatory cytokine secretion in BALF (IL-6, CCL2, CXCL1 ≥71±4%, p<0.05). LPS-induced p38 activation was decreased (69%, p<0.05) while JNK activation was increased (2-fold, p<0.05) in a stiffness-dependent manner with no change in ERK activation in TRPV4 KO BMDMs. Inhibition of p38 (SB203580, BIRB796) decreased phagocytosis whereas inhibition of JNK (SP600125) decreased cytokine secretion (IL-6, CCL2, and CXCL1) after LPS (2 fold, p<0.05). DUSP1/MKP1 (MAPK phosphatase) protein was reduced by 3-fold in TRPV4 KO and inhibition of DUSP1 decreased phagocytosis and selectively increased activation of JNK. These data are the first to demonstrate new roles for macrophage TRPV4 in regulating innate immunity in a mechanosensitive manner, through MAPK activation switching.

TRPV4 Protects the Lung from Bacterial Pneumonia via MAPK Molecular Pathway Switching

Mechanical cell-matrix interactions can drive the innate immune responses to infection; however, the molecular underpinnings of these responses remain elusive. This study was undertaken to understand the molecular mechanism by which the mechanosensitive cation channel, transient receptor potential vanilloid 4 (TRPV4), alters the in vivo response to lung infection. For the first time, to our knowledge, we show that TRPV4 protects the lung from injury upon intratracheal Pseudomonas aeruginosa in mice. TRPV4 functions to enhance macrophage bacterial clearance and downregulate proinflammatory cytokine secretion. TRPV4 mediates these effects through a novel mechanism of molecular switching of LPS signaling from predominant activation of the MAPK, JNK, to that of p38. This is accomplished through the activation of the master regulator of inflammation, dual-specificity phosphatase 1. Further, TRPV4's modulation of the LPS signal is mechanosensitive in that both upstream activation of p38 and its downstream biological consequences depend on pathophysiological range extracellular matrix stiffness. We further show the importance of TRPV4 on LPS-induced activation of macrophages from healthy human controls. These data are the first, to our knowledge, to demonstrate new roles for macrophage TRPV4 in regulating innate immunity in a mechanosensitive manner through the modulation of dual-specificity phosphatase 1 expression to mediate MAPK activation switching.

Translocation of TRPV4-PI3Kγ complexes to the plasma membrane drives myofibroblast transdifferentiation

Myofibroblasts are key contributors to pathological fibrotic conditions of several major organs. The transdifferentiation of fibroblasts into myofibroblasts requires both a mechanical signal and transforming growth factor-β (TGF-β) signaling. The cation channel transient receptor potential vanilloid 4 (TRPV4) is a critical mediator of myofibroblast transdifferentiation and in vivo fibrosis through its mechanosensitivity to extracellular matrix stiffness. Here, we showed that TRPV4 promoted the transdifferentiation of human and mouse lung fibroblasts through its interaction with phosphoinositide 3-kinase γ (PI3Kγ), forming nanomolar-affinity, intracellular TRPV4-PI3Kγ complexes. TGF-β induced the recruitment of TRPV4-PI3Kγ complexes to the plasma membrane and increased the activities of both TRPV4 and PI3Kγ. Using gain- and loss-of-function approaches, we showed that both TRPV4 and PI3Kγ were required for myofibroblast transdifferentiation as assessed by the increased production of α-smooth muscle actin and its incorporation into stress fibers, cytoskeletal changes, collagen-1 production, and contractile force. Expression of various mutant forms of the PI3Kγ catalytic subunit (p110γ) in cells lacking PI3Kγ revealed that only the noncatalytic, amino-terminal domain of p110γ was necessary and sufficient for TGF-β-induced TRPV4 plasma membrane recruitment and myofibroblast transdifferentiation. These data suggest that TGF-β stimulates a noncanonical scaffolding action of PI3Kγ, which recruits TRPV4-PI3Kγ complexes to the plasma membrane, thereby increasing myofibroblast transdifferentiation. Given that both TRPV4 and PI3Kγ have pleiotropic actions, targeting the interaction between them could provide a specific therapeutic approach for inhibiting myofibroblast transdifferentiation.

Abstract 254: Kinase-dead Pi3kγ Expression in the Cardiac Myocytes Regulates Fibroblast Signaling and Myofibroblast Differentiation

Phosphoinositide 3 Kinase γ (PI3Kγ) is an anti-apoptotic molecule acting through Akt pathway. Even though, role of PI3Kγ in cardiac fibrosis has been established, the mechanistic details by which PI3Kγ regulates cardiac myofibroblast differentiation are not clear. Myofibroblasts are hallmark of tissue fibrosis, characterized by smooth muscle α-actin (αSMA) over-expression. We have previously shown that αSMA abundance in cardiac lysates from PI3Kγ null mice (PI3Kγ -/- ) showed significant baseline and pressure overload [Transverse Aortic Constriction (TAC)] induced upregulation compared to wildtype (WT), indicating that loss of PI3Kγ predisposes the hearts towards fibrosis. Furthermore, isolated cardiac fibroblasts (CF) from PI3Kγ -/- exhibited a myofibroblast phenotype with αSMA in stress fibers. Moreover, cardiomyocyte-specific over-expression of kinase-dead PI3Kγ (PI3Kγ inact ) in the global PI3Kγ -/- (PI3Kγ inact /PI3Kγ -/- ) reduced αSMA abundance and myofibroblast differentiation suggesting unique kinase-independent function of PI3Kγ in myocyte-initiated pathway that drives CF to become myofibroblasts. Conditioned media experiments showed that PI3Kγ -/- myocytes release pro-fibrotic factors and PI3Kγ inact /PI3Kγ -/- myocytes release fibrosis protective factors. We have previously observed that PI3Kγ regulated MAPK signaling in fibroblasts in a kinase-independent manner by sequestering PP2A association and activity. Previous studies have shown that fibroblast growth factor mediated activation of the signaling pathway downregulates αSMA and that this inhibition of αSMA expression is through negative regulation by extracellular regulated kinase (ERK). Consistent with these previous observations, PI3Kγ possibly mediates αSMA and myofibroblast differentiation through regulation of ERK signaling in the fibroblasts. Intriguingly, we observed presence of PI3Kγ when lysates of isolated CF from PI3Kγ inact /PI3Kγ -/- were immunoblotted for PI3Kγ. These data indicate that a unique communication between myocytes and fibroblasts regulated by PI3Kγ, leads to a compensatory mechanism that results in expression of PI3Kγ in the fibroblasts, thereby regulating fibroblast signaling in myofibroblast differentiation.

Abstract 528: Cell Specific Signaling Regulated by PI3Kγ Modulates Myofibroblast Differentiation

Phosphoinositide 3 Kinase γ (PI3Kγ) regulates anti-apoptotic Akt signaling. Previous studies have established a role for PI3Kγ in cardiac fibrosis, a key underlying cause of heart failure. However, less is known about the mechanism by which PI3Kγ regulates cardiac myofibroblast differentiation, hallmark of tissue fibrosis, characterized by smooth muscle α-actin (αSMA) overexpression. Measurement of αSMA abundance in cardiac lysates from PI3Kγ null mice (PI3Kγ -/- ) showed significant baseline and pressure overload induced upregulation compared to wildtype (WT), indicating that loss of PI3Kγ predisposes the hearts towards fibrosis. Furthermore, isolated cardiac fibroblasts (CF) from PI3Kγ -/- exhibited a myofibroblast phenotype with αSMA in stress fibers. Correspondingly, immunoblotting showed significantly higher αSMA in PI3Kγ -/- CF than WT. It has been previously shown that fibroblast growth factor mediated activation of the signaling pathway downregulates αSMA and that this inhibition of αSMA expression is through negative regulation by extracellular regulated kinase (ERK). To understand whether PI3Kγ regulates ERK signaling in a cell specific manner and thereby possibly αSMA, the phosphorylation of ERK by insulin stimulation were compared in CF isolated from WT and PI3Kγ -/- . Intriguingly, there was significant loss of ERK phosphorylation in CF from PI3Kγ -/- when compared to CF from WT. However, ERK phosphorylation was not altered in cardiomyocytes (CM) due to the absence of PI3Kγ. Confirming this differential regulation of ERK in CM and CF, there was no change in the association of ERK and protein phosphatase 2A (PP2A) in CM of WT and PI3Kγ -/- . However, there was increased association of PP2A with ERK in CF of PI3Kγ -/- when compared to CF from WT. This is because in the CM, ERK is regulated by Dual Specificity Phosphatase (DUSP8). However, in the CF, PP2A is the major regulator of ERK and thus PI3Kγ plays a major role in ERK signaling in CF. Consistent with previous observations, we found that association of DUSP8 with ERK was not observed in CF. Moreover, ERK-DUSP8 interaction was not dependent on the presence or absence of PI3Kγ. These data indicate that PI3Kγ regulates signaling pathways in a cell specific manner with respect to cardiac remodeling.

The Mechanosensitive Ion Channel, Transient Receptor Potential Vanilloid 4 (TRPV4) in Macrophages Regulates the Host Defense Response to Bacterial Pneumonia

Macrophage phagocytosis and cytokine production are sensitive to the surrounding matrix and thereby mediate host defense and lung tissue injury. The consequences and mechanism of this matrix sensitivity are unknown. We have determined that the mechanosensitive channel, TRPV4, responds to extracellular matrix biophysical properties and thereby modulates the macrophage response to pathogens. We undertook this study to determine the in vivo consequences and the intracellular signaling pathway by which TRPV4 modulates macrophage responses. In order to evaluate the role of TRPV4 in chronic pneumonia/lung injury, WT and TRPV4 KO mice were administered agarose bead embedded-Pseudomonas aeruginosa. Loss of TRPV4 led to decreased phagocytosis by macrophages (6-fold), and increased lung injury as measured by inflammatory cell infiltration (≥ 80 ± 3%), vascular permeability (total protein ≥ 63 ± 6%), and cytokine secretion (IL-1β ≥ 71 ± 4%). In vivo, lung alveolar macrophages predominantly expressed TRPV4 and were the key phagocytic cell, as assessed by FACS and immunofluorescence. Known LPS signaling pathways were investigated in vitro. Loss of TRPV4 abrogates LPS-induced p38 activation and phagocytosis of E. coli particles. Additionally, loss of TRPV4 increased basal pro-/active IL-1β expression (2-fold), thereby enhancing IL-1β secretion independent of caspase 1 cleavage and blockade. These findings demonstrate that TRPV4 is important for bacterial clearance, lung injury and cytokine production in macrophages. TRPV4 mediates these effects through p38 and through upregulation of IL-1β protein expression in an inflammasome-independent manner. The results implicate macrophage TRPV4 as a key component of the host defense.

Abstract 109: PI3Kγ Regulates Release of Myocyte-derived Factors Responsible for Myofibroblast Differentiation and Cardiac Fibrosis

Phosphoinositide 3 Kinase γ (PI3Kγ) is a lipid kinase that regulates downstream anti-apoptotic Akt signaling. Thus, pressure overload in PI3Kγ null (PI3Kγ -/- ) mice leads to significant cardiac fibrosis, a key underlying cause of fatal heart failure. Classical hallmark of tissue fibrosis is differentiation of fibroblasts to myofibroblasts characterized by smooth muscle α-actin (αSMA) overexpression. However, less is known about the role of PI3Kγ in cardiac myofibroblast differentiation. Assessment of αSMA expression in cardiac lysates from WT and PI3Kγ -/- showed significant baseline upregulation in PI3Kγ -/- showing that loss of PI3Kγ predisposes the hearts towards fibrosis. To directly confirm that PI3Kγ -/- cardiac fibroblasts (CF) exhibit a myofibroblast phenotype, CF were isolated from hearts of WT and PI3Kγ -/- and assessed by immunostaining for αSMA in stress fibers. Greater number of CF from PI3Kγ -/- exhibited αSMA in stress fibers than CF from WT. Correspondingly, immunoblotting showed significantly higher expression of αSMA in PI3Kγ -/- CF compared to WT showing enhanced myofibroblast differentiation by PI3Kγ -/- fibroblasts. Surprisingly, abundance of αSMA protein is significantly reduced in the hearts of mice with cardiomyocyte-specific expression of kinase-dead PI3Kγ (PI3Kγ inact ) in the PI3Kγ -/- (PI3Kγ inact /PI3Kγ -/- ) suggesting that myocytes derived factors responsible for myofibroblast differentiation are regulated by kinase-independent function of PI3Kγ. To directly evaluate the PI3Kγ-dependent cardiomyocyte derived factors responsible for myofibroblast differentiation; fibroblasts were treated with conditioned media derived from primary adult cardiomyocytes from WT, PI3Kγ -/- and PI3Kγ inact /PI3Kγ -/- mice. Conditioned media derived from PI3Kγ -/- showed pro-fibrotic effects, while that from PI3Kγ inact /PI3Kγ -/- showed fibrosis protective biological activity compared to WT. These findings reveal that kinase-independent function of PI3Kγ is a key regulator of the myocyte-initiated pathway that ultimately drives myofibroblast conversion. Proteomic analysis of conditioned media identified several pro-fibrotic factors that are regulated by PI3Kγ, the results of which will be discussed.

The Role of Transient Receptor Potential Vanilloid 4 in Pulmonary Inflammatory Diseases

Ion channels/pumps are essential regulators of organ homeostasis and disease. In the present review, we discuss the role of the mechanosensitive cation channel, transient receptor potential vanilloid 4 (TRPV4), in cytokine secretion and pulmonary inflammatory diseases such as asthma, cystic fibrosis (CF), and acute lung injury/acute respiratory distress syndrome (ARDS). TRPV4 has been shown to play a role in lung diseases associated with lung parenchymal stretch or stiffness. TRPV4 indirectly mediates hypotonicity-induced smooth muscle contraction and airway remodeling in asthma. Further, the literature suggests that in CF TRPV4 may improve ciliary beat frequency enhancing mucociliary clearance, while at the same time increasing pro-inflammatory cytokine secretion/lung tissue injury. Currently it is understood that the role of TRPV4 in immune cell function and associated lung tissue injury/ARDS may depend on the injury stimulus. Uncovering the downstream mechanisms of TRPV4 action in pulmonary inflammatory diseases is likely important to understanding disease pathogenesis and may lead to novel therapeutics.

Correlation of higher levels of soluble TNF-R1 with a shorter survival, independent of age, in recurrent glioblastoma

The circulating levels of soluble tumor necrosis factor receptor-1 (sTNF-R1) and sTNF-R2 are altered in numerous diseases, including several types of cancer. Correlations with the risk of progression in some cancers, as well as systemic manifestations of the disease and therapeutic side-effects, have been described. However, there is very little information on the levels of these soluble receptors in glioblastoma (GBM). Here, we report on an exploratory retrospective study of the levels of sTNF-Rs in the vascular circulation of patients with GBM. Banked samples were obtained from 112 GBM patients (66 untreated, newly-diagnosed patients and 46 with recurrent disease) from two institutions. The levels of sTNF-R1 in the plasma were significantly lower in patients with newly-diagnosed or recurrent GBM than apparently healthy individuals and correlated with the intensity of expression of TNF-R1 on the tumor-associated endothelial cells (ECs) in the corresponding biopsies. Elevated levels of sTNF-R1 in patients with recurrent, but not newly-diagnosed GBM, were significantly associated with a shorter survival, independent of age (p = 0.02) or steroid medication. In contrast, the levels of circulating sTNF-R2 were significantly higher in recurrent GBM than healthy individuals and there was no significant correlation with expression of TNF-R2 on the tumor-associated ECs or survival time. The results indicate that larger, prospective studies are warranted to determine the predictive value of the levels of sTNF-R1 in patients with recurrent GBM and the factors that regulate the levels of sTNF-Rs in the circulation in GBM patients.

Abstract 423: Kinase Independent Signaling of PI3Kγ Mediates Cardiac Fibrosis

Phosphoinositide 3 Kinase γ (PI3Kγ) belongs to a family of lipid kinases genetic deletion of which leads to pressure overload induced cardiac fibrosis in mice. However, the mechanism by which PI3Kγ mediates cardiac fibrosis is unknown. Cardiac fibrosis is a key underlying cause of fatal heart failure. A well-known fibrogenic mechanism is the generation of myofibroblasts, which are characterized by overexpression of smooth muscle α-actin (αSMA). Myofibroblast is a fibrosis-effector cell that produces pro-fibrotic cytokines and exuberant extracellular matrix that leads to cardiac fibrosis. To evaluate the role of PI3Kγ in fibrotic phenotype, cardiac tissue lysates from 3 months old WT and PI3Kγ null (PI3Kγ -/- ) mice were assessed for the expression of αSMA. Interestingly, there is significant up-regulation of αSMA in PI3Kγ -/- in comparison to littermate controls (WT) even at baseline suggesting that loss of PI3Kγ predisposes the hearts towards fibrosis. To directly confirm that PI3Kγ -/- cardiac fibroblasts (CF) exhibit a myofibroblast phenotype even at baseline, CF were isolated from hearts of WT and PI3Kγ -/- mice and assessed for myofibroblast phenotype by immunostaining for αSMA in stress fibers. Fluorescence microscopy on the CF from PI3Kγ -/- mice showed intense immunostaining for αSMA with greater number of cells exhibiting αSMA in stress fibers when compared to CF from WT mice. Consistently, immunoblotting showed significantly higher αSMA protein levels in PI3Kγ -/- CF compared to WT CF suggesting that PI3Kγ -/- fibroblasts are “primed” to undergo myofibroblast differentiation. To determine the role of kinase-independent function of PI3Kγ in vivo, we generated unique mice lines with cardiomyocyte-specific expression of either kinase-dead PI3Kγ (PI3Kγ inact ) or constitutively active PI3Kγ ( Myr PI3Kγ) in the global PI3Kγ -/- (PI3Kγ inact /PI3Kγ -/- or Myr PI3Kγ/PI3Kγ -/- ) and measured αSMA. Surprisingly, abundance of αSMA protein is significantly reduced in PI3Kγ inact /PI3Kγ -/- when compared to WT and PI3Kγ -/- mice. These data reveal that kinase-independent function of PI3Kγ is a key component in the myocyte-initiated pathway that ultimately drives CF to become myofibroblasts uncovering a novel mechanism of regulating pro-fibrotic signals.

Matrix-driven Myosin II Mediates the Pro-fibrotic Fibroblast Phenotype

Pro-fibrotic mesenchymal cells are known to be the key effector cells of fibroproliferative disease, but the specific matrix signals and the induced cellular responses that drive the fibrogenic phenotype remain to be elucidated. The key mediators of the fibroblast fibrogenic phenotype were characterized using a novel assay system that measures fibroblast behavior in response to actual normal and fibrotic lung tissue. Using this system, we demonstrate that normal lung promotes fibroblast motility and polarization, while fibrotic lung immobilizes the fibroblast and promotes myofibroblast differentiation. These context-specific phenotypes are surprisingly both mediated by myosin II. The role of myosin II is supported by the observation of an increase in myosin phosphorylation and a change in intracellular distribution in fibroblasts on fibrotic lung, as compared with normal lung. Moreover, loss of myosin II activity has opposing effects on protrusive activity in fibroblasts on normal and fibrotic lung. Loss of myosin II also selectively inhibits myofibroblast differentiation in fibroblasts on fibrotic lung. Importantly, these findings are recapitulated by varying the matrix stiffness of polyacrylamide gels in the range of normal and fibrotic lung tissue. Comparison of the effects of myosin inhibition on lung tissue with that of polyacrylamide gels suggests that matrix fiber organization drives the fibroblast phenotype under conditions of normal/soft lung, while matrix stiffness drives the phenotype under conditions of fibrotic/stiff lung. This work defines novel roles for myosin II as a key regulatory effector molecule of the pro-fibrotic phenotype, in response to biophysical properties of the matrix.

TRPV4 Mechanosensitive Ion Channel Regulates Lipopolysaccharide-Stimulated Macrophage Phagocytosis

Macrophage phagocytosis of particles and pathogens is an essential aspect of innate host defense. Phagocytic function requires cytoskeletal rearrangements that depend on the interaction between macrophage surface receptors, particulates/pathogens, and the extracellular matrix. In the present study we determine the role of a mechanosensitive ion channel, transient receptor potential vanilloid 4 (TRPV4), in integrating the LPS and matrix stiffness signals to control macrophage phenotypic change for host defense and resolution from lung injury. We demonstrate that active TRPV4 mediates LPS-stimulated murine macrophage phagocytosis of nonopsonized particles (Escherichia coli) in vitro and opsonized particles (IgG-coated latex beads) in vitro and in vivo in intact mice. Intriguingly, matrix stiffness in the range seen in inflamed or fibrotic lung is required to sensitize the TRPV4 channel to mediate the LPS-induced increment in macrophage phagocytosis. Furthermore, TRPV4 is required for the LPS induction of anti-inflammatory/proresolution cytokines. These findings suggest that signaling through TRPV4, triggered by changes in extracellular matrix stiffness, cooperates with LPS-induced signals to mediate macrophage phagocytic function and lung injury resolution. These mechanisms are likely to be important in regulating macrophage function in the context of pulmonary infection and fibrosis.

TRPV4 mediates myofibroblast differentiation and pulmonary fibrosis in mice

Idiopathic pulmonary fibrosis (IPF) is a fatal fibrotic lung disorder with no effective medical treatments available. The generation of myofibroblasts, which are critical for fibrogenesis, requires both a mechanical signal and activated TGF-β; however, it is not clear how fibroblasts sense and transmit the mechanical signal(s) that promote differentiation into myofibroblasts. As transient receptor potential vanilloid 4 (TRPV4) channels are activated in response to changes in plasma membrane stretch/matrix stiffness, we investigated whether TRPV4 contributes to generation of myofibroblasts and/or experimental lung fibrosis. We determined that TRPV4 activity is upregulated in lung fibroblasts derived from patients with IPF. Moreover, TRPV4-deficient mice were protected from fibrosis. Furthermore, genetic ablation or pharmacological inhibition of TRPV4 function abrogated myofibroblast differentiation, which was restored by TRPV4 reintroduction. TRPV4 channel activity was elevated when cells were plated on matrices of increasing stiffness or on fibrotic lung tissue, and matrix stiffness-dependent myofibroblast differentiation was reduced in response to TRVP4 inhibition. TRPV4 activity modulated TGF-β1-dependent actions in a SMAD-independent manner, enhanced actomyosin remodeling, and increased nuclear translocation of the α-SMA transcription coactivator (MRTF-A). Together, these data indicate that TRPV4 activity mediates pulmonary fibrogenesis and suggest that manipulation of TRPV4 channel activity has potential as a therapeutic approach for fibrotic diseases.

Urokinase-type plasminogen activator receptor (uPAR) ligation induces a raft-localized integrin signaling switch that mediates the hypermotile phenotype of fibrotic fibroblasts

The urokinase-type plasminogen activator receptor (uPAR) is a glycosylphosphatidylinositol-linked membrane protein with no cytosolic domain that localizes to lipid raft microdomains. Our laboratory and others have documented that lung fibroblasts from patients with idiopathic pulmonary fibrosis (IPF) exhibit a hypermotile phenotype. This study was undertaken to elucidate the molecular mechanism whereby uPAR ligation with its cognate ligand, urokinase, induces a motile phenotype in human lung fibroblasts. We found that uPAR ligation with the urokinase receptor binding domain (amino-terminal fragment) leads to enhanced migration of fibroblasts on fibronectin in a protease-independent, lipid raft-dependent manner. Ligation of uPAR with the amino-terminal fragment recruited α5β1 integrin and the acylated form of the Src family kinase, Fyn, to lipid rafts. The biological consequences of this translocation were an increase in fibroblast motility and a switch of the integrin-initiated signal pathway for migration away from the lipid raft-independent focal adhesion kinase pathway and toward a lipid raft-dependent caveolin-Fyn-Shc pathway. Furthermore, an integrin homologous peptide as well as an antibody that competes with β1 for uPAR binding have the ability to block this effect. In addition, its relative insensitivity to cholesterol depletion suggests that the interactions of α5β1 integrin and uPAR drive the translocation of α5β1 integrin-acylated Fyn signaling complexes into lipid rafts upon uPAR ligation through protein-protein interactions. This signal switch is a novel pathway leading to the hypermotile phenotype of IPF patient-derived fibroblasts, seen with uPAR ligation. This uPAR dependent, fibrotic matrix-selective, and profibrotic fibroblast phenotype may be amenable to targeted therapeutics designed to ameliorate IPF.