Focal adhesion kinase (FAK)-related non-kinase inhibits myofibroblast differentiation through differential MAPK activation in a FAK-dependent manner

New insights into the pathogenesis and treatment of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is the most common and lethal of the idiopathic interstitial pneumonias. There are currently no effective pharmacological therapies approved for the treatment of IPF. Despite the focus on targeting fibrogenic pathways, recent clinical trials have been largely disappointing. Progress is being made in elucidating key cellular processes and molecular pathways critical to IPF pathogenesis, and this should facilitate the development of more effective therapeutics for this recalcitrant disease. Emerging pathobiological concepts include the role of aging and cellular senescence, oxidative stress, endoplasmic reticulum stress, cellular plasticity, microRNAs and mechanotransduction. Therapeutic approaches that target molecular pathways to modulate aberrant cellular phenotypes and promote tissue homeostasis in the lung must be developed. Heterogeneity in biological and clinical phenotypes of IPF warrants a personalized medicine approach to diagnosis and treatment of this lung disorder.

Fibronectin splice variants: understanding their multiple roles in health and disease using engineered mouse models

The extracellular matrix (ECM) is a highly dynamic network of proteins, glycoproteins, and proteoglycans. Numerous diseases result from mutation in genes coding for ECM proteins, but only recently it has been reported that mutations in the fibronectin (FN) gene were associated with a human disorder. FN is one of the main components of the ECM. It generates protein diversity through alternative splicing of a single pre-mRNA, having at least 20 different isoforms in humans. The precise function of these protein isoforms has remained obscure in most cases. Only in the recent few years, it was possible to shed light on the multiple roles of the alternatively spliced FN isoforms. This substantial progress was achieved basically with the knowledge derived from engineered mouse models bearing subtle mutations in specific FN domains. These data, together with a recent report associating mutations in the FN gene to a form of glomerulopathy, clearly show that mutations in constitutive exons or misregulation of alternatively spliced domains of the FN gene may have nonlethal pathological consequences. In this review, we focus on the pathological consequences of mutations in the FN gene, by connecting the function of alternatively spliced isoforms of fibronectin to human diseases.

The involvement of integrin β1 signaling in the migration and myofibroblastic differentiation of skin fibroblasts on anisotropic collagen-containing nanofibers

Utilization of nanofibrous matrices for skin wound repair holds great promise due to their morphological and dimensional similarity to native extracellular matrix (ECM). It becomes highly desired to understand how various nanofibrous matrices regulate skin cell behaviors and intracellular signaling pathways, important to tuning the functionality of tissue-engineered skin grafts and affecting the wound healing process. In this study, the phenotypic expressions of normal human dermal fibroblasts (NHDFs) on collagen-containing nanofibrous matrices with either isotropic (i.e., fibers collected randomly with no alignment) or anisotropic (i.e., fibers collected with alignment) fiber organizations were studied by immunostaining, migration assay and molecular analyses. Results showed that both nanofibrous matrices supported the attachment and growth of NHDFs similarly, while showing different cell morphology with distinct variation in focal adhesion formation and distribution. Anisotropic nanofibers significantly triggered the integrin β1 signaling pathway in NHDFs as evidenced by an increase of active integrin β1 (130 kD mature form) and phosphorylation of focal adhesion kinase (FAK) at Tyr-397. Anisotropic matrices also promoted the migration of NHDFs along the fibers, while neutralization of the integrin β1 activity abolished this promotion. Moreover, the fibroblast-to-myofibroblast differentiation was greatly enhanced for the NHDFs cultured on anisotropic nanofibrous matrices over a period of 48 h. Inhibition of cellular integrin β1 activity by neutralizing antibody eliminated this enhancement. These findings suggest the important role of integrin β1 signaling pathway in regulating the nanofiber-induced fibroblast phenotypic alteration and providing insightful understanding of the possible application of collagen-containing nanofibrous matrices for skin regeneration.

Survivin expression induced by endothelin-1 promotes myofibroblast resistance to apoptosis

Fibrosis of the lungs and other organs is characterized by the accumulation of myofibroblasts, effectors of wound-repair that are responsible for the deposition and organization of new extracellular matrix (ECM) in response to tissue injury. During the resolution phase of normal wound repair, myofibroblast apoptosis limits the continued deposition of ECM. Mounting evidence suggests that myofibroblasts from fibrotic wounds acquire resistance to apoptosis, but the mechanisms regulating this resistance have not been fully elucidated. Endothelin-1 (ET-1), a soluble peptide strongly associated with fibrogenesis, decreases myofibroblast susceptibility to apoptosis through activation of phosphatidylinositol 3'-OH kinase (PI3K)/AKT. Focal adhesion kinase (FAK) also promotes myofibroblast resistance to apoptosis through PI3K/AKT-dependent and -independent mechanisms, although the role of FAK in ET-1 mediated resistance to apoptosis has not been explored. The goal of this study was to investigate whether FAK contributes to ET-1 mediated myofibroblast resistance to apoptosis and to examine potential mechanisms downstream of FAK and PI3K/AKT by which ET-1 regulates myofibroblast survival. Here, we show that ET-1 regulates myofibroblast survival by Rho/ROCK-dependent activation of FAK. The anti-apoptotic actions of FAK are, in turn, dependent on activation of PI3K/AKT and the subsequent increased expression of Survivin, a member of the inhibitor of apoptosis protein (IAP) family. Collectively, these studies define a novel mechanism by which ET-1 promotes myofibroblast resistance to apoptosis through upregulation of Survivin.

Delayed stress fiber formation mediates pulmonary myofibroblast differentiation in response to TGF-β

Myofibroblast differentiation induced by transforming growth factor-β (TGF-β) and characterized by de novo expression of smooth muscle (SM)-specific proteins is a key process in wound healing and in the pathogenesis of fibrosis. We have previously shown that TGF-β-induced expression and activation of serum response factor (SRF) is required for this process. In this study, we examined the signaling mechanism for SRF activation by TGF-β as it relates to pulmonary myofibroblast differentiation. TGF-β stimulated a profound, but delayed (18-24 h), activation of Rho kinase and formation of actin stress fibers, which paralleled SM α-actin expression. The translational inhibitor cycloheximide blocked these processes without affecting Smad-dependent gene transcription. Inhibition of Rho kinase by Y-27632 or depolymerization of actin by latrunculin B resulted in inhibition TGF-β-induced SRF activation and SM α-actin expression, having no effect on Smad signaling. Conversely, stabilization of actin stress fibers by jasplakinolide was sufficient to drive these processes in the absence of TGF-β. TGF-β promoted a delayed nuclear accumulation of the SRF coactivator megakaryoblastic leukemia-1 (MKL1)/myocardin-related transcription factor-A, which was inhibited by latrunculin B. Furthermore, TGF-β also induced MKL1 expression, which was inhibited by latrunculin B, by SRF inhibitor CCG-1423, or by SRF knockdown. Together, these data suggest a triphasic model for myofibroblast differentiation in response to TGF-β that involves 1) initial Smad-dependent expression of intermediate signaling molecules driving Rho activation and stress fiber formation, 2) nuclear accumulation of MKL1 and activation of SRF as a result of actin polymerization, and 3) SRF-dependent expression of MKL1, driving further myofibroblast differentiation.

The role of tumour stroma in colorectal cancer invasion and metastasis

Colorectal cancer (CRC) is a major cause of mortality in western society with a 5-year survival of approximately 50%. Metastasis to the liver and lungs is the principal cause of death and occurs in up to 25% of patients at presentation. Despite advances in available techniques for treating metastases, the majority of patients remain incurable and existing adjuvant therapies such as chemotherapy are only of limited effectiveness. Understanding the molecular mechanisms underlying the metastatic process may allow us to identify those at greatest risk of recurrence and discover new tumour targets to prevent disease progression. It is now apparent that tumour stroma plays an important role in promoting tumour progression. A pronounced desmoplastic reaction was associated with a reduced immune response and has been shown to be an independent poor prognostic indicator in CRC and cancer recurrence. Determining the cause(s) and effect(s) of this stromal response will further our understanding of tumour cell/stromal interactions, and will help us identify prognostic indicators for patients with CRC. This will not only allow us to target our existing treatments more effectively, we also aim to identify novel and more specific therapeutic targets for the treatment of CRC which will add to our current therapeutic options.

Tumor necrosis factor-alpha regulates the Hypocretin system via mRNA degradation and ubiquitination

Recent studies recognize that Hypocretin system (also known as Orexin) plays a critical role in sleep/wake disorders and feeding behaviors. However, little is known about the regulation of the Hypocretin system. It is also known that tumor necrosis factor alpha (TNF-α) is involved in the regulation of sleep/wake cycle. Here, we test our hypothesis that the Hypocretin system is regulated by TNF-α. Prepro-Hypocretin and Hypocretin receptor 2 (HcrtR2) can be detected at a very low level in rat B35 neuroblastoma cells. In response to TNF-α, Prepro-Hypocretin mRNA and protein levels are down-regulated, and also HcrtR2 protein level is down-regulated in B35 cells. To investigate the mechanism, exogenous rat Prepro-Hypocretin and rat HcrtR2 were overexpressed in B35 cells. In response to TNF-α, protein and mRNA of Prepro-Hypocretin are significantly decreased (by 93% and 94%, respectively), and the half-life of Prepro-Hypocretin mRNA is decreased in a time- and dose-dependent manner. The level of HcrtR2 mRNA level is not affected by TNF-α treatment; however, HcrtR2 protein level is significantly decreased (by 86%) through ubiquitination in B35 cells treated with TNF-α. Downregulation of cellular inhibitor of apoptosis protein-1 and -2 (cIAP-1 and -2) abrogates the HcrtR2 ubiquitination induced by TNF-α. The control green fluorescent protein (GFP) expression is not affected by TNF-α treatment. These studies demonstrate that TNF-α can impair the function of the Hypocretin system by reducing the levels of both Prepro-Hypocretin and HcrtR2.

Transforming growth factor-β-induced epithelial-mesenchymal transition facilitates epidermal growth factor-dependent breast cancer progression

TGF-β and EGF play critical roles in regulating the metastasis of aggressive breast cancers, yet the impact of epithelial-mesenchymal transition (EMT) induced by TGF-β in altering the response of breast cancer cells to EGF remains unknown. We show here that murine metastatic 4T1 breast cancer cells formed compact, and dense spheroids when cultured under 3-dimensional (3D) conditions, which contrasted sharply to the branching phenotypes exhibited by their nonmetastatic counterparts. Using the human MCF10A series, we show that epithelial-type and nonmetastatic breast cancer cells were unable to invade to EGF, while their mesenchymal-type and metastatic counterparts readily invaded to EGF. Furthermore, EMT induced by TGF-β was sufficient to manifest spheroid morphologies, a phenotype that increased primary tumor exit and invasion to EGF. Post-EMT invasion to EGF was dependent upon increased activation of EGFR and p38 MAPK, all of which could be abrogated either by pharmacological (PF-271) or genetic (shRNA) targeting of focal adhesion kinase (FAK). Mechanistically, EMT induced by TGF-β increased cell surface levels of EGFR and prevented its physical interaction with E-cadherin, leading instead to the formation of oncogenic signaling complexes with TβR-II. Elevated EGFR expression was sufficient to transform normal mammary epithelial cells, and to progress their 3D morphology from that of hollow acini to branched structures characteristic of nonmetastatic breast cancer cells. Importantly, we show that TGF-β stimulation of EMT enabled this EGFR-driven breast cancer model to abandon their inherent branching architecture and form large, undifferentiated masses that were hyper-invasive to EGF and displayed increased pulmonary tumor growth upon tail vein injection. Finally, chemotherapeutic targeting of FAK was sufficient to revert the aggressive behaviors of these structures. Collectively, this investigation has identified a novel EMT-based approach to neutralize the oncogenic activities of EGF and TGF-β in aggressive and invasive forms of breast cancer.

Loss of beta1-integrin enhances TGF-beta1-induced collagen expression in epithelial cells via increased alphavbeta3-integrin and Rac1 activity

Transforming growth factor β (TGF-β) promotes tissue fibrosis via the receptor-specific Smad pathway and non-canonical pathways. We recently reported that TGF-β1-stimulated collagen expression by cultured kidney cells requires integrin-dependent activation of focal adhesion kinase (FAK) and consequent ERK MAP kinase activity leading to Smad3 linker region phosphorylation. Here, we defined a role for αvβ3-integrin in this non-canonical pathway. A human kidney tubular cell line in which β1-integrin was knocked down (β1-k/d) demonstrated enhanced type I collagen mRNA expression and promoter activity. A second shRNA to either αv-integrin or β3-integrin, but not to another αv-binding partner, β6-integrin, abrogated the enhanced COL1A2 promoter activity in β1-k/d cells. Although αvβ3-integrin surface expression levels were not different, αvβ3-integrins colocalized with sites of focal adhesion significantly more in β1-k/d cells, and activated αvβ3-integrin was detected only in β1-k/d cells. Further, the collagen response was decreased by a function-blocking antibody or a peptide inhibitor of αvβ3-integrin. In cells lacking αvβ3-integrin, the responses were attenuated, whereas the response was enhanced in αvβ3-overexpressing cells. Rac1 and ERK, previously defined mediators for this non-canonical pathway, showed increased activities in β1-k/d cells. Finally, inhibition of αvβ3-integrin decreased Rac1 activity and COL1A2 promoter activity in β1-k/d cells. Together, our results indicate that decreasing β1 chain causes αvβ3-integrin to become functionally dominant and promotes renal cell fibrogenesis via Rac1-mediated ERK activity.

miR-21 mediates fibrogenic activation of pulmonary fibroblasts and lung fibrosis

Uncontrolled extracellular matrix production by fibroblasts in response to tissue injury contributes to fibrotic diseases, such as idiopathic pulmonary fibrosis (IPF), a progressive and ultimately fatal process that currently has no cure. Although dysregulation of miRNAs is known to be involved in a variety of pathophysiologic processes, the role of miRNAs in fibrotic lung diseases is unclear. In this study, we found up-regulation of miR-21 in the lungs of mice with bleomycin-induced fibrosis and also in the lungs of patients with IPF. Increased miR-21 expression was primarily localized to myofibroblasts. Administration of miR-21 antisense probes diminished the severity of experimental lung fibrosis in mice, even when treatment was started 5-7 d after initiation of pulmonary injury. TGF-beta1, a central pathological mediator of fibrotic diseases, enhanced miR-21 expression in primary pulmonary fibroblasts. Increasing miR-21 levels promoted, whereas knocking down miR-21 attenuated, the pro-fibrogenic activity of TGF-beta1 in fibroblasts. A potential mechanism for the role of miR-21 in fibrosis is through regulating the expression of an inhibitory Smad, Smad7. These experiments demonstrate an important role for miR-21 in fibrotic lung diseases and also suggest a novel approach using miRNA therapeutics in treating clinically refractory fibrotic diseases, such as IPF.

Downregulation of FAK-related non-kinase mediates the migratory phenotype of human fibrotic lung fibroblasts

Fibroblast migration plays an important role in the normal wound healing process; however, dysregulated cell migration may contribute to the progressive formation of fibrotic lesions in the diseased condition. To examine the role of focal-adhesion-kinase (FAK)-related non-kinase (FRNK) in regulation of fibrotic lung fibroblast migration, we examined cell migration, FRNK expression, and activation of focal adhesion kinase (FAK) and Rho GTPase (Rho and Rac) in primary lung fibroblasts derived from both idiopathic pulmonary fibrosis (IPF) patients and normal human controls. Fibrotic (IPF) lung fibroblasts have increased cell migration when compared to control human lung fibroblasts. FRNK expression is significantly reduced in IPF lung fibroblasts, while activation of FAK, Rho and Rac is increased in IPF lung fibroblasts. Endogenous FRNK expression is inversely correlated with FAK activation and cell migration rate in IPF lung fibroblasts. Forced exogenous FRNK expression abrogates the increased cell migration, and blocked the activation of FAK and Rho GTPase (Rho and Rac), in IPF lung fibroblasts. These data for the first time provide evidence that downregulation of endogenous FRNK plays a role in promoting cell migration through FAK and Rho GTPase in fibrotic IPF lung fibroblasts.

Human lung parenchyma but not proximal bronchi produces fibroblasts with enhanced TGF-beta signaling and alpha-SMA expression

Given the contribution various fibroblast subsets make to wound healing and tissue remodeling, the concept of lung fibroblast heterogeneity is of great interest. However, the mechanisms contributing to this heterogeneity are unknown. To this aim, we compared molecular and biophysical characteristics of fibroblasts concurrently isolated from normal human proximal bronchi (B-FBR) and distal lung parenchyma (P-FBR). Using quantitative RT-PCR, spontaneous expression of more than 30 genes related to repair and remodeling was analyzed. All P-FBR lines demonstrated significantly increased basal α-smooth muscle actin (α-SMA) mRNA and protein expression levels when compared with donor-matched B-FBR. These differences were not associated with sex, age, or disease history of lung tissue donors. In contrast to B-FBR, P-FBR displayed enhanced transforming growth factor (TGF)-β/Smad signaling at baseline, and inhibition of either ALK-5 or neutralization of endogenously produced and activated TGF-β substantially decreased basal α-SMA protein in P-FBR. Both B-FBR and P-FBR up-regulated α-SMA after stimulation with TGF-β1, and basal expression levels of TGF-β1, TGF-βRI, and TGF-βRII were not significantly different between fibroblast pairs. Blockade of metalloproteinase-dependent activation of endogenous TGF-β did not significantly modify α-SMA expression in P-FBR. However, resistance to mechanical tension of these cells was significantly higher in comparison with B-FBR, and added TGF-β1 significantly increased stiffness of both cell monolayers. Our data suggest that in contrast with human normal bronchial tissue explants, lung parenchyma produces mesenchymal cells with a myofibroblastic phenotype by intrinsic mechanisms of TGF-β activation in feed-forward manner. These results also offer a new insight into mechanisms of human fibroblast heterogeneity and their function in the airway and lung tissue repair and remodeling.

FAK mediates the activation of cardiac fibroblasts induced by mechanical stress through regulation of the mTOR complex

AIMS

Cardiac fibroblasts are activated by mechanical stress, but the underlying mechanisms involved remain poorly understood. In this study, we investigated whether focal adhesion kinase (FAK) plays a role in the activation of cardiac fibroblasts in response to cyclic stretch.

METHODS AND RESULTS

Neonatal (NF-P3/80--third passage, 80% confluence) and adult (AF-P1/80--first passage, 80% confluence) rat cardiac fibroblasts were exposed to cyclic stretch (biaxial, 1 Hz), which enhanced FAK phosphorylation at Tyr397. Proliferation (anti-5-bromo-2'-deoxyuridine and anti-Ki67 nuclear labelling), differentiation into myofibroblasts (expression of alpha-smooth muscle actin--alpha-SMA), and the activity of matrix metalloproteinase-2 were equally enhanced in stretched NF-P3/80 and AF-P1/80. Treatment with the integrin inhibitor RGD peptide impaired FAK phosphorylation and increased apoptosis (TUNEL) in non-stretched and stretched NF-P3/80, whereas FAK silencing induced by small interfering RNA modestly enhanced apoptosis only in stretched cells. RGD peptide or FAK silencing suppressed the activation of NF-P3/80 invoked by cyclic stretch. In addition, NF-P3/80 depleted of FAK were defective in AKT Ser473, TSC-2 Thr1462, and S6 kinase Thr389 phosphorylation induced by cyclic stretch. The activation of NF-P3/80 invoked by cyclic stretch was prevented by pre-treatment with the mammalian target of rapamycin (mTOR) inhibitor rapamycin, whereas supplementation with the amino acid, leucine, activated S6K and rescued the stretch-induced activation of NF-P3/80 depleted of FAK.

CONCLUSIONS

These findings demonstrate a critical role for the mTOR complex, downstream from FAK, in mediating the activation of cardiac fibroblasts in response to mechanical stress.

TAE226-mediated inhibition of focal adhesion kinase interferes with tumor angiogenesis and vasculogenesis

Neoangiogenesis plays an important role in tumor growth and metastasis. Evaluation of new anti-angiogenic targets may broaden the armament for future therapeutic concepts. Focal adhesion kinase (FAK), expressed in endothelial and tumor cells, is essential for adhesion and mobility of adherent cells. In the current study we analyzed the anti-angiogenic properties of the FAK inhibitor TAE226 on the proliferation of blood outgrowth endothelial cell (OEC) and differentiation of endothelial progenitor cells (EPC), derived from peripheral blood CD133(+) cells, tube formation and on neovascularization in a HT29 xenotransplant model. The effects of TAE226 were compared to those of the rapamycin analogue RAD001. The combination of both drugs was also studied. We showed that HT29 tumor cells and OEC were most sensitive to the action of TAE226 compared to EPC in vitro. In contrast, RAD001 affected the proliferation of both types of endothelial cells stronger than that of HT29 cells. Furthermore we could show that TAE226 inhibited tube formation in a dose dependent manner. In a HT29 subcutaneous tumor model TAE226 and RAD001 diminished MVD at commonly employed doses to a similar degree. Combination of both compounds did not show synergy in vitro or in vivo. Since TAE226 has been shown to inhibit the PI3 kinase, Akt kinase, mTor pathway, addition of RAD001 may not increase this effect. In conclusion, we have shown that treatment with TAE leads to a reduction of neoangiogenesis in vitro and in a mouse model. The effects are mediated by inhibition of angiogenesis and vasculogenic OEC and EPC.

Basic fibroblast growth factor induces down-regulation of alpha-smooth muscle actin and reduction of myofibroblast areas in open skin wounds

To examine the effects of basic fibroblast growth factor (bFGF) on the inhibition of alpha-smooth muscle actin (alpha-SMA) expression in dermal fibroblasts, we have established two dermal myofibroblastic cell lines positive for alpha-SMA (rat myofibroblasts [RMF] and rat myofibroblast-like [RMFL] cells) and one fibroblastic cell line negative for alpha-SMA (rat fibroblasts cells) as a model of fibroblast differentiation. In contrast to the increased expression of alpha-SMA in RMF and RMFL cells, irrespective of transforming growth factor-beta1 treatment, bFGF induced a decrease in alpha-SMA expression in the myofibroblastic cells and the reduced expression patterns of alpha-SMA differed between cells, as demonstrated by Western blot and reverse transcription polymerase chain reaction analyses. Along with the inhibition of alpha-SMA expression by bFGF, the RMF and RMFL cells also showed different activated expression of extracellular signal-regulated kinase 1/2, suggesting the involvement of extracellular signal-regulated kinase 1/2 activation in the down-regulation of alpha-SMA expression in myofibroblasts. Furthermore, an in vivo study demonstrated that bFGF administration markedly decreases the area that is positive for alpha-SMA expression in the treated wounds after day 18. In contrast, bFGF administration significantly increased the number of terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) staining and alpha-SMA-positive cells at days 10 and 14, and reduced the double-positive cells rapidly after day 18. Collectively, the current investigation identified bFGF as a potent stimulator for the reduction of the myofibroblastic area in vivo, presumably because of its effects on the down-regulation of alpha-SMA expression as well as rapid induction of apoptosis in myofibroblasts.

Periostin mediates vascular smooth muscle cell migration through the integrins alphavbeta3 and alphavbeta5 and focal adhesion kinase (FAK) pathway

Smooth muscle cell (SMC) migration involves interactions of integrin receptors with extracellular matrix (ECM) and is an important process of neointimal formation in atherosclerosis and restenosis after vascular interventions. Previous studies have shown that periostin (PN), a novel ECM protein, is upregulated in rat carotid artery after balloon injury, and growth factor-stimulated expression of PN promotes SMC migration in vitro. Here, we address the mechanism by which PN-integrin interaction mediates SMC migration in vitro. Aortic SMCs isolated from PN null mice exhibited a significantly reduced ability to migrate and proliferate in vitro. Endogenous PN protein was absent and very low in the culture medium from the primary cultures of PN-/- and wildtype SMCs, respectively. In both types of SMCs, adenovirus-mediated overexpression of HA-tagged PN to a similar extent, which induced a robust cell migration concomitantly with an increase in beta3-integrin expression and phosphorylation of FAK (Tyr397). Furthermore, in cultured human SMCs, specific integrin blocking antibodies showed that interactions of PN-alphanubeta3 and PN-alphanubeta5, but not PN-beta1 integrins, are required for SMC migration. Inhibition of FAK signaling by overexpression of an endogenous FAK inhibitor termed FRNK (FAK-related nonkinase) significantly attenuated FAK (Tyr397) phosphorylation and the SMC migration induced by PN. These results reveal a mechanism whereby PN mediates vascular SMC migration through an interaction with alphaV-integrins (mainly alphanubeta3) and subsequent activation of FAK pathway.

Urokinase receptor mediates lung fibroblast attachment and migration toward provisional matrix proteins through interaction with multiple integrins

Fibroblasts from patients with pulmonary fibrosis express higher levels of the receptor for urokinase, and the extent of fibrosis in some animal models exhibits a dependence on the urokinase receptor. Recent observations have identified the urokinase receptor as a trans-interacting receptor with consequences on signaling and cell responses that vary depending on its interacting partner, the relative levels of expression, and the state of cellular transformation. We undertook this study to define the urokinase-type plasminogen activator cellular receptor (u-PAR)-integrin interactions and to determine the functional consequences of such interactions on normal human lung fibroblast attachment and migration. u-PAR colocalizes in lammelipodia/filopodia with relevant integrins that mediate fibroblast attachment and spreading on the provisional matrix proteins vitronectin, fibronectin, and collagens. Inhibitory antibody studies have revealed that human lung fibroblasts utilize alpha(v)beta(5) to attach to vitronectin, predominantly alpha(5)beta(1) (and alpha(v)beta(3)) to attach to fibronectin, and alpha(1)beta(1), alpha(2)beta(1), and alpha(3)beta(1) to attach to collagen. Blocking studies with alpha-integrin subunit decoy peptides and u-PAR neutralizing antibodies indicate that u-PAR modulates the integrin-mediated attachment to purified provisional matrix proteins, to anti-integrin antibodies, or to fibroproliferative lesions from fibrotic lungs. Furthermore, these decoy peptides blunt fibroblast spreading and migration. We show that u-PAR can interact with multiple alpha-integrins but with a preference for alpha(3). Taken together, these data demonstrate that u-PAR may interact with multiple integrins in normal human lung fibroblasts thereby promoting attachment, spreading, and migration. Modulation of fibroblast invasion would be expected to lead to amelioration of fibroproliferative diseases of the lung.

Emerging concepts in the pathogenesis of lung fibrosis

Fibrogenesis is an often-deadly process with increasing world-wide incidence and limited therapeutic options. Pulmonary fibrogenesis involves remodeling of the distal airspace and parenchyma of the lung, and is characterized by excessive extracellular matrix deposition and accumulation of apoptosis-resistant myofibroblasts. Recent studies have added significantly to our understanding of the complex mechanisms involved in lung fibrogenesis. Emerging concepts in this field include the critical role of the epithelium, particularly type II pneumocytes, in the initiation and perpetuation of fibrosis in response to either endogenous or exogenous stress; a growing awareness of alternative activation of macrophages in tissue remodeling; growing appreciation of the alternative origins and phenotypic plasticity of fibroblasts; the roles of epigenetic reprogramming and context-dependent signaling in profibrotic phenotype alterations; and recognition of the importance of cross talk and convergence of intracellular signaling pathways. In vitro, in vivo, and in silico approaches support a paradigm of "disordered re-development" of the lung. Designing effective antifibrotic interventions will require accurate understanding of the complex interactions among the genetic, environmental, epigenetic, biochemical, cellular, and contextual abnormalities that promote pulmonary fibrogenesis.