PAK proteins and YAP-1 signalling downstream of integrin beta-1 in myofibroblasts promote liver fibrosis

Fibrosis due to extracellular matrix (ECM) secretion from myofibroblasts complicates many chronic liver diseases causing scarring and organ failure. Integrin-dependent interaction with scar ECM promotes pro-fibrotic features. However, the pathological intracellular mechanism in liver myofibroblasts is not completely understood, and further insight could enable therapeutic efforts to reverse fibrosis. Here, we show that integrin beta-1, capable of binding integrin alpha-11, regulates the pro-fibrotic phenotype of myofibroblasts. Integrin beta-1 expression is upregulated in pro-fibrotic myofibroblasts in vivo and is required in vitro for production of fibrotic ECM components, myofibroblast proliferation, migration and contraction. Serine/threonine-protein kinase proteins, also known as P21-activated kinase (PAK), and the mechanosensitive factor, Yes-associated protein 1 (YAP-1) are core mediators of pro-fibrotic integrin beta-1 signalling, with YAP-1 capable of perpetuating integrin beta-1 expression. Pharmacological inhibition of either pathway in vivo attenuates liver fibrosis. PAK protein inhibition, in particular, markedly inactivates the pro-fibrotic myofibroblast phenotype, limits scarring from different hepatic insults and represents a new tractable therapeutic target for treating liver fibrosis

Arp2/3- and Cofilin-coordinated Actin Dynamics Is Required for Insulin-mediated GLUT4 Translocation to the Surface of Muscle Cells

Insulin increases GLUT4 at the muscle cell surface, and this process requires actin remodeling. We show that a dynamic cycle of actin polymerization and severing is induced by insulin, governed by Arp2/3 and dephosphorylation of cofilin, respectively. The cycle is self-perpetuating and is essential for GLUT4 translocation

Integrin alpha 11 in the regulation of the myofibroblast phenotype: implications for fibrotic diseases

Tissue fibrosis, characterized by excessive accumulation of aberrant extracellular matrix (ECM) produced by myofibroblasts, is a growing cause of mortality worldwide. Understanding the factors that induce myofibroblastic differentiation is paramount to prevent or reverse the fibrogenic process. Integrin-mediated interaction between the ECM and cytoskeleton promotes myofibroblast differentiation. In the present study, we explored the significance of integrin alpha 11 (ITGA11), the integrin alpha subunit that selectively binds to type I collagen during tissue fibrosis in the liver, lungs and kidneys. We showed that ITGA11 was co-localized with α-smooth muscle actin-positive myofibroblasts and was correlatively induced with increasing fibrogenesis in mouse models and human fibrotic organs. Furthermore, transcriptome and protein expression analysis revealed that ITGA11 knockdown in hepatic stellate cells (liver-specific myofibroblasts) markedly reduced transforming growth factor β-induced differentiation and fibrotic parameters. Moreover, ITGA11 knockdown dramatically altered the myofibroblast phenotype, as indicated by the loss of protrusions, attenuated adhesion and migration, and impaired contractility of collagen I matrices. Furthermore, we demonstrated that ITGA11 was regulated by the hedgehog signaling pathway, and inhibition of the hedgehog pathway reduced ITGA11 expression and fibrotic parameters in human hepatic stellate cells in vitro, in liver fibrosis mouse model in vivo and in human liver slices ex vivo. Therefore, we speculated that ITGA11 might be involved in fibrogenic signaling and might act downstream of the hedgehog signaling pathway. These findings highlight the significance of the ITGA11 receptor as a highly promising therapeutic target in organ fibrosis

Stepping up regulatory mechanisms of GLUT4 traffic in L6 skeletal muscle cellss

Insulin increases glucose uptake into muscle and fat by enhancing GLUT4 glucose transporter externalization; a process requiring input from Akt and actin. Downstream of phosphatidylinositol-3-kinase, insulin signaling bifurcates into Akt and actin activating arms. Akt-mediated phosphorylation of the Rab-GAP AS160 is required for gain in surface GLUT4 by insulin. However, little is known of the mechanism(s) by which AS160 and/or actin dynamics modulate GLUT4 traffic in muscle. We recently showed that GLUT4 arrival and/or fusion can be regulated by insulin signaling molecules and phospholipids. Using ‘rounded up’ L6 myoblasts stably expressing GLUT4myc, we find that transient expression of a non-phosphorylatable mutant of AS160 (AS160-4P) abrogates the surface fusion of GLUT4myc and partially reduces its sub-membranous accumulation. In contrast, tetanus toxin-mediated cleavage of VAMP2 inhibits GLUT4myc fusion but not arrival to the plasma membrane. Conversely, disrupting actin dynamics with Latrunculin B or silencing expression of a cytoskeletal protein a-actinin4 precludes the insulin-induced cortical build-up of GLUT4myc. These data suggest that AS160 and actin dynamics impinge on distinct stages of insulin-regulated GLUT4 traffic: AS160 may contribute to peripheral retention and is essential for GLUT4myc vesicle docking/fusion. It will be interesting to note which Rabs facilitate these AS160-dependent events. Actin dynamics instead may allow GLUT4 vesicle movement to the cell surface and/or its retention, presumably via cortical anchoring mechanisms involving a-actinin4, whilst VAMP2 has a major role in GLUT4 vesicle fusion. Indeed, defects in AS160 phosphorylation and actin dynamics are associated with insulin resistant states. Thus, discerning which steps of GLUT4 traffic are modulated by these inputs may help elucidate strategies to bypass insulin resistance

Glycated collagen induces a11 integrin expression through TGF-ß2 and Smad3

The adhesion of cardiac fibroblasts to the glycated collagen interstitium in diabetics is associated with de novo expression of the a11 integrin, myofibroblast formation and cardiac fibrosis. We examined how methylglyoxal-glycated collagen regulates a11 integrin expression. In cardiac fibroblasts plated on glycated collagen but not glycated fibronectin, there was markedly increased a11 integrin and a-smooth muscle actin expression. Compared with native collagen, binding of purified a11b1 integrin to glycated collagen was reduced by fourfold, which was consistent with reduced fibroblast attachment to glycated collagen. Glycated collagen strongly enhanced the expression of TGF-b2 but not TGF-b1 or TGF-b3. The increased expression of TGF-b2 was inhibited by triple helical collagen peptides that mimic the a11b1 integrin binding site on type I collagen. In cardiac fibroblasts transfected with a11 integrin luciferase promoter constructs, glycated collagen activated the a11 integrin promoter. Analysis of a11 integrin promoter truncation mutants showed a novel Smad2/3 binding site located between 809 and 1300 nt that was required for promoter activation. We conclude that glycated collagen in the cardiac interstitium triggers an autocrine TGF-b2 signaling pathway that stimulates a11 integrin expression through Smad2/3 binding elements in the a11 integrin promoter, which is important for myofibroblast formation and fibrosis.This work was sponsored by the Canadian Institutes of Health Research

Fibroblast α11β1 Integrin Regulates Tensional Homeostasis in Fibroblast/A549 Carcinoma Heterospheroids

We have previously shown that fibroblast expression of α11β1 integrin stimulates A549 carcinoma cell growth in a xenograft tumor model. To understand the molecular mechanisms whereby a collagen receptor on fibroblast can regulate tumor growth we have used a 3D heterospheroid system composed of A549 tumor cells and fibroblasts without (α11+/+) or with a deletion (α11-/-) in integrin α11 gene. Our data show that α11-/-/A549 spheroids are larger than α11+/+/A549 spheroids, and that A549 cell number, cell migration and cell invasion in a collagen I gel are decreased in α11-/-/A549 spheroids. Gene expression profiling of differentially expressed genes in fibroblast/A549 spheroids identified CXCL5 as one molecule down-regulated in A549 cells in the absence of α11 on the fibroblasts. Blocking CXCL5 function with the CXCR2 inhibitor SB225002 reduced cell proliferation and cell migration of A549 cells within spheroids, demonstrating that the fibroblast integrin α11β1 in a 3D heterospheroid context affects carcinoma cell growth and invasion by stimulating autocrine secretion of CXCL5. We furthermore suggest that fibroblast α11β1 in fibroblast/A549 spheroids regulates interstitial fluid pressure by compacting the collagen matrix, in turn implying a role for stromal collagen receptors in regulating tensional hemostasis in tumors. In summary, blocking stromal α11β1 integrin function might thus be a stroma-targeted therapeutic strategy to increase the efficacy of chemotherapy