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Integrins control many cell functions, including generation of reactive oxygen species (ROS) and regulation of collagen synthesis. Mesangial cells, found in the glomerulus of the kidney, are able to produce large amounts of ROS via the NADPH oxidase. We previously demonstrated that integrin ␣1-null mice develop worse fibrosis than wild-type mice following glomerular injury and this is due, in part, to excessive ROS production by ␣1-null mesangial cells. In the present studies, we describe the mechanism whereby integrin ␣1-null mesangial cells produce excessive ROS. Integrin ␣1-null mesangial cells have constitutively increased basal levels of activated Rac1, which result in its increased translocation to the cell membrane, excessive ROS production, and consequent collagen IV deposition. Basal Rac1 activation is a direct consequence of ligand-independent increased epidermal growth factor receptor (EGFR) phosphorylation in ␣1-null mesangial cells. Thus, our study demonstrates that integrin ␣1␤1-EGFR cross talk is a key step in negatively regulating Rac1 activation, ROS production, and excessive collagen synthesis, which is a hallmark of diseases characterized by irreversible fibrosis.Integrin ␣1␤1, a major collagen binding receptor, is expressed in different cell types, including fibroblasts (45), endothelial cells (8), and mesangial cells in the glomerulus of the kidney (30, 53). Integrin ␣1␤1 confers the ability of cells to bind to collagenous substrata, including collagens I and IV (4, 45), and to proliferate on these substrata (45). Moreover, cells expressing integrin ␣1␤1 sense extracellular levels of collagen and downregulate its synthesis at both transcriptional and translational levels (4,14). Finally, we demonstrated that integrin ␣1␤1 also downregulates the production of reactive oxygen species (ROS) (4, 58).Following renal injury, mice lacking integrin ␣1␤1 develop more extensive glomerular fibrosis characterized by excessive accumulation of collagen type IV compared to wild-type (WT) mice (4, 58). Increased fibrosis is due to both a direct effect of the lack of integrin ␣1␤1-mediated downregulation of collagen IV synthesis and excessive ROS production by ␣1-null mesangial cells.Constitutive production of ROS by mesangial cells, a major cell type found in the glomerulus of the kidney, originates from an intrinsic NADPH oxidase (26, 48) that normally functions at a low level and increases in response to inflammatory stimuli, high glucose, or stress (25,34,35,37,55). The NADPH oxidase, highly characterized in phagocytes, is a multicomponent enzyme complex that consists of the membrane-bound cytochrome b 558 (p22 phox and gp91 phox ) and cytoplasmic proteins (p40 phox , p47 phox , p67 phox ) that translocate to the membrane following cellular stimulation to produce superoxide (3, 9, 47). A multicomponent phagocyte-like NADPH oxidase is also a major source of ROS in many nonphagocytic cells, including mesangial cells. In the phagocyte-like NADPH oxidase, the catalytic subunits are termed Nox proteins, with ...

The collagen IV binding receptor integrin A1B1 has been shown to regulate lung cancer due to its proangiogenic properties; however, it is unclear whether this receptor also plays a direct role in promoting primary lung tumors. To investigate this possibility, integrin A1-null mice were crossed with KrasLA2 mice that carry an oncogenic mutation of the Kras gene (G12D) and develop spontaneous primary tumors with features of non-small cell lung cancer. We provide evidence that KrasLA2/A1-null mice have a decreased incidence of primary lung tumors and longer survival compared with KrasLA2/A1 wild-type controls. Tumors from KrasLA2/A1-null mice were also smaller, less vascularized, and exhibited reduced cell proliferation and increased apoptosis, as determined by proliferating cell nuclear antigen and terminal deoxynucleotidyl-transferase-mediated dUTP nick-end staining, respectively. Moreover, tumors from the KrasLA2/A1-null mice showed diminished extracellular signal-regulated kinase (ERK) but enhanced p38 mitogenactivated protein kinase activation. Primary lung tumor epithelial cells isolated from KrasLA2/A1-null mice showed a significant decrease in anchorage-independent colony formation, collagen-mediated cell proliferation, ERK activation, and, most importantly, tumorigenicity when injected into nude mice compared with KrasLA2/A1 wild-type tumor cells. These results indicate that loss of the integrin A1 subunit decreases the incidence and growth of lung epithelial tumors initiated by oncogenic Kras, suggesting that both Kras and integrin A1B1 cooperate to drive the growth of non-small cell lung cancer in vivo. [Cancer Res 2008;68(15):6127-9]

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