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Overexpression of α 1 β 1 integrin directly affects rat mesangial cell behavior. Background Glomerular mesangial cell (MC) proliferation, hypertrophy, and abnormal matrix remodeling characterized by increased expression of fibronectin, laminin and collagen type IV, and neoexpression of collagen I and III are the main biological features of progressive glomerulonephritis (GN). Especially, persistent pathological matrix remodeling may lead to glomerular scar formation (glomerular scarring). We reported recently that α 1 β 1 integrin, a major collagen receptor for MCs, may be a potential adhesion molecule for MC-mediated pathological collagen matrix remodeling in GN. Methods To address further the direct role of α 1 β 1 integrin in MC behavior, such as cell growth and matrix remodeling, α 1 β 1 integrin was overexpressed in MCs by transfecting an expression vector containing a full-length rat α 1 integrin cDNA. Flow cytometry and immunoprecipitation analysis were applied for selection of transfectants with a stable expression of the α 1 integrin subunit. The effect of α 1 β 1 integrin overexpression on MC biology was examined with a 3 H-thymidine incorporation assay, flow cytometric analysis of cell size and DNA content, Western blot analysis of a cyclin-dependent-kinase inhibitor, p27 Kip1 , α-smooth muscle actin expression, and a collagen gel contraction assay. Results The α 1 transfectants displayed a dramatic inhibition of 3 H-thymidine incorporation as compared with the mock transfectants. Increased expression of the α 1 subunit inversely correlated with cell cycle progression and paralleled the expression of p27 Kip1 and α-smooth muscle actin, as well as the cell size in MCs. In addition, the α 1 -transfectants were able to enhance collagen matrix reorganization effectively. Conclusion These results indicate that MC-α 1 β 1 integrin expression is a critical determinant of MC phenotypes, including cell growth, cell size, and collagen matrix remodeling ability, and thereby contributes to scar matrix remodeling (sclerosis) in GN.