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Distinct structural forms of type I collagen modulate cell cycle regulatory proteins in mesangial cells. Background Extracellular matrix molecules profoundly regulate cell behavior, including proliferation. In glomerulonephritis, type I collagen accumulates in the mesangium and is constantly structurally modified and degraded during the course of the disease. Methods We studied how two structurally distinct forms of type I collagen, monomer versus polymerized fibrils, affect cell proliferation, mitogen-activated protein kinase (MAPK) activation, and expression of G 1 -phase regulatory proteins in cultured rat mesangial cells (MCs). To analyze the possible involvement of collagen-binding integrins in type I collagen-derived growth signals further, distribution patterns of integrin chains were examined by immunocytochemistry. Results Polymerized type I collagen completely prevented the increase of DNA synthesis and cell replication induced by 5% fetal calf serum (FCS) or 25 ng/mL platelet-derived growth factor (PDGF) in MCs on monomer type I collagen. Protein expression of cyclins D1 and E was markedly down-regulated in MCs plated on polymerized type I collagen for eight hours in 5% FCS, as compared with MCs on monomer type I collagen. Incubation with 5% FCS reduced expression of the cdk-inhibitor protein p27 Kip1 on monomer but not on polymerized type I collagen. Moreover, polymerized type I collagen markedly reduced cyclin E-associated kinase activity in the presence of 5% FCS. Polymerized type I collagen diminished the PDGF-induced phosphorylation and nuclear translocation of p42/p44 MAPK, but did not affect phosphorylation of PDGF β-receptors. In MCs plated on monomer type I collagen, α 1 , α 2 , and β 1 integrin chains were recruited into focal contacts. However, on polymerized type I collagen, α 2 and β 1 , but not α 1 , integrin chains were condensed into focal contacts. Conclusions The growth-inhibitory effect of polymerized type I collagen is characterized by rapid changes of expression and/or activation of MAPK and G 1 -phase regulators and could result from the lack of α 1 β 1 integrin signaling in MCs on polymerized type I collagen. Conceivably, deposition of polymerized type I collagen might reflect a reparative response to control MC replication in glomerular inflammation.