Regulation of the hepatocyte cell cycle by type I collagen matrix: role ofcyclin D1

Rat hepatocytes adherent to a rigid film of type I collagen will spread and enter S phase, while those attached to collagen gel or a dried collagen su bstrate remain round and quiescent. The current studies were initiated to d etermine the mechanism by which these different substrates differentially i nfluence cell cycle progression. Cyclin D1 mRNA and protein expression and associated kinase activity was low on dried collagen relative to collagen h im. In contrast, cyclin E and cdk2 protein levels were similar on the two s ubstrates, Although cyclin E and cdk2 were present, cells on dried collagen lacked cdk2 kinase activity. p27 protein levels did not differ between dri ed collagen and film, but more p27 was associated with cdka in cells on dri ed collagen than those on collagen film. Cyclin D1 expression on collagen f ilm was inhibited by cytochalasin D and exoenzyme C3, suggesting a role for the GTP-binding protein, Rho, in regulating cyclin D1 expression. Cyclin D 1 over-expression induced hepatocytes into S phase in the absence of cell s hape change on dried collagen or collagen gel, These results demonstrate a novel, substrate-dependent mechanism for cyclin D1 expression in hepatocyte s, and also demonstrate that cyclin D1 overexpression allows shape-independ ent S phase entry.