Hepatocyte growth factor releases mink epithelial cells from transforming growth factor beta 1-induced growth arrest by restoring Cdk6 expression andcyclin E-associated Cdk2 activity

Transforming growth factor beta (TGF-beta) potently suppresses Mv1Lu mink e pithelial cell growth, whereas hepatocyte growth factor (HGF) counteracts T GF-beta-mediated growth inhibition and induces Mv1Lu cell proliferation (J. Taipale and J. Keski-Oja, J, Biol. Chem. 271:4342-4348, 1996). By addressi ng the cell cycle regulatory mechanisms involved in NGF-mediated release of Mv1Lu cells from TGF-beta inhibition,we show that increased DNA replicatio n is accompanied by phosphorylation of the retinoblastoma protein and alter native regulation of cyclin-Cdk-inhibitor complexes. While TGF-beta treatme nt decreased the expression of Cdk6, this effect was counteracted by HGF, f ollowed by partial restoration of cyclin D2-associated kinase activity. Not ably, HGF failed to prevent TGF-beta induction of p15 and its association w ith Cdk6. However, HGF reversed the TGF-beta-mediated decrease in Cdk6-asso ciated p27 and cyclin D2-associated Cdk6, suggesting that HGF modifies the TGF-beta response at the level of G(1) cyclin complex formation. Counteract ion of TGF-beta regulation of Cdk6 by HGF may in turn a affect the associat ion of p27 with Cdk2-cyclin E complexes. Though HGF did not differentially regulate the total levels of p27 in TGF-beta-treated cells, p27 immunodeple tion experiments suggested that upon treatment with both growth factors, le ss p27 is associated with Cdk2-cyclin E complexes, in parallel with restora tion of the active form of Cdk2 and the associated kinase activity. The res ults demonstrate that HGF intercepts TGF-beta cell cycle regulation at mult iple points, affecting both G(1) and G(1)-S cyclin kinase activities.