EFFECT OF TGF-BETA-1 ON CELL-CYCLE REGULATORY PROTEINS IN LPS-STIMULATED NORMAL MOUSE B-LYMPHOCYTES

The cell cycle events accompanying TGF-beta 1-induced growth arrest of normal mouse resting B lymphocytes stimulated by LPS were investigate d, We showed that TGF-beta 1 prevents the retinoblastoma protein (pRb) phosphorylation and induces growth arrest in mid- to late G(1). To ex plore the molecular basis of the effect of TGF-beta 1, we analyzed the in vitro kinase activities of cyclin/cyclin-dependent kinase (cdk) co mplexes involved in the progression through G(1) phase and in the G(1) /S transition, by using the glutathione S-transferase-pRb fusion prote in as a substrate. Cdk2-associated kinase activity was strongly induce d in mitogen-treated B cells, It was dramatically inhibited by TGF-bet a 1 as were the cyclin E- and cyclin A-dependent kinase activities, TG F-beta 1 treatment had no significant effect on the expression of two G(1)/S phase proteins, cyclin E and cdk2, In contrast, the appearance of cyclin A, occuring in late G(1) phase, was almost totally inhibited by TGF-beta 1, We also showed that expression of the cdk inhibitor pr otein p27(Kip1) decreased as cells progressed through the G(1) phase, An accumulation of p27 was found in TGF-beta 1-treated cells, showing that TGF-beta 1 prevented LPS-induced decline of p27, Finally we found that the lack of kinase activity associated with cyclin E/cdk2 comple xes was correlated with increased amounts of cdk2- and cyclin E-bound p27, Overall, these results suggest that both cyclin A and cdk2 may be active participants in the TGF-beta 1-induced cell cycle arrest in no rmal mouse B cells and indicate the involvement of p27 in this mechani sm.