DIFFERENTIAL CONTROL OF CYCLINS D1 AND D3 AND THE CDK INHIBITOR P27(KIP1) BY DIVERSE SIGNALING PATHWAYS IN SWISS 3T3 CELLS

Quiescent Swiss 3T3 cells can be induced to re-enter the cell cycle by stimulation of a variety of growth factor-dependent signal transducti on cascades. We have utilised this cell system to investigate the poin t of convergence of mitogenic signalling by analysing the changes that distinct mitogens induce in the components of the cell cycle regulato ry machinery (the G(1) cyclins, cdks and their inhibitors). In the pre sence of insulin, activation of cAMP-dependent protein kinase caused a dramatic post-transcriptional down-regulation of p27(Kip1), increase in cyclin D3 but had little effect on cyclin D1 levels, whilst activat ion of protein kinase C had a more modest effect on cyclin D3 and p27( Kip1) but caused a striking elevation in the expression of cyclin D1, The neuropeptide bombesin, when combined with insulin, caused increase d expression of cyclin D1 and down-regulation of p27(Kip1) mRNA and pr otein. Thus each combination of mitogenic agents had different effects on the components responsible for regulating the orderly progression of the cell cycle. This outcome is incompatible with a single route to mitogenesis and demonstrates that different mitogens remain distinct in the signalling responses they initiate, only converging at the leve ls of the expression of the D-type cyclins and the inhibitor p27(Kip1) .