Convergence of mitogenic signalling cascades from diverse classes of receptors at the cyclin D-cyclin-dependent kinase-pRb-controlled G1 checkpoint.

The commitment of mammalian cells in late G1 to replicate the genome and divide in response to mitogenic growth factors operating via tyrosine kinase receptors depends on phosphorylation of the retinoblastoma protein (pRb), a process controlled by cyclin D-associated cyclin-dependent kinases (cdks) and their inhibitors. This study addressed the issue of whether also other mitogenic signalling cascades require activation of cyclin D-associated kinases or whether any mitogenic pathway can bypass the cyclin D-pRb checkpoint. We show that mitogenic signal transduction pathways from three classes of receptors, the membrane tyrosine kinase receptors activated by serum mitogens or epidermal growth factor, estrogen receptors triggered by estradiol, and the cyclic AMP-dependent signalling from G-protein-coupled thyrotropin receptors, all converge and strictly require the cyclin D-cdk activity to induce S phase in human MCF-7 cells and/or primary dog thyrocytes. Combined microinjection and biochemical approaches showed that whereas these three mitogenic cascades are sensitive to the p16 inhibitor of cdk4/6 and/or cyclin D1-neutralizing antibody and able to induce pRb kinase activity, their upstream biochemical routes are distinct as demonstrated by their differential sensitivity to lovastatin and requirements for mitogen-activated protein kinases whose sustained activation is seen only in the growth factor-dependent pathway. Taken together, these results support the candidacy of the cyclin D-cdk-pRb interplay for the convergence step of multiple signalling cascades and a mechanism contributing to the restriction point switch.