Phosphorylation of the three Rb protein family members is a common step of the cAMP-, the growth factor, and the phorbol ester-mitogenic cascades but is not necessary for the hypertrophy induced by insulin.

Thyrotropin (TSH) through the cAMP cascade and in the presence of insulin induces the proliferation of dog thyroid cells. In this work, it is shown that TSH via cAMP causes the phosphorylation of the three members of the pRb family, pRb, p107, and p130, with the same kinetics as those observed when these cells are stimulated by mitogens acting through a tyrosine kinase receptor or through activation of kinase C. It is the first described point of convergence of cAMP-dependent and -independent mitogenic pathways in dog thyrocytes and suggests that the phosphorylation of the three proteins may be involved in the initiation of DNA synthesis in these cells. We also show that insulin, which induces hypertrophy and is permissive for the TSH mitogenic action, does not provoke the phosphorylation of any pRb family member, suggesting that none of these phosphorylations is required for this effect.