Phosphatidylinositol 3-kinase, protein kinase B and ribosomal S6 kinases in the stimulation of thyroid epithelial cell proliferation by cAMP and growth factors in the presence of insulin

The proliferation of most normal cells depends on the cooperation of severa l growth factors and hormones, each with a specific role, but the key event s involved in the action of each necessary stimulant remain largely unchara cterized. In the present study, the pathways involved in the mechanism(s) o f co-operation have been investigated in primary cultures of dog thyroid ep ithelial cells. In this physiologically relevant system, thyroid stimulatin g hormone (TSH) acting through cAMP, epidermal growth factor (EGF) and phor bol esters (such as PMA) induce DNA synthesis. Their effect requires stimul ation of the insulin-like growth factor-1 (IGF-1) receptor by either IGF-1 or insulin, which are not themselves mitogenic agents. In contrast, hepatoc yte growth factor (HGF) is itself fully mitogenic. The results of the study demonstrate that cAMP, EGF, HGF and PMA stimulate p70 ribosomal S6 kinase (p70 S6 kinase). However, insulin/IGF-1 also stimulate p70 S6 kinase. Thus stimulation of p70 S6 kinase might be necessary, but is certainly not suffi cient, for the induction of DNA synthesis and is not specific for any stimu lated pathway. In contrast, phosphatidylinositol 3-kinase (PI 3-kinase) and protein kinase B (PKB) activation by insulin and HGF is strong and sustain ed, whereas it is weak and transient with EGF and absent in the presence of TSH or PMA. These findings suggest that: (i) stimulation of PI 3-kinases a nd/or PKB is not involved in the cAMP-dependent pathways leading to thyrocy te proliferation, or in the action of PMA, (ii) the stimulation of the PI 3 -kinase/PKB pathway may account for the permissive action of insulin/IGF-1 in the proliferation of these cells, and (iii) the stimulation of this path way by HGF may explain why this agent does not require insulin or IGF-1 for its mitogenic action.