THYROTROPIN-INDUCED MITOGENESIS IS RAS DEPENDENT BUT APPEARS TO BYPASS THE RAF-DEPENDENT CYTOPLASMIC KINASE CASCADE

Cellular growth control requires the coordination and integration of m ultiple signaling pathways which are likely to be activated concomitan tly. Mitogenic signaling initiated by thyrotropin (TSH) in thyroid cel ls seems to require two distinct signaling pathways, a cyclic AMP (cAM P)-dependent signaling pathway and a Ras-dependent pathway. This is a paradox, since activated cAMP-dependent protein kinase disrupts Ras-de pendent signaling induced by growth factors such as epidermal growth f actor and platelet-derived growth factor. This inhibition may occur by preventing Raf-1 protein kinase from binding to Ras, an event thought to be necessary for the activation of Raf-1 and the subsequent activa tion of the mitogen-activated protein (MAP)/extracellular signal-regul ated kinase (ERK) kinases (MEKs) and MAP kinase (MAPK)/ERKs. Here we r eport that serum-stimulated hyperphosphorylation of Raf-1 was inhibite d by TSH treatment of Wister rat thyroid cells, indicating that in thi s cell line, as in other cell types, increases in intracellular cAMP l evels inhibit activation of downstream kinases targeted by Ras. Ras-st imulated expression of genes containing AP-1 promoter elements was sim ilarly inhibited by TSH. On the other hand, stimulation of thyroid cel ls with TSH resulted in stimulation of DNA synthesis which was Ras dep endent but both Raf-1 and MEK independent. We also show that Ras-stimu lated DNA synthesis required the use of this kinase cascade in untreat ed quiescent cells hut not in TSH-treated cells. These data suggest th at in TSH-treated thyroid cells, Ras might be able to signal through e ffecters other than the well-studied cytoplasmic kinase cascade.