TSH and cAMP do not signal mitogenesis through Ras activation

Ras activation by receptor tyrosine kinases or serpentine receptors is gene rally considered to be essential for G1 phase progression and mitogenesis. In the physiologically relevant model of primary dog thyrocytes, the accumu lation of the GTP-bound form of Res constituted an early convergence point of various mitogenic or comitogenic stimuli including EGF, HGF, phorbol est ers, insulin and carbachol. By contrast, the basal level of GTP-Ras was sli ghtly reduced by TSH and forskolin and did not increase during the TSH/cAMP -dependent progression into G1 phase. This rules out a role for the activat ion of Res as a signal in the mitogenesis elicited by TSH via cAMP in these cells. (C) 2000 Academic Press.