DIFFERENTIAL-EFFECTS OF PROTEIN-KINASE-A ON RAS EFFECTOR PATHWAYS

Ras mutants with the ability to interact with different effecters have played a critical role in the identification of Ras-dependent signali ng pathways. We used two mutants, Ras(S35) and Ras(G37), which differ in their ability to bind Raf-1, to examine Ras-dependent signaling in thyroid epithelial cells.Wistar rat thyroid cells are dependent upon t hyrotropin (TSH) for growth. Although TSH-stimulated mitogenesis requi res Ras, TSH activates protein kinase A (PKA) and downregulates signal ing through Raf and the mitogen-activated protein kinase (MAPK) cascad e. Cells expressing Ras(S35), a mutant which binds Raf, or Ras(G37), a mutant which binds RalGDS, exhibited TSH-independent proliferation. R as(S35) stimulated morphological transformation and anchorage-independ ent growth. Ras(G37) stimulated proliferation but not transformation a s measured by these indices. TSH exerted markedly different effects on the Ras mutants and transiently repressed MAPK phosphorylation in Ras (S35)-expressing cells. In contrast, TSH stimulated MAPK phosphorylati on and growth in cells expressing Ras(G37). The Ras mutants, in turn, exerted differential effects on TSH signaling. Ras(S35) abolished TSH- stimulated changes in cell morphology and thyroglobulin expression, wh ile Ras(G37) had no effect on these activities. Together, the data ind icate that cross talk between Ras and PKA discriminates between distin ct Ras effector pathways.