DIFFERENTIAL-EFFECTS OF CAMP ON THE MAP KINASE CASCADE - EVIDENCE FORA CAMP-INSENSITIVE STEP THAT CAN BYPASS RAF-1

Because cAMP exerts opposite effects on cell proliferation in differen t cell types, we undertook to study its effect on the mitogen-activate d protein kinase (MAPK) pathway in three cell lines (Rat-1, Swiss-3T3, and COS-7) chosen for their different mitogenic responses to cAMP. We measured the effect of cAMP on MAPK, MEK, and Raf-1 activities after stimulation by agonists acting through a tyrosine kinase receptor (epi dermal growth factor) or a G protein-coupled receptor (lysophosphatidi c acid). In Rat-1 cells we found that cAMP strongly inhibited all thre e activities (MAPK, MEK, and Raf-1), in good agreement with its effect on cell proliferation in these cells. In Swiss-3T3 and COS-7 cells, o n the contrary, cAMP did not inhibit epidermal growth factor- and lyso phosphatidic acid-induced stimulation of MAPK and MEK activities, and even stimulated MAPK activity slightly on its own. Again these results are in good agreement with the proliferative effect of cAMP in Swiss- 3T3 cells. Raf-1 activity, on the other hand, was inhibited by cAMP in Swiss-3T3 and COS-7 as it was in Rat-1 cells. This result indicates t hat signaling pathways in Swiss-3T3 and COS-7 cells can activate MEK a nd MAPK in a Raf-1-independent and cAMP-insensitive manner. Our result s add to growing evidence for the existence of Ras- and/or Raf-1-indep endent pathways leading to MEK and MAPK activation.