Differential effects on cAMP on the MAP kinase cascade: evidence for a cAMP-insensitive step that can bypass Raf-1.
Because cAMP exerts opposite effects on cell proliferation in different cell types, we undertook to study its effect on the mitogen-activated 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 (epidermal growth factor) or a G protein-coupled receptor (lysophosphatidic acid). In Rat-1 cells we found that cAMP strongly inhibited all three 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, on the contrary, cAMP did not inhibit epidermal growth factor- and lysophosphatidic 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 hand, was inhibited by cAMP in Swiss-3T3 and COS-7 as it was in Rat-1 cells. This result indicates that signaling pathways in Swiss-3T3 and COS-7 cells can activate MEK and MAPK in a Raf-1-independent and cAMP-insensitive manner. Our results add to growing evidence for the existence of Ras- and/or Raf-1-independent pathways leading to MEK and MAPK activation.
Faure
M
M
Department of Pharmacology, University of California, San Francisco 94143, USA.
Bourne
H R
HR
D016475:3T3 Cells; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D002460:Cell Line; D002522:Chlorocebus aethiops; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology*; D004789:Enzyme Activation; D004815:Epidermal Growth Factor / Q000494:pharmacology; D008246:Lysophospholipids / Q000494:pharmacology; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1; D020929:Mitogen-Activated Protein Kinase Kinases; D008934:Mitogens / Q000494:pharmacology; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf; D051381:Rats; D015398:Signal Transduction / Q000502:physiology*
D008246:Lysophospholipids; D008934:Mitogens; D011518:Proto-Oncogene Proteins; D005576:Colforsin; D004815:Epidermal Growth Factor; D000242:Cyclic AMP; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482127:Map2k1 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases