Ras mediates the cAMP-dependent activation of extracellular signal-regulated kinases (ERKs) in melanocytes.
In melanocytes and melanoma cells, cAMP activates extracellular signal-regulated kinases (ERKs) and MEK-1 by an unknown mechanism. We demonstrate that B-Raf is activated by cAMP in melanocytes. A dominant-negative mutant of B-Raf, but not of Raf-1, blocked the cAMP-induced activation of ERK, indicating that B-Raf is the MEK-1 upstream regulator mediating this cAMP effect. Studies using Clostridium sordelii lethal toxin and Clostridium difficile toxin B have suggested that Rap-1 or Ras might transduce cAMP action. We show that Ras, but not Rap-1, is activated cell-specifically and mediates the cAMP-dependent activation of ERKs, while Rap-1 is not involved in this process in melanocytes. Our results suggest a novel, cell-specific mechanism involving Ras small GTPase and B-Raf kinase as mediators of ERK activation by cAMP. Also, in melanocytes, Ras or ERK activation by cAMP is not mediated through protein kinase A activation. Neither the Ras exchange factor, Son of sevenless (SOS), nor the cAMP-responsive Rap-1 exchange factor, Epac, participate in the cAMP-dependent activation of Ras. These findings suggest the existence of a melanocyte-specific Ras exchange factor directly regulated by cAMP.
Buscà
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INSERM U385, Faculté de Médecine, Avenue de Valombrose, 06107 Nice Cédex 2, France. busca@unice. fr.
Abbe
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P
Mantoux
F
F
Aberdam
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E
Peyssonnaux
C
C
Eychène
A
A
Ortonne
J P
JP
Ballotti
R
R
D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism; D020935:MAP Kinase Signaling System; D008544:Melanocytes / Q000378:metabolism*; D008546:Melanoma, Experimental; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008954:Models, Biological; D016716:PC12 Cells; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D051381:Rats; D020664:Son of Sevenless Proteins / Q000235:genetics / Q000378:metabolism; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism*
D020662:Guanine Nucleotide Exchange Factors; C115955:RAP1GDS1 protein, human; C116122:RAPGEF3 protein, human; D020664:Son of Sevenless Proteins; D000242:Cyclic AMP; D019908:Proto-Oncogene Proteins c-raf; D017868:Cyclic AMP-Dependent Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D020930:MAP Kinase Kinase Kinases; D018631:ras Proteins