Modulation of the erythropoietin-induced proliferative pathway by cAMP in vascular smooth muscle cells

We previously reported thaterythropoietin (Epo) has a mitogenic effect on rat vascular smoothmuscle cells (VSMC) and that activation of the mitogen-activatedprotein kinase (MAPK) cascade is an important mediator for Epo-inducedmitogenesis. An increase in intracellular cAMP has an antiproliferativeeffect on VSMC. We therefore hypothesized that cAMP effectors inhibitEpo-induced MAPK activation in rat VSMC. When we exposed VSMC torecombinant human Epo (rHuEpo), DNA synthesis was increased. Forskolin(Fsk) or cilostazol (Cil) decreased the DNA synthesis stimulated by rHuEpo. Coincubation with Rp-cAMPS triethylamine canceledthe suppression of DNA synthesis and MAPK activity by Fsk. Both rHuEpo and phorbol 12-myristate 13-acetate upregulated phosphorylations of MEKand MAPK. Pretreatment with Fsk inhibited these phosphorylations. Protein kinase C inhibitors also suppressed MEK and MAPKphosphorylations. Moreover, Fsk induced phosphorylation of Raf-1 atserine-259. These results indicated that cAMP inhibited Epo-inducedMAPK activation and that this suppression might be regulated upstreamor at Raf-1. The results also suggested that these agents, which couldaccumulate cAMP, might be protective for Epo-stimulated direct action.