INHIBITORS OF CYCLIC-NUCLEOTIDE PHOSPHODIESTERASE ISOZYMES TYPE-III AND TYPE-IV SUPPRESS MITOGENESIS OF RAT MESANGIAL CELLS

We studied interactions between the mitogen-activated protein kinase ( MAPK) signalling pathway and cAMP-protein kinase (PKA) signaling pathw ay in regulation of mitogenesis of mesangial cells (MC) determined by [H-3] thymidine incorporation, with or without added EGF. Forskolin or dibutyryl cAMP strongly (by 60-70%) inhibited [3H]thymidine incorpora tion into MC. Cilostamide, lixazinone or cilostazol selective inhibito rs of cAMP-phosphodiesterase (PDE) isozyme PDE-III, inhibited mitogene sis to similar extent as forskolin and DBcAMP and activated in situ PK A, but without detectable increase in cAMP levels, Cilostamide and cil ostazol were more than three times more effective at inhibiting mesang ial mitogenesis than rolipram and denbufylline, inhibitors of isozyme PDE-IV, even though PDE-IV was two times more abundant in MC than was PDE-III, On the other hand, when incubated with forskolin, rolipram-en hanced cAMP accumulation was far greater (10-100x) than with cilostami de. EGF increased MAPK activity (+300%); PDE isozyme inhibitors which suppressed mitogenesis also inhibited MAPK, PDE isozyme inhibitors als o suppressed PDGF-stimulated MC proliferation, We conclude that cAMP i nhibits the mitogen-dependent MAPK-signaling pathway probably by decre asing the activity of Raf-1 due to PKA-catalyzed phosphorylation, Furt her, we surmise that minor increase in the cAMP pool metabolized by PD E-III is intimately related to regulation of mesangial proliferation, Thus, PDE isozyme inhibitors have the potential to suppress MC prolife ration by a focused effect upon signaling pathways.