Compartmentalization of cAMP signaling in mesangial cells by phosphodiesterase isozymes PDE3 and PDE4. Regulation of superoxidation and mitogenesis

Some major pathobiologic processes in renal mesangial cells, elicited in response to immunoinflammatory stimuli, are modulated via cAMP-protein kinase A (PKA) signaling pathways; namely, generation of reactive oxygen metabolites (ROM) and accelerated proliferation of mesangial cells. We investigated the role of cAMP phosphodiesterase (PDE) isozymes in these regulatory mechanisms. Generation of ROM in cultured rat mesangial cells was inhibited by selective inhibitors of PDE4, rolipram and denbufylline, whereas PDE3 inhibitors, cilostamide and lixazinone, had no effect. Conversely, cilostamide or lixazinone suppressed mitogenic synthesis of DNA in mesangial cells, but 1 μM rolipram or 1 μM denbufylline showed no inhibitory effect. The efficacy of PDE isozyme inhibitors (IC₅₀) to suppress [₃H]thymidine incorporation or ROM generation paralleled IC₅₀ values for inhibition of cAMP PDE. Incubation of mesangial cells with either rolipram alone or with cilostamide alone increased significantly in situ activity of PKA in mesangial cells, assessed by (-cAMP/+cAMP) PKA activity ratio, and the stimulatory effects were additive. Results indicate that in mesangial cells a cAMP pool that is metabolized by PDE4 activates PKA and thereby inhibits ROM generation; another cAMP pool that is metabolized by PDE3 activates another PKA (isozyme or pool) which suppresses proliferation of mesangial cells. We propose that in mesangial cells, a cAMP-PKA pathway that regulates mitogenesis is determined by activity of PDE3, whereas another cAMP-PKA pathway is directed by activity of PDE4 and controls ROM generation. Therefore, two PDE isozymes within one cell type compartmentalize distinct cAMP signaling pathways.