Mycophenolic acid reduces renin-angiotensin-system activity in cultured mouse medullary thick ascending limb cells

Mycophenolate mofetil (MMF) is a promising immunosuppressive agent. The intra-renal reninangiotensin system (RAS) plays an important role in the regulation of intra-renal hemodynamics. Mycophenolic acid (MPA) is the bioactive metabolite of MMF. The interaction between MPA and intra-renal RAS is still unclear. We hypothesized that MPA might affect intra-renal RAS activity. We chose models of cultured mouse medullary thick ascending limb (mTAL) cells for the experiments, as the mTAL is one of the major sites of intra-renal RAS. We investigated the angiotensin-converting enzyme (ACE) activity by means of enzymatic assay, and the angiotensin-receptor activity by means of a binding study with radiolabeled angiotensin II, and measured the intracellular calcium concentration in cultured cells treated with and without MPA. ACE activity changed neither in cells incubated with MPA nor in those treated without MPA. The binding study also indicated decreased angiotensin-II binding in MPA-treated (MPA 10^-7 M) cells, up to 43.7%. The decreased intracellular calcium concentration in MPA-treated cells further confirmed the MPA-inhibitory effect on the angiotensin receptor. We conclude that MPA reduces intra-renal RAS activity mainly through the decrease of AT1 receptor activity without affecting ACE activity. The results suggest that the inhibitory effect of MPA in the intra-renal RAS might play a role in the extra-immunosuppressive effect of MMF.