Short-term treatment of mesangial cells with phorbol 12-myristate 13-acetate (PMA) decreases angiotensin II-induced InsP3 formation, but potentiates hormone-stimulated arachidonic acid release and prostaglandin (PG) E2 synthesis. Long-term treatment with PMA augments hormone-stimulated InsP3 generation (after 8 h treatment), but eliminates angiotensin II-induced arachidonic acid release and PGE2 formation (after 24 h treatment). By using specific antibodies it is observed that mesangial cells express two protein kinase C (PKC) isoenzymes, PKC-alpha and -epsilon. No PKC-beta and -gamma isoenzymes are detected. On exposure to PMA a complete depletion of PKC-alpha is observed within 8 h. In contrast, down-regulation of PKC-epsilon to 10-20% of that found in control cells requires a 24 h treatment with PMA. These results may imply that PKC-alpha mediates feedback inhibition of phosphoinositide hydrolysis, whereas PKC-epsilon is a candidate for regulating PG synthesis in mesangial cells.
