THROMBOXANE STIMULATION OF MESANGIAL CELL FIBRONECTIN SYNTHESIS IS SIGNALED BY PROTEIN-KINASE-C AND MODULATED BY CGMP

Thromboxane (TX) has been implicated in the pathogenesis of glomerulos clerosis in several models of glomerular injury. In the present study, we examined the role of the protein kinase C (PKC) signalling system in expression of the action of the TXA(2)/PGH(2) analogue U-46619 to s timulate fibronectin (Fn) synthesis in cultured rat mesangial cells (M C), and the influence of cGMP on this MC response. U-46619 activated P KC and enhanced Fn synthesis in MC in a time and concentration depende nt fashion. Both responses to U-46619 were blocked by GF 109203X, a se lective inhibitor of PKC activity, as well as by calphostin C and stau rosporine, PKC inhibitors structurally distinct from GFX. Down-regulat ion of PKC by prior sustained exposure of MC to 0.5 mu M phorbol myris tate acetate similarly blocked increases in Fn synthesis induced by U- 46619. The TXA(2)PGH(2) receptor antagonist Sq-29548 also prevented ac tivation of PKC and stimulation of Fn synthesis by U-46619, consistent with transduction of these responses via specific high affinity TXA(2 )/PGH(2) receptors on MC. Addition of exogenous 8-Br-cGMP or stimulati on of endogenous cGMP generation with atrial natriuretic peptide (ANP) suppressed both U-46619 activation of PKC and stimulation of Fn synth esis. cGMP did not alter TXA(2)/PGH(2) receptor number or affinity in MC, but significantly suppressed phorbol ester activation of PKC. Thus , cGMP inhibition of U-46619 actions is expressed at steps distal to T X receptor binding and may involve effects at and proximal to activati on of PKC. Interactions between the PKC and cGMP cellular signalling s ystems may be important determinants of MC matrix protein production i n response to TX.