RO31-8220 INHIBITS PROTEIN-KINASE-C TO BLOCK THE PHORBOL ESTER-STIMULATED RELEASE OF CHOLINE-METABOLITES AND ETHANOLAMINE-METABOLITES FROM C6 GLIOMA-CELLS - P70 S6 KINASE AND MAPKAP KINASE-1-BETA DO NOT FUNCTION DOWNSTREAM OF PKC IN ACTIVATING PLD

The use of bisindolylmaleimide derivatives of staurosporine as selecti ve inhibitors of protein kinase C (PKC) is in doubt following the repo rt by Alessi [FEBS Lett. 402 (1997) 121-123] that Ro31-8220 and GF1092 03X are potent in vitro inhibitors of p70 S6 kinase and mitogen-activa ted protein kinase-activated protein kinase-1 beta, as well as of PKC, Here we show that the phorbol ester-stimulated release of choline- an d ethanolamine-metabolites from C6 glioma cells due to phospholipid hy drolysis by phospholipase D (PLD) is not inhibited by rapamycin or PD9 8059, specific inhibitors respectively of p70 S6 kinase and MAPKK (MEK ) and thus of MAPKAP kinase-1 beta but is still completely blocked by Ro31-8220. We conclude therefore that p70(S6k) and MAPKAP kinase-1 bet a as well as MAPK are not involved in signalling pathways downstream o f PKC that regulate phorbol ester-stimulated phospholipid turnover and that the inhibitory action of Ro31-8220 occurs by blocking PKC which regulates at least one pathway to PLD activation. The PI-3 kinase inhi bitor, wortmannin, inhibits the phorbol ester-stimulated release of et hanolamine-but not choline-metabolites from C6 cells suggesting that d ifferent PLD isoforms regulate the turnover of PtdEth and PtdCho in C6 cells. Both PLD isoforms are activated via PKC but the PtdEth-PLD is also regulated via a wortmannin-sensitive pathway. (C) 1997 Federation of European Biochemical Societies.