INHIBITION OF G1 CYCLIN EXPRESSION IN NORMAL RAT-KIDNEY CELLS BY INOSTAMYCIN, A PHOSPHATIDYLINOSITOL SYNTHESIS INHIBITOR

We previously reported that inostamycin, an inhibitor of CDP-DG:inosit ol transferase, inhibited cell proliferation in normal rat kidney (NRK ) cells by blocking cell cycle progression at the G(1) phase. In the p resent paper, we report the effect of inostamycin on the serum-induced activation of Ser/Thr protein kinases that are involved in G(1) progr ession. In quiescent NRK cells mitogen-activated protein kinase (MAP k inase) and casein kinase II mere activated within 15 min after serum a ddition. Neither activation was affected by the treatment with inostam ycin. However, in the inostamycin-treated cell, cyclin-dependent kinas e 2 (CDK2) failed to be activated after serum stimulation. Since serum -induced expression of cyclin E was also suppressed by inostamycin, th is inhibitor would appear to block CDK2 activation by inhibiting cycli n E expression, Furthermore, inostamycin also inhibited cyclin D1 expr ession induced by serum; and consequently, hyperphosphorylation of ret inoblastoma protein (pRB) by RB-kinases such as CDK4 and CDK2 was abol ished, which would result in elimination of functional inactivation of pRB. Thus, early G(1) arrest in NRK cells by inostamycin is due to th e inhibition of cyclin D1 and E expressions.