The addition of serum to quiescent normal rat kidney (NRK) cells induced phosphatidylinositol (PI) synthesis after 4 h and DNA synthesis after 16 h. Inostamycin, an inhibitor of CDP-DG:inositol transferase, added within 4 h, inhibited both the serum-induced PI synthesis and S phase entry. By contrast, inostamycin added 8 h after release from quiescence did not inhibit the S phase entry. Inostamycin did not affect the GS/M/G1 transition, and incubation with inostamycin induced cell synchronization at early G1 phase. Thus, PI synthesis is involved in the regulation of S phase induction.
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