Screening of phosphatidylinositol turnover inhibitors and regulation of cell cycle progression

Phosphatidylinositol (PI) turnover is considered to be involved in the regulation of cell growth. The enzymes for PI turnover include phospholipase C (PLC), PI4-kinase and PI synthase. We have isolated pholipeptin and fluvirucin B₂ from microorganisms and akaterpin from a marine sponge as PLC γ inhibitors. We also isolated echiguanines from Streptomyces as P14-kinase inhibitors. Since echiguanines did not inhibit the enzyme in situ, we synthesized their ribosylated derivatives that were effective in cultured cells. We previously isolated inostamycin from Streptomyces as an inhibitor of PI synthase. We found that inostamycin induced G₁ block in cycling NRK cells. Inostamycin inhibited the serum-induced S-phase induction in quiescent NRK cells. Inostamycin was found to decrease serum-induced expression of cyclin D and cyclin E, without inhibiting the activation of MAP kinase. It also inhibited serum-induced activation of CDK2 and phosphorylation of pRB. Thus, PI synthesis was suggested to be involved in regulation of serum- induced S-phase induction by modulating G₁ cyclin expression.