LEPTOLSTATIN FROM STREPTOMYCES SP SAM1595, A NEW GAP PHASE-SPECIFIC INHIBITOR OF THE MAMMALIAN-CELL CYCLE .1. SCREENING, TAXONOMY, PURIFICATION AND BIOLOGICAL-ACTIVITIES

Leptolstatin, a new inhibitor of the progression of G1 and G2 phases of the mammalian cell cycle, was discovered through a unique screening system, in which effects of microbial metabolites on the cell cycle progression of the cultured rat fibroblasts were monitored by flow cytometry. The new inhibitor was extracted from the fermentation broth of Streptomyces sp. SAM1595 with ethyl acetate, and purified by silica gel column chromatography and HPLC. Leptolstatin showed a strong cytostatic effect on rat normal fibroblasts 3Y1 with an IC50 value of 0.4 ng/ml, but its antimicrobial activity was very weak. A 24-hour treatment of the fibroblast cells with 10 ng/ml of leptolstatin caused an arrest at G1 or G2 phase, as determined by flow cytometry. When the G2-arrested cells were freed from leptolstatin, those containing 4C DNA entered a new S phase without intervening M phase, resulting in the formation of proliferative tetraploid cells.