Disruption of cytoplasmic microtubules by colchicine, colcemid or vinblastine induced the initiation of DNA synthesis and cell division in quiescent cultures of rat embryo fibroblast cell line, 3Y1, in the absence of growth factors. The microtubule-disruption-induced DNA synthesis was so marked that we could initiate analysis of the mechanism. Incubation of quiescent 3Y 1 cells with high concentrations of vinblastine or those of vinblastine plus colcemid, which formed large tubulin paracrystals concomitant with the depolymerization of cytoplasmic microtubules, initiated DNA synthesis in the cells. Because these treatments did not increase the free tubulin level in the cells, but rather decreased it, an increase in the free tubulin level may not be required for initiation of DNA synthesis induced by microtubule-disrupting agents. Culture fluid of the 3Y1 cells incubated with colchicine did not stimulate the initiation of DNA synthesis when added to other quiescent 3Y1 cells after conversion of the colchicine in thc culture fluid to lumicolchicine by ultraviolet irradiation. This suggested that colchicine treatment did not induce secretion of any mitogenic factors from 3Y1 cells. These results suggest that disruption of normal microtubules may elicit intracellular signals leading to cell proliferation.Cytoplasmic microtubules function in a wide variety of ways as a structural component and motility machinery. Cytoplasmic microtubules have also been suggested to play a regulatory role in cell growth, based on the observation that microtubule-depolymerizing agents have inhibitory or stimulatory effects on cell cycle traverse of quiescent cultures. Vasiliev et al. reported that colcemid stimulated the initiation of DNA synthesis in primary cultures of density-arrested mouse embryo fibroblast-like cells in the presence of 10% bovine serum [I]. On the other hand, colchicine inhibited the initiation of DNA synthesis in some cell lines [2][3][4][5][6][7][8]. But, as for confluently cultured, well-arrested fibroblastic cells, colchicine always potentiated the effect of growth factors on initiation of DNA synthesis [9-131. Crossin and Carney demonstrated that colchicine treatment alone initiated DNA synthesis in somc primary cultures in serum-free meclium [14, IS], though the stimulation of thymidine incorporation by colchicine was no more than 2-_?-fold, probably because the cultures might not be completely rendered quiescent.We have found in this study that microtubule-disrupting agents in the absence of any growth factors induce effectively the initiation of DNA synthesis in quiescent cultures of the rat embryo fibroblast cell line, 3Y1. Using this system we could further investigate how microtubule depolymerization relates to the induction of DNA synthesis. Recent excellent experiments done by Cleveland and co-workers [16, 171 and others [I 81 have established that cytoplasmic free tubulin concentration could be a signal for regulating the level of tubulinCorrespondence to E. Nishida, Department of Biophysics and ...