Depolymerization of cellular tubulin during DNA biosynthesis initiated with growth factors in 3T3 cells

The results indicated that total cellular tubulin increased markedly when the cultured 3T3 cells entered DNA biosynthesis initiated by growth factors including EGF, FDGF, insulin, PDGF and vasopressin, while the polymerized microtubutles remained unchanged. Such an increase in cellular tubulin was mainly unpolymerized free tubulin, being twice or thrice as much as that of control cells with a good dose-dependent manner. This was consistent with the pattern of increasing [3H]TdR incorporation into cells and DNA content shown by flow cytometry. The study of time course showed that the increase of free tubulin occurred between 6 and 20 h after incubation with the growth factors, just when the cells entered DNA biosynthesis from G0/G1 phase. We suggest that the increase of free tubulin may be one of the important signals related to the regulation of the initiation of DNA biosynthesis.