The role of cytoplasmic microtubules in the regulation of the activity of peptide growth factors

Abstract

The enhancement of DNA stimulation by peptide growth factors in colchicine-treated cultures of Swiss 3T3 mouse fibroblasts appears to be associated with disassembly of cytoplasmic microtubules. Other antitubulin agents (colcemid, vinblastine, podophyllotoxin, nocodazole, and maytansine) also potentiate the effects of growth promoting factors such as insulin and EGF. The cytoplasmic microtubules are disrupted by antitubulin agents at concentrations very similar to those required for depolymerization of mitotic spindle microtubules. In the presence of highly radioactive [ 3H]thymidine, DNA synthesis can be stimulated in quiescent mouse fibroblasts without completion of the cell cycle. This makes possible studies of the potentiating effects of colchicine during the prereplicative period of the cycle uncomplicated by effects of the antitubulin agent on the mitotic spindle. The enhancing effects of colchicine can be demonstrated with sparse cultures of 3T3 fibroblasts as well as with 3T6 cells. Maytansine and colchicine need not be present during the initial period of interaction of peptides with the cell to produce enhancement of DNA synthesis. The enhancement can be obtained after transient exposure of cells to fetal bovine serum. The potentiating effects of antitubulin agents such as nocodazole, podophyllotoxin, and maytansine can be easily reversed whereas the enhancement with colchicine persists despite washing of cells. The stimulatory effects of insulin and EGF persist in cultures treated with antitubulin agents. The findings support the premise that after internalization the receptor-mitogen complex may remain active within endocytic vesicles when cells are exposed to colchicine.