Density-dependent stimulation and inhibition of cell growth by agents that disrupt microtubules

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Abstract
Agents that disrupt microtubules, such as colchicine [C], inhibit the growth stimulation of lymphocytes and arrested fibroblasts. Enhanced stimulation of fibroblasts in the presence of the same drugs was also reported. This conflict is resolved by demonstrating that the stimulatory and inhibitory effects of microtubule disruption depend upon the density and the cell type of the treated cultures. The analysis included an examination of 3 variables: cell density (sparse or confluent), cell type (resting fibroblasts from mouse or chicken embryos or from the permanent 3T3 mouse fibroblast line) and treatment with C and related drugs in the presence or absence of various mitogens such as serum, insulin and epidermal growth factor. C augmented mitogenesis in confluent cultures of all cell types. C by itself appeared to be mitogenic only for confluent chicken embryo fibroblasts. In sparse cultures with minimal cell-cell counts, however, there were differences between embryonic cells and the 3T3 cell line. In confirmation of previous reports, disruption of microtubules by C inhibited the mitogenic stimulation of sparse cultures of embryonic chicken or mouse fibroblasts. In contrast, fibroblasts of the permanent 3T3 line in sparse cultures were stimulated by some mitogens despite the presence of C. The augmentative effects of C on the stimulation of confluent cultures were synergistic with the mitogens, and C allowed response to otherwise submitogenic doses of growth factors. Kinetic studies indicated that the stimulatory and inhibitory effects of C are separable and that both can operate simultaneously. Apparently the regulation of growth by nutrient deprivation and the regulation by density dependence proceed at least in part by different mechanisms and microtubular integrity can be associated with the expression of either negative or positive controls on cell growth, depending upon the confluence or lineage of the cells in culture.