Role of surface modulating assemblies in growth control of normal and transformed fibroblasts.

Abstract

Cellular microtubules, microfilaments and surface receptors were postulated to form a surface modulating assembly that regulates surface receptor mobility and cell growth. To test this hypothesis, we examined 3 agents known to affect cell growth [colchicine, concanavalin A (Con A) and the src gene product of Rous sarcoma virus] were examined for their effects on chick embryo fibroblasts. Individual cells from serum-starved normal fibroblast populations became committed to enter S phase at various times over a 12 h period after exposure to serum. Colchicine and other microtubule-disrupting agents blocked entry into S phase at a point close to the commitment point for each cell. The lectin Con A also blocked entry into the S phase when present in doses sufficient to modulate surface receptor mobility. Succinyl-Con A, which does not induce surface modulation, had no effect. Both Con A and colchicine blocked the appearance of cytoplasmic factors capable of stimulating DNA replication in a cell-free system. To study endogenous effects on the surface modulating assembly, fibroblasts were infected with a Rous sarcoma virus (tsNY68) having a temperature-sensitive mutation in the transforming (src) gene. Microtubular and microfilamentous structures of the surface modulating assembly are direct or indirect targets of the src gene product with consequent reduction in the capacity of Con A to induce surface modulation. TsNY68-infected fibroblasts shifted to the nonpermissive temperature acquired normal microtubular morphology more rapidly (2 h) than cells grown at the permissive temperature in the presence of protein synthesis inhibitors (7.5 h). The src gene product may act directly on the surface modulating assembly rather than via the nucleus or at the level of protein synthesis. Transformation of the surface modulating assembly was partly blocked by treatment of the infected cells with Con A but not succinyl-Con A. Both Con A and colchicine inhibited entry into the S phase following a shift from nonpermissive to permissive growth conditions. These observations are consistent with the surface modulating assembly acting as a signal regulator in growth control.