The influence of contact-inhibited growth and of agents which alter cell morphology on the levels of G- and F-actin in cultured cells.

The amounts of G-actin and F-actin were measured in cultured cells grown under various conditions. The percent of total actin as F-actin in monolayer cultures of asynchronous cells was 72.4% in Chinese hamster ovary (CHO) cells, 57.7% in HeLa cells, 69.8% in V79 cells, and 79.5% in 1080 cells. Actin comprises 2.4-3.1% of the total protein in these cell lines. Treatment of cells with 20 microM cytochalasin B (CB) caused different cytological effects from treatment with 10 microM colchicine, but the effects characteristic of each drug were observed throughout the range of cell lines used. Of the five cell lines treated with CB only the V79 and CHO cells showed a decrease (5-8%) in the level of F-actin. Colchicine treatment of HeLa cells resulted in a 13% increase in the percent F-actin, but similar treatment of CHO cells caused no significant change in F-actin. Therefore, a change in the steady state level of F-actin is not necessary for the observed cell shape change. The F-actin levels in CHO cells treated with 7 mM procaine decreased from 72 to 65% over the first 15 min of exposure, a time during which the cells rounded. After continuous exposure of the cells to procaine for 1 h, the F-actin percentage returned to control levels and the cells, though abnormal in appearance, flattened on the culture dish. The relationship between the level of F-actin and cell density was studied on the culture dish. The relationship between the level of F-actin and cell density was studied in the 10T 1/2 cells, a cell line which demonstrates density-dependent growth regulation. While contact inhibition was accompanied by a decrease in the F-actin percentage (from greater than 95% to about 60%) in one strain of 10T1/2 cells, two other strains of the same cells progressed from log phase growth (highly motile) to late plateau phase (non-motile, contact-inhibited) with a constant level of F-actin (about 60%). A spontaneous transformant of this cell line, which no longer demonstrated contact-inhibited cell growth, also maintained the same constant F-actin (60%). Thus, the maintenance of contact-inhibited growth control does not appear to depend upon the net distribution of actin between the globular and filamentous forms.