Abstract
Mitogenic stimulation of density‐arrested C3H 10T1/2 mouse fibroblasts by serum or purified platelet‐derived growth factor (PDGF) was potently inhibited by retinyl acetate (RAc; IC~50~ = 0.1 μg/ml, 0.3 × 10^−6^ M) when administered during the frist 2 hours of mitogen exposure. This inhibitory effect of RAc coincided with a period early in the cell growth‐division cycle when density‐arrested C3H 10T1/2 cells stimulated by PDGF were found to require Physiological levels of extracellular Ca^2+^ for the transition from G~0~ to G~1~ of the cell cycle. To determine if the inhibitory effect of RAc was mediated through alterations in the Ca^2+^ signaling pathway induced by mitogens, we examined Fura‐2‐loaded fibroblasts for changes in the Ca^2+^ response elicited by PDGF. Addition of PDGF (5 ng/ml) induced a transient increase in the [Ca^2+^]i that was not significantly effected by the extracellular Ca^2+^ concentration. Treatment of cells with RAc caused a concentration‐ and time‐dependent inhibition of this PDGF‐stimulated Ca^2+^ flux (IC~50~ = 0.45 μg/ml or 1.5 × 10^−6^ M; t~1/2~ = 15 min), whereas release of intracellularly stored Ca^2+^ by thrombin was unaffected by RAc (1.2 μg/ml, 4 × 10^−6^ M). Treatment with RAc did not significantly affect PDGF binding to cell surface receptors or the generation of inositol phosphates. These results suggest that the mechanism by which RAc inhibits PDGF‐ or serum‐induced mitogenesis is through modulation of the Ca^2+^ signal stimulated by PDGF, and thereby depriving the cell of a rise in intracellular Ca^2+^ necessary for progression through the cell cycle.