Cell cycle versus density dependence of smooth muscle alpha actin expression in cultured rat aortic smooth muscle cells

Cultured smooth muscle cells (SMC) undergo induction of smooth muscle (SM) alpha actin at confluency. Since confluent cells exhibit contact inhibition of growth, this finding suggests that induction of SM alpha actin may be associated with cell cycle withdrawal. This issue was further examined in the present study using fluorescence-activated cell sorting of SMC undergoing induction at confluency and by examination of the effects of FBS and platelet-derived growth factor (PDGF) on SM alpha actin expression in postconfluent SMC cultures that had already undergone induction. Cell sorting was based on DNA content or differential incorporation of bromodeoxyuridine (Budr). The fractional synthesis of SM alpha actin in confluent cells was increased two- to threefold compared with subconfluent log phase cells, but no differences were observed between confluent cycling (Budr+) and noncycling (Budr-) cells. In cultures not exposed to Budr, confluent cycling S + G2 cells exhibited similar induction. These data indicate that cell cycle withdrawal is not a prerequisite for the induction of SM alpha actin synthesis in SMC at confluency. Growth stimulation of postconfluent cultures with either FBS or PDGF resulted in marked repression of SM alpha actin synthesis but the level of repression was not directly related to entry into S phase in that PDGF was a more potent repressor of SM alpha actin synthesis than was FBS despite a lesser mitogenic effect. This differential effect of FBS versus PDGF did not appear to be due to transforming growth factor-beta present in FBS since addition of transforming growth factor-beta had no effect on PDGF-induced repression. Likewise, FBS (0.1-10.0%) failed to inhibit PDGF-induced repression. Taken together these data demonstrate that factors other than replicative frequency govern differentiation of cultured SMC and suggest that an important function of potent growth factors such as PDGF may be the repression of muscle-specific characteristics.