Cell-density-dependent regulation of actin gene expression due to changes in actin treadmilling

The actins are essential cytoskeletal proteins required for the survival and growth of cells. The transitions between soluble (G-actin) and filamentous (F-actin) forms of actin (actin treadmilling) are complexly regulated. Here we show that the expression of the cytoplasmic beta-actin and gamma-actin genes is down-regulated in mouse fibroblasts when the cell density of the culture increases. Conversely, a dense culture replated at lower density results in increases in actin mRNA levels within a few hours. Concomitant with these changes in mRNA levels, we observe increased depolymerization of actin microfilaments at higher densities resulting in an elevated G-actin to F-actin ratio. By using actin polymerization inhibitors, we show that the density-dependent change in actin gene expression is dependent on changes in the ratio of G-actin vs. F-actin levels. Therefore, actin treadmilling and actin gene regulation are not coregulated by cell density, but represent a linear signal transduction pathway in which actin treadmilling regulates actin gene transcription. The physiological transition represented by the growth of a sparse fibroblast population into a confluent and growth-arrested population represents a useful model for the study of how the actin treadmill exerts its action on the gene expression program of cells.