Joint requirement for Rac and ERK activities underlies the mid-G1 phase induction of cyclin D1 and S phase entry in both epithelial and mesenchymal cells.

Cyclin D1 gene induction is a key event in G1 phase progression. Our previous studies indicated that signaling to cyclin D1 is cell type-dependent because the timing of cyclin D1 gene expression in MCF10A mammary epithelial cells and mesenchymal cells such as fibroblasts and vascular smooth muscle cells is very different, with epithelial cells first expressing cyclin D1 in early rather than mid-G1 phase. In this report, we induced a mesenchymal phenotype in MCF10A cells by long-term exposure to TGF-beta and used the control and transitioned cells to examine cell type specificity of the signaling pathways that regulate cyclin D1 gene expression. We show that early-G1 phase cyclin D1 gene expression in MCF10A cells is under the control of Rac, whereas mid-G1 phase cyclin D1 induction requires parallel signaling from Rac and ERK, both in the control and transitioned cells. This combined requirement for Rac and ERK signaling is associated with an increased requirement for intracellular tension, Rb phosphorylation, and S phase entry. A similar co-regulation of cyclin D1 mRNA by Rac and ERK is seen in primary mesenchymal cells. Overall, our results reveal two mechanistically distinct phases of Rac-dependent cyclin D1 expression and emphasize that the acquisition of Rac/ERK co-dependence is required for the mid-G1 phase induction of cyclin D1 associated with S phase entry.