Regulation of cyclin D1 expression by Rac, the actin cytoskeleton, and extracellular matrix compliance

Induction of cyclin D1 in G1-phase of the cell cycle requires simultaneous signaling from integrins and growth factor receptors. Our lab has previously shown that integrins and Rho GTPases regulate cyclin D1 induction in fibroblasts by sustaining ERK activity. In MEFs, in the absence of Rho activity, Rae induces cyclin D1 in early G1-phase. In this study, I examine the mechanism by which Rae induces cyclin D1 and I show that the Rae pathway is parallel to signaling from NF-κB and is also ERK-independent. ^ The second aim of this study is to investigate how extracellular matrix (ECM) compliance regulates the cell cycle. ECM compliance regulates progression through G1 phase in MEFs, affects the expression and function of cyclin D1, but does not alter the G1 phase profiles of the cdk inhibitors, p21 or p27. Interestingly, ERK activation is unaffected by changes in ECM compliance. Rae signaling plays a major role in the induction of cyclin D1 in MCF10A mammary epithelial cells, and we now show that this pathway functions efficiently in early-G1 phase on high compliance substrata characteristic of mammary tissue. However, mid-G1 phase cyclin D1 induction in MCF10As is ERK and compliance dependent similar to MEFs. Moreover, when MCF10A cells undergo an epithelial-mesenchymal transition (EMT), they lose Rac-mediated early-G1 cyclin D1 expression and their signaling to cyclin D1 reverts to the mid-G1 phase ERK-dependent pathway that is strongly compliance-regulated. We conclude that despite the existence of cell-type specific signaling pathways to cyclin D1 (ERK vs. Rac), there remains a universal compliance-dependent mechanism for cyclin D1 induction.