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

Abstract

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-␤ 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 codependence is required for the mid-G1 phase induction of cyclin D1 associated with S phase entry.

Forkhead box M1B is a determinant of rat susceptibility to hepatocarcinogenesis and sustains ERK activity in human HCC

“…Accordingly, restriction of FOXM1 expression by siRNA, in the HuH6 cell line, led to G 2 –M phase arrest. FOXM1 inhibition was also associated with G 0 –G 1 arrest which could depend on modulation of the activity of FOXM1 targets, including pERK1/2 downregulation,36 and possible activation of G 1 inhibitors such as p21 WAF1 and p27 KIP1 14 – 17. Thus, the present investigation shows FOXM1 regulation by genes controlling the susceptibility to HCC, and underlines the role of uncontrolled progression through the cell cycle as an effector mechanism of these genes, determining the susceptible phenotype.…”

The diverse roles of Rac signaling in tumorigenesis

“…108 An alternative splice variant of Rac1, Rac1b, which encodes a highly active isoform, was also increased in colon cancer. 109 Rather than changes at the protein level, the activity of another isoform of Rac, Rac3, was found ERK-independent cyclin D1 expression was not seen in mesenchymal cells (and was not sufficient to induce Rb phosphorylation or S-phase entry), but the mid-G 1 induction is conserved in both epithelial and mesenchymal cells, 88 highlighting the potential differences in Rac-dependent cell cycle events in different cell types.…”

“…87 A more recent study using MCF10A cells confirmed that Rac can induce cyclin D1 in early G 1 independently of ERK, but also showed that mid-G 1 phase induction of cyclin D1 required parallel signaling from both ERK and Rac. 88 Interestingly, the early many opportunities to develop novel therapeutic strategies to target Rac signaling at different stages of tumor cell metastasis.…”

Rac1 Drives Melanoblast Organization during Mouse Development by Orchestrating Pseudopod- Driven Motility and Cell-Cycle Progression

“…Both cell-cell and cell-matrix adhesion signal to increase cyclin D1 levels for progression through the cell cycle and Rac1, together with ERK regulates this (Fournier et al , 2008; Klein et al , 2008). Rac1 is tightly regulated in cytokinesis, both temporally and spatially and is inhibited at the cleavage furrow by centralspindlin, a GTPase activator (GAP) (Canman et al, 2008).…”