NFκB-Independent Signaling to the Cyclin D1 Gene by Rac

Abstract

AbbREviAtionsNFκB nuclear factor κB ERK extracellular signal-regulated kinase P-ERK dually phosphorylated ERK MEF mouse embryo fibroblast QPCR quantitative RT-PCR ACKnowlEdGEMEntsWe thank Jessie Villanueva and Hanqin Lei for generating some of the cyclin D1 promoterluciferase constructs described in this paper. This work was supported by NIH grants GM069064 and CA72639 to R.K.A. AbstRACtIn this report we characterize the mechanism of Rac-mediated cyclin D1 gene expression in mouse embryonic fibroblasts. Activated Rac strongly stimulated cyclin D1 gene transcription but did not alter the half-life of cyclin D1 mRNA. Inhibition of NFκB signaling with the IκB super-repressor blocked the Rac-dependent expression of cyclin D1 mRNA, and this effect was selective since ERK-dependent cyclin D1 mRNA induction was minimally affected by super-repressor expression. However, we found that p65 activity in this system was induced by serum and not by activated Rac. Moreover, mouse cyclin D1 promoter-luciferase assays showed that Rac stimulated cyclin D1 gene expression without activating NFκB and that an essential Rac-regulated promoter element is located far upstream or downstream of the cyclin D1 transcription start site. We conclude that, in MEFs, Rac-mediated induction of cyclin D1 mRNA requires activation of a parallel NFκB pathway whereas ERK induces cyclin D1 transcription independent of NFκB.

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

“…We do not yet know if the Rac-dependent signaling pathway(s) that leads to early-G1 phase cyclin D1 mRNA and mid-G1 phase cyclin D1 gene expression are the same or different. Evidence that they are different includes the observation that Rac-dependent induction of cyclin D1 in early-G1 phase requires concomitant signaling by NF-B, whereas its induction of cyclin D1 in mid-G1 phase is independent of NF-B (17).…”

“…However, most studies on ERK and Rac signaling to cyclin D1 have either been performed in different cell types (epithelial versus mesenchymal) or under conditions of Rac overexpression (3,4,16,17). Thus, little is known about the potential interplay between endogenous ERK and Rac activities as it relates to expression of the cyclin D1 gene within a single cell type.…”

Rac-dependent cyclin D1 gene expression regulated by cadherin- and integrin-mediated adhesion

“…Consistent with the redundant effects of integrins and E-cadherins on cyclin D1 expression in MCF10A cells, we find that both of these adhesion receptors use a Rac-dependent signaling pathway to induce cyclin D1 mRNA. We and others have previously shown that both ERK and Rac can induce cyclin D1 gene expression, but most of the studies with the Rac pathway have relied on overexpression of an activated Rac allele (Gjoerup et al, 1998;Joyce et al, 1999;Klein et al, 2007;Page et al, 1999;Westwick et al, 1997). We previously showed that endogenous Rac can induce cyclin D1 mRNA in fibroblasts, but this pathway is only seen upon deliberate inhibition of Rho (Welsh et al, 2001).…”

“…RTKs and integrins also regulate the activation of Rac (RAC1), and integrin signaling additionally controls the coupling of Rac to its downstream targets (del Pozo et al, 2000). Although cyclin D1 is induced downstream of activated Rac (Joyce et al, 1999;Klein et al, 2007;Page et al, 1999), endogenous Rac signaling to cyclin D1 is not readily detected in fibroblasts because the pathway is inhibited by Rho (Welsh et al, 2001). …”

Differential Activation of ERK and Rac Mediates the Proliferative and Anti-proliferative Effects of Hyaluronan and CD44

“…In several cell types, a sustained ERK activity between 3 and 6 h after mitogen stimulation results in the induction of cyclin D1 mRNA in mid-G 1 phase, at least when cells are re-entering G 1 phase from quiescence (22,23). A second common way to induce cyclin D1 mRNA is through Rac (24,25). Rac signaling to cyclin D1 requires NF-B and results in a somewhat earlier induction of cyclin D1 within the G 1 phase (23,26).…”