RGS6 suppresses Ras-induced cellular transformation by facilitating Tip60-mediated Dnmt1 degradation and promoting apoptosis

Abstract

The RAS protooncogene plays a central role in regulation of cell proliferation, and point mutations leading to oncogenic activation of Ras occur in a large number of human cancers. Silencing of tumor suppressor genes by DNA methyltransferase 1 (Dnmt1) is essential for oncogenic cellular transformation by Ras, and Dnmt1 is over-expressed in numerous human cancers. Here we provide new evidence that the pleiotropic Regulator of G protein Signaling (RGS) family member RGS6 suppresses Ras-induced cellular transformation by facilitating Tip60-mediated degradation of Dmnt1 and promoting apoptosis. Employing mouse embryonic fibroblasts (MEFs) from wild type (WT) and RGS6−/− mice, we found that oncogenic Ras induced up-regulation of RGS6, which in turn blocked Ras-induced cellular transformation. RGS6 functions to suppress cellular transformation in response to oncogenic Ras by down regulating Dnmt1 protein expression leading to inhibition of Dnmt1-mediated anti-apoptotic activity. Further experiments showed that RGS6 functions as a scaffolding protein for both Dnmt1 and Tip60 and is required for Tip60-mediated acetylation of Dnmt1 and subsequent Dnmt1 ubiquitylation and degradation. The RGS domain of RGS6, known only for its GAP activity toward Gα subunits, was sufficient to mediate Tip60 association with RGS6. This work demonstrates a novel signaling action for RGS6 in negative regulation of oncogene-induced transformation and provides new insights into our understanding of the mechanisms underlying Ras-induced oncogenic transformation and regulation of Dnmt1 expression. Importantly, these findings identify RGS6 as an essential cellular defender against oncogenic stress and a potential therapeutic target for developing new cancer treatments.

RGS6 as a Novel Therapeutic Target in CNS Diseases and Cancer

“…Further studies demonstrated that RGS6 is not only a tumor suppressor itself but is also induced by oncogenic Ras and blocks Ras-induced cellular transformation through a novel DNMT1-dependent mechanism (110). In this study, initial experiments confirmed that cellular transformation induced by oncogenic Ras and dominant negative p53 was increased in RGS6−/− MEFs compared to RGS6+/+ MEFs.…”

“…These experiments thus identified a critical role of RGS6 in regulating DNMT1 expression and preventing its oncogenic actions. Subsequent experiments in this study showed that RGS6 promotes DNMT1 degradation by scaffolding DNMT1 and the acetyltransferase Tip60 to facilitate DNMT1 acetylation which is followed by its subsequent ubiquitylation and degradation (110) (Fig 6b).…”

“…R7BP R7 family-binding protein, DMAP1 DNA methyltransferase1 associated protein 1, Met methylation, Ub ubiquitin, Ac acetylation, ERE estrogen response element. Image adapted from references (28,30,110) undergo malignant transformation, may serve a crucial role in defending against oncogenic or genotoxic stress. Figure 6b details the current model of RGS6 tumor suppression, in which RGS6 blocks cellular transformation and tumorigenesis by facilitating activation of the DDR and apoptosis, effectively halting HER2-, ER-and carcinogen-induced cellular proliferation.…”

Laccaic Acid A Is a Direct, DNA-competitive Inhibitor of DNA Methyltransferase 1

“…Because Rgs6 functions as a negative regulator of Dnmt1 (27), we tested whether LCA can inhibit Ras-induced proliferative and apoptotic phenotypes in a rgs6 knock-out model. In these studies, Dnmt1 was strongly overexpressed and drove cellular phenotypes that were effectively reversed by LCA but not by the control anthraquinone (59). These data indicate that LCA is a novel DNA-competitive inhibitor of Dnmt1 with the potential to improve the understanding of biology of DNA methylation and the enzymology of Dnmt isozymes and serve as a lead compound for combination therapy agents.…”

Development and translational imaging of a TP53 porcine tumorigenesis model

“…-puro retroviruses for 2 days, using methods of virus preparation and infection identical to those reported previously (71,72). Infected cells were selected with 1 μg/ml puromycin for 2 days and replated at identical cell numbers; 5-8 days later, induction of senescence was measured by cell counting, quantitative RT-PCR for porcine CDKN1A mRNA expression, staining for SA-βgal activity (73), and phase-contrast microscopy.…”