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Cellular response to oncogenic ras involves induction of the Cdk4 and Cdk6 inhibitor p15(INK4b)
M Malumbres1, I Pérez De Castro, M I Hernández
1Department of Pathology and Kaplan Comprehensive Cancer Center, New York University Medical Center, New York, New York 10016, USA.
Abstract:
The cell cycle inhibitor p15(INK4b) is frequently inactivated by homozygous deletion together with p16(INK4a) and p19(ARF) in some types of tumors. Although the tumor suppressor capability of p15(INK4b) is still questioned, it has been found to be specifically inactivated by hypermethylation in hematopoietic malignancies in the absence of p16(INK4a) alterations. Here we show that, in vitro, p15(INK4b) is a strong inhibitor of cellular transformation by Ras. Surprisingly, p15(INK4b) is induced in cultured cells by oncogenic Ras to an extent similar to that of p16(INK4a), and their expression is associated with premature G(1) arrest and senescence. Ras-dependent induction of these two INK4 genes is mediated mainly by the Raf-Mek-Erk pathway. Studies with activated and dominant negative forms of Ras effectors indicate that the Raf-Mek-Erk pathway is essential for induction of both the p15(INK4b) and p16(INK4a) promoters, although other Ras effector pathways can collaborate, giving rise to a stronger response. Our results indicate that p15(INK4b), by itself, is able to stop cell transformation by Ras and other oncogenes such as Rgr (a new oncogene member of the Ral-GDS family, whose action is mediated through Ras). In fact, embryonic fibroblasts isolated from p15(INK4b) knockout mice are susceptible to transformation by the Ras or Rgr oncogene whereas wild-type embryonic fibroblasts are not. Similarly, p15(INK4b)-deficient mouse embryo fibroblasts are more sensitive than wild-type cells to transformation by a combination of the Rgr and E1A oncogenes. The cell cycle inhibitor p15(INK4b) is therefore involved, at least in some cell types, in the tumor suppressor activity triggered after inappropriate oncogenic Ras activation in the cell.
Insights
The cell cycle inhibitor p15(INK4b) acts as a tumor suppressor by halting cellular transformation induced by oncogenic Ras. This protein
Area of Science:
- Oncology
- Molecular Biology
- Cell Cycle Regulation
Background:
- The cell cycle inhibitor p15(INK4b) is often inactivated in tumors, with its tumor suppressor role being debated.
- p15(INK4b) inactivation occurs via homozygous deletion with p16(INK4a) and p19(ARF), or hypermethylation in hematopoietic malignancies.
- The specific role of p15(INK4b) in tumor suppression, particularly in response to oncogenic signaling, requires further elucidation.
Purpose of the Study:
- To investigate the role of p15(INK4b) as an inhibitor of cellular transformation mediated by oncogenic Ras.
- To determine the mechanism of p15(INK4b) induction by oncogenic Ras and its contribution to cell cycle arrest.
- To assess the functional significance of p15(INK4b) in preventing oncogene-induced transformation using preclinical models.
Main Methods:
- In vitro studies assessing p15(INK4b) inhibition of Ras-mediated cellular transformation.
- Analysis of p15(INK4b) and p16(INK4a) gene induction by oncogenic Ras and its dependence on the Raf-Mek-Erk pathway.
- Transformation assays using embryonic fibroblasts from p15(INK4b) knockout and wild-type mice.
Main Results:
- p15(INK4b) strongly inhibits cellular transformation by Ras in vitro.
- Oncogenic Ras induces p15(INK4b) and p16(INK4a) expression via the Raf-Mek-Erk pathway, leading to G1 arrest and senescence.
- p15(INK4b) knockout embryonic fibroblasts are susceptible to transformation by Ras and other oncogenes, unlike wild-type cells.
Conclusions:
- p15(INK4b) functions as a potent tumor suppressor by halting oncogene-induced cellular transformation.
- The Raf-Mek-Erk pathway mediates the induction of p15(INK4b) and p16(INK4a) in response to oncogenic Ras.
- p15(INK4b) plays a critical role in the cellular defense against oncogenic activation, particularly involving the Ras pathway.