Cellular response to oncogenic ras involves induction of the Cdk4 and Cdk6 inhibitor p15(INK4b).
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.
Malumbres
M
M
Department of Pathology and Kaplan Comprehensive Cancer Center, New York University Medical Center, New York, New York 10016, USA.
Pérez De Castro
I
I
Hernández
M I
MI
Jiménez
M
M
Corral
T
T
Pellicer
A
A
D016475:3T3 Cells; D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins; D002471:Cell Transformation, Neoplastic; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16; D018844:Cyclin-Dependent Kinases / Q000502:physiology*; D004789:Enzyme Activation / Q000502:physiology; D005786:Gene Expression Regulation / Q000502:physiology; D051379:Mice; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D011518:Proto-Oncogene Proteins; D015398:Signal Transduction / Q000502:physiology; D025521:Tumor Suppressor Proteins; D018631:ras Proteins / Q000502:physiology*
D002352:Carrier Proteins; C493968:Cdkn2b protein, mouse; D018797:Cell Cycle Proteins; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16; D011518:Proto-Oncogene Proteins; D025521:Tumor Suppressor Proteins; D017346:Protein Serine-Threonine Kinases; C495902:Cdk4 protein, mouse; C495912:Cdk6 protein, mouse; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; D018631:ras Proteins