Role of GTPases and GTPase regulatory proteins in oncogenesis.
The GTPases comprise a superfamily of GTP-binding proteins with intrinsic GTPase activity. Some members of this family representing either heterotrimeric or small G-proteins are involved in the transmission of mitogenic signals. Mutations that lead to constitutively activated G-proteins have been shown to contribute to malignant transformation. These genes represent, therefore, putative oncogenes. Examples are the gsp and gip2 oncogenes, encoding GTPase deficient alpha-subnits of Gs or Gi-2 proteins. Representatives from the family of small G-proteins are the products of the Harvey-, Kirsten- or N-ras oncogenes. These oncogenes, which are frequently expressed in human malignancies, code for proteins (p21ras) that are locked in the activated GTP-bound state because their GTPase is refractory to the ras-specific GTPase activating protein (GAP). In other cases p21ras-GTP levels have been found to be elevated as a result of an increase in GDP/GTP exchange rate. In neurofibromatosis v. Recklinghausen, a mutated gene (NF1) is detectable. The protein encoded by NF1 contains a GAP homology region, binds p21ras-GTP, and stimulates the hydrolysis of p21ras-bound GTP. Both ras-GAP (p120 GAP) and NF1-GAP are inhibited by acidic lipids. Elevated levels of these lipids may exert growth-stimulatory or perhaps tumor-promoting activity by increasing p21ras GTP. The function of transforming p21ras is under control of tumor suppressor genes. Putative suppressor genes isolated from revertants from ras-transformed cells include rsp-1 and the ras recision gene (rrg). Experimentally, an overexpression of Rap 1A/Krev-1 is able to antagonize transformation by p21ras. This mechanism also may be relevant under normal conditions. p53 also is capable of inhibiting transforming p21ras. It is postulated that p105-RB exerts a similar anti-ras effect. The mechanism by which retinoic acid suppresses transformation by ras is discussed. Current strategies for a pharmacological interference of p21ras function are described.
Grunicke
H H
HH
Institute of Medical Chemistry and Biochemistry, University of Innsbruck, Austria.
Maly
K
K
D000818:Animals; D020558:GTP Phosphohydrolases / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000235:genetics / Q000502:physiology*; D016147:Genes, Tumor Suppressor; D006801:Humans; D009369:Neoplasms / Q000188:drug therapy / Q000209:etiology*; D009857:Oncogenes
D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins