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pubmed:abstractText |
G2A is a heptahelical cell surface protein that has recently been described as a potential tumor suppressor, based on its ability to counteract transformation of pre-B cells and fibroblasts by Bcr-Abl, an oncogenic tyrosine kinase. We have isolated cDNAs encoding G2A in the course of screening libraries for clones that cause oncogenic transformation of NIH3T3 fibroblasts. When expressed at high levels in NIH3T3 cells by retroviral transduction, G2A induced a full range of phenotypes characteristic of oncogenic transformation, including loss of contact inhibition, anchorage-independent survival and proliferation, reduced dependence on serum, and tumorigenicity in mice. When expressed by transfection, G2A greatly enhanced the ability of a weakly oncogenic form of Raf-1 to transform NIH3T3 cells. These results demonstrate that G2A is potently oncogenic both on its own and in cooperation with another oncogene. Expression of G2A in fibroblasts and endothelial cells resulted in changes in cell morphology and cytoskeleton structure that were equivalent to those induced by the G protein subunit Galpha13. Transformation of NIH3T3 cells via G2A expression was completely suppressed by co-expression of LscRGS, a GTPase activating protein that suppresses signaling by Galpha12 and Galpha13. Hyperactivity of Galpha12 or Galpha13 has previously been shown to result in activation of Rho GTPases. G2A expression resulted in activation of Rho, and transformation via G2A was suppressed by a dominant negative form of RhoA. These results indicate that G2A may be directly coupled to Galpha13, and that it is the activation of this Rho-activating Galpha protein which is responsible for the ability of G2A to transform fibroblasts.
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