Abstract 31: Activated phosphatidylinositol-3 kinase: An oncogene that increases gap junctional, intercellular communication

Abstract

Abstract Gap junctions are channels that connect the cytoplasm of adjacent cells. Gap junctional, intercellular communication (GJIC) is blocked in cells transformed by oncogenes such as the middle Tumor antigen of polyoma virus (mT), an oncoprotein which associates with and is tyrosine-phosphorylated by cSrc family members. Specific phosphotyrosines provide docking sites for the phosphotyrosine binding domain of Shc (mT-tyr250) and the SH2 domain of the phosphatidylinositol 3-kinase (mT-tyr315), resulting in the activation of their downstream signaling cascades, Ras/Raf/Erk and PI-3 kinase/Akt, respectively. To examine the effect of these mT-initiated pathways upon gap junctional communication and neoplasia, GJIC was assessed in mT-mutant-expressing, rat liver epithelial T51B cells which normally have extensive GJIC, using a novel technique of in situ electroporation we developed. The results show that although low levels of wt-mT are sufficient to interrupt gap junctional communication, GJIC suppression still requires an intact tyr250 site, that is activation of the Ras pathway. In sharp contrast, activation of the PI-3 kinase pathway is not required for GJIC suppression. It is remarkable that T51B cells expressing a mutant deficient in binding of Shc, hence activation of the Ras pathway, are still morphologically transformed and do grow into tumors in syngeneic rats, albeit with an altered morphology. These results indicate that GJIC suppression requires Ras but not PI-3 kinase/Akt signalling, and is independent of full neoplastic conversion in rat liver epithelial cells. Since PI3k activation by mT increases GJIC, we examined whether PI3k might have a positive role upon GJIC. The results showed that PI3k pharmacological inhibition eliminates GJIC, concomitant with apoptosis induction. We next expressed a form of PI3k rendered constitutively active through the addition of a myristylation signal (myr-PI3k). The results demonstrated that myr-PI3k causes an increase in GJIC. Therefore PI3k, although in an activated form it can act as an oncogene, it plays a positive role upon gap junctional, intercellular communication. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 31. doi:1538-7445.AM2012-31