RASSF1A has been demonstrated to be a tumor suppressor, while RASSF1C is now emerging as a growth promoting protein in breast and lung cancer cells. To further highlight the dual functionality of the RASSF1 gene, we have compared the effects of RASSF1A and RASSF1C on cell proliferation and apoptosis in the presence of TNF-α. Overexpression of RASSF1C in breast and lung cancer cells reduced the effects of TNF-α on cell proliferation, apoptosis, and MST1/2 phosphorylation, while overexpression of RASSF1A had the opposite effect. We also assessed the expression of RASSF1A and RASSF1C in breast and lung tumor and matched normal tissues. We found that RASSF1A mRNA levels are significantly higher than RASSF1C mRNA levels in all normal breast and lung tissues examined. In addition, RASSF1A expression is significantly downregulated in 92% of breast tumors and in 53% of lung tumors. Conversely, RASSF1C was upregulated in 62% of breast tumors and in 47% of lung tumors. Together, these findings suggest that RASSF1C, unlike RASSF1A, is not a tumor suppressor but instead may play a role in stimulating survival in breast and lung cancer cells. 1. Introduction RASSF1A and RASSF1C are the two major isoforms encoded by the RASSF1 gene. RASSF1A is a key tumor suppressor. It is inactivated in the vast majority of human cancers, including breast and lung cancers [1–5]. Recently, we and others demonstrated that the RASSF1C isoform promotes cell proliferation and migration and attenuates apoptosis in breast and lung cancers [6–10]. This suggests that RASSF1C and RASSF1A have opposing effects on cancer cells. This is further supported by recent cohort studies in human lung and pancreatic cancers showing that elevated RASSF1C expression correlates significantly with poor patient survival [11, 12]. To shed more light on this newly emerging concept of RASSF1 dual functionality, we have undertaken studies which compare the impact of the RASSF1A and RASSF1C isoforms on TNF-α-induced apoptosis. RASSF1A is intimately involved in the Hippo and MOAP1/Bax proapoptotic pathways. RASSF1A promotes apoptosis through activation of both MST kinases in the Hippo pathway and activation of Bax in the MOAP/Bax pathway through TNF-α receptor activation. RASSF1A promotes and maintains phosphorylation of MST1, and also activates Bax leading to the induction/activation of downstream of proapoptotic genes such as puma and caspases 3 and 7 genes [13–17]. Like RASSF1A, RASSF1C is able to bind MST1/2 proteins. As such, we reasoned that RASSF1C could potentially interfere with MST1/2 activation and
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