P-Akt/SNF5 signaling interactions in rhabdoid tumors

4544 Chromatin remodeling factors are important regulators of cell growth and differentiation. Alterations in these factors have been shown to lead to the development of human cancers. Expression of SNF5/Baf47, a member of the SWI/SNF (mating type switch/sucrose nonfermenting) complex, is lost in more than 80% of malignant rhabdoid tumors (MRT). Our lab previously demonstrated that re-introduction of SNF5 into A204 and TM87-16 MRT cell lines induces growth arrest. In A204 cells, SNF5 interacts directly at the p16 and p21 promoters to up-regulate expression. However, re-introduction of SNF5 does not induce a growth arrest in all SNF5 null rhabdoid tumor cell lines, including G401. Therefore we are investigating whether the SNF5 induced p21 response is functional in these cells. SNF5 does induce p21 expression in G401. However, transiently transfected p21 predominantly localizes in the cytoplasm of G401 as compared to solely localizing in the nucleus of A204. Because P-Akt has been shown to phosphorylate p21, causing its exclusion from the nucleus, we characterized Akt expression in these rhabdoid cell lines. We discovered high levels of P-Akt expression in G401 and TTC709 as compared to A204 and TM87-16 cells. As the growth inhibitory activity of p21 is associated with nuclear localization, we hypothesized that high levels of P-Akt and altered p21 localization may be responsible for impaired SNF5 sensitivity. In support of this model, we found that wild type p21 does not induce as great a growth arrest in G401 or TTC709 as compared to A204 or TM87-16 cells. In contrast, a p21 mutant incapable of being phosphorylated by P-Akt, induces a growth arrest in G401 and TTC709 similar to that found in A204 and TM87-16 cells. However, as p21 has some inhibitory effect on G401 and TTC709 cells, impaired p21 activity may not be solely responsible for lack of SNF5 induced growth arrest. We also generated stable control and dominant negative DN-Akt expressing G401 clones. We found DN-Akt G401 clones exhibit a 50% decrease in cellular proliferation and colony formation when transfected with SNF5 as compared to control. Therefore, blocking P-Akt allows SNF5 to induce a partial growth arrest in G401. These studies provide a more comprehensive approach to inhibiting rhabdoid tumor cell growth. Moreover, we also found that P-Akt expression is high in some human primary rhabdoid tumors in vivo. Therefore, this work may give insight into the development of rhabdoid tumors because their cellular origin remains unknown.