Abstract 2970: Reduced levels of both SNF2L and its isoform SNF2LT mediate the absence of a DNA damage response and resulting apoptosis in the quiescent lymphovascular emboli of inflammatory breast cancer

Previous studies with a human xenograft model of inflammatory breast cancer, MARY-X, demonstrated that overexpressed E-cadherin moderated the formation of the lymphovascular embolus in vivo and tumoral spheroids of super-high density in vitro. In vitro, cells in MARY-X spheroids grow very slowly and can not be cultured as attached monolayers despite the use of numerous cellular and extracellular matrix substrates. Our previous data showed that exogenous growth factors could not stimulate the growth of the spheroids in vitro. The spheroid state itself ultimately may be indirectly responsible for the resistance of the spheroids to chemotherapy since the cells within the spheroids are not actively dividing. However the spheroids also resist apoptosis and there may be other mechanisms in play responsible for their quiescent state. Our laboratory has been studying genes whose expression inversely correlates with E-cadherin expression and observed that both SNF2L and its truncated isoform, SNF2LT, chromatin-remodeling ATPases, are expressed at very low to absent levels in MARY-X and its derived spheroids. Both SNF2L and its truncated isoform, SNF2LT, are nearly ubiquitously expressed in most malignant and normal cell lines, but our previous studies have shown that only malignant cell lines are sensitive to their separate knockdowns. Single knockdowns of either SNF2L or SNF2LT caused DNA damage, a DNA damage response, cell cycle arrest and apoptosis in a number of highly malignant cell lines including Hela, the MDA-MB-468 and −231. Both single and dual knockdown similarly caused DNA damage measured by COMET as well as by increased p-H2AX. However single v dual knockdown had opposite effects on the DNA damage response, cell cycle arrest and apoptosis. Single knockdown of either SNF2L or SNF2LT increased phosphorylated p53 and its downstream genes, GADD45A, 14-3-3σ and p21 whereas double knockdown had no effect. Single knockdown caused a G 2 /M arrest with upregulation of p-cdc2 whereas double knockdown had no effect. Single knockdown caused profound apoptosis with increased Apaf-1/caspase 9 whereas double knockdown actually increased cellular proliferation. The results suggest that the ratio of SNF2L/SNF2LT determines how a cancer cell responds to DNA damage, either by initiating a damage response which leads to apoptosis or ignoring the DNA damage and continuing to proliferate. Since both SNF2L and SNF2LT are expressed at extremely low levels in MARY-X and its derived spheroids, this mimics the situation of dual knockdown in the other cell lines and hence would be permissive to the occurrence of DNA damage but not a concomitant DNA damage response nor the induction of apoptosis. This quiescent state then contributes to subsequent tumor progression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2970.