Abstract 2442: EWS/FLI1 activates GLI1 indirectly through GLI2 in Ewing Sarcoma

Abstract

Abstract GLI1 expression in Ewing tumors has been shown to be enhanced by EWS/FLI1 expression in both model transformation systems and in Ewing cell lines. In Ewing cells, GLI1 has been shown to produce significant phenotypic effects consistent with a role in Ewing tumor formation and propagation. Also consistent with an important role is the demonstrated correspondence between the transcriptional target sets of EWS/FLI1 and of GLI1 in Ewing tumor cells. The principal physiologic developmental regulator of GLI1 is the diffusible Sonic Hedgehog ligand which binds the cell surface tumor suppressor Patched with consequent activation of the transmembrane protein Smoothened. In Ewing tumors, the enhancement of GLI1 expression has been shown to be Hedgehog independent and to occur downstream of Smoothened. Indeed, evidence has been published that EWS/FLI1 directly activates the GLI1 promoter. While there is experimental evidence that GLI1 inhibition may be clinically useful in Ewing tumors, preclinical evidence for one inhibitory approach has been disappointing. Interestingly, direct transcriptional activation of GLI1 has not been demonstrated to be of significance in other most other physiologic or pathologic contexts. We speculated that direct transcriptional activation of GLI1 by EWS/FLI1 might be only part of the story in Ewing tumors. Since GLI2 is the most potent physiologic transcriptional activator of GLI1, we undertook to investigate the role of GLI2 in Hedgehog/GLI (HH-GLI) signaling in Ewing Tumors. We found that GLI2 is broadly expressed in Ewing tumors. Based on GLI2 overexpression and on shRNA knockdown of GLI2, we found that GLI2 expression increases GLI1 expression in Ewing cell lines. Via GLI2 knockdown, we have found that GLI2 has biologic effects similar to or greater than GLI1 in Ewing cells. Finally, we demonstrate that EWS/FLI1 enhances nuclear localization of GLI2, suggesting an additional indirect mechanism by which EWS/FLI1 activates HH-GLI signaling and GLI1 expression in Ewing tumors. These data indicate that GLI2 is an active player in HH-GLI pathway activation in Ewing tumors. They also suggest that measures which target GLI2 may have equal or greater efficacy to those aimed at GLI1. Citation Format: Laura Christensen, William A. May. EWS/FLI1 activates GLI1 indirectly through GLI2 in Ewing Sarcoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2442.