Abstract 3494: Liver X receptor activation represents a novel strategy to block Hedgehog signaling in pancreatic adenocarcinoma and enhance the antitumor effect of gemcitabine
Authors: Rasheed Zeshaan A. , Rajeshkumar NV , Jung Clinton , Huang Ally , Agarwal Jasmin , Oliviero Elizabeth , Rudek Michelle , Zhao Ming , He Ping , DeJesus-Acosta Ana , Laheru Daniel , Maitra Anirban , Parhami Farhad , Matsui William 
Published: August 2012

Background: The Hedgehog (Hh) signaling pathway plays a pathogenic role in many human cancers, including pancreatic adenocarcinoma, and several pathway antagonists have begun clinical testing. All of these novel agents target Smoothened (Smo), a key regulator of Hh signaling, but reported emergence of resistance mutations questions the durability of this approach. Oxysterols, oxidized derivatives of cholesterol, can modulate Hh signaling either through direct interaction with Smo or via cross-talk with liver X receptors (LXR) that serve as their natural receptors. Thus, oxysterols may represent mechanistically novel Hh inhibitors, especially if they involve LXRs. Methods: We examined the effects of LXR activation on the in vivo growth of pancreatic cancer. Nude mice bearing pancreatic cancer xenografts derived from primary clinical specimens were treated with a vehicle control, gemcitabine, the non-steroidal LXR agonist TO901317, or the combination of gemcitabine and TO901317 for 4-6 weeks. The LXR agonist TO901317 was used since it lacks the ability to directly bind to Smo. Tumor volumes were measured to assess response, and LXR and Hh pathway activities were quantified by real-time RT-PCR for Hh (GLI1, PTCH1) and LXR (ABCA1) target gene expression. Human specific primers were used to detect changes in tumor cells, whereas mouse-specific primers were used to study stroma. Intratumoral levels of gemcitabine triphosphate were quantified by LC-MS-MS. Results: Administration of TO901317 alone did not impact tumor growth compared to control treated animals similar to previous findings with Smo antagonists. Treatment with gemcitabine alone decreased tumor growth, However the addition of TO901317 to gemcitabine significantly enhanced this effect to promote tumor regression. In vivo treatment with TO901317 successfully activated LXR in both human tumor cells and murine stromal cells as evidenced by increased expression of ABCA1. Moreover, TO901317 inhibited expression of the Hh targets GLI1 and PTCH1 5- and 2-fold, respectively, in stromal cells and 5-fold in tumor cells. Previous studies (Olive et al, Science, 2009) demonstrated that Hh pathway inhibition improves drug delivery in vivo. Therefore, we quantified intratumoral levels of gemcitabine triphosphate, the active metabolite of gemcitabine, and found that it was significantly elevated in tumors exposed to TO901317 (P&lt;0.03). Conclusions: The LXR agonist TO901317 enhances the cytotoxic effects of gemcitabine to promote tumor regression in a human xenograft model of pancreatic adenocarcinoma. Therefore, LXR agonists, such as TO901317 and oxysterols, may represent a novel strategy to target pathologic Hh signaling within both tumor cells as well as the surrounding stroma.

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 3494. doi:1538-7445.AM2012-3494