Targeted Inhibition Of Hedgehog (Hh) Signal Transduction In Rhabdomyosarcoma (Rms) Reduces Cell Survival In Vitro And Tumor Growth In Vivo

RMS is the most common sarcoma of childhood. Approximately 60% of embryonal RMS (ERMS) and more rarely alveolar RMS (ARMS) express HH pathway components. However, roles for the HH signal transduction pathway in RMS biology and as a therapeutic target in RMS have been incompletely characterized. Therefore, we determined if RMS cells lines have HH signaling pathways that are responsive to pathway activation or inhibition in vitro. We demonstrated expression of HH pathway genes (Indian HH, Desert HH, PTCH1, SMO, GLI1, GLI2, and GLI3) by qRT PCR in ERMS (Rh18, Ruch2, SMS-CTR, and RD) and ARMS (Rh 30 and Rh41) cells. We showed high GLI1 protein expression, the downstream mediator of HH signaling, by Western blot in Rh18, Rh30, and Rh41 cells but not in RD or Rh28 cells. Exposure of Rh41 cells to Sonic HH-containing conditioned media caused a 12-fold increase in PTCH1 expression and exposure of Rh28 cells to Sonic HH peptide increased GLI1 expression 1.5-fold, suggesting intact HH signal transduction in some RMS cell lines. Down-regulation of HH pathway activity with the GLI1 inhibitor GANT61 reduced Rh41 and RD cell viability by MTT assays in a dose dependent manner. Reduced cell viability appeared to result at least in part from down-regulation of the anti-apoptotic GLI1 target gene, BCL2. These results suggest a role for HH signaling in RMS cell survival. Next, we used subcutaneous xenografts (with either Rh30 cells, n = 4; Rh41cells, n = 3; or RD cells, n = 3) to assess whether the HH pathway represents a potentially important therapeutic target in RMS in vivo. Mice were treated with either cyclopamine (subcutaneous SMO inhibitor), GANT61 (subcutaneous GLI1 inhibitor), or Vismodegib (oral SMO inhibitor) for 16 days once the tumor volume was 250 mm3. The study endpoints were HH pathway inhibition (qRT PCR), tumor volume (mm3), proliferation (BrdU immunostain), and apoptosis (Caspase immunostain) on day 17 of treatment. Cyclopamine and GANT61 reduced GLI1, PTCH1, and BCL2 expression in 7/10 and 4/10 mice, respectively. We observed partial responses to cyclopamine and GANT61 in Rh41 xenograft mice; GANT61 caused significant regression in tumor volume from baseline (p = 0.03). We did not detect changes in proliferation or apoptosis post-therapy by immunohistochemistry. Our results suggest that targeted inhibition of the HH signaling pathway with small-molecule inhibitors sometimes inhibits RMS xenograft tumor growth in vivo and support further RMS xenograft studies, assessing HH pathway inhibitors in combination with chemotherapeutic agents. Citation Format: Joon Won Yoon, Christopher Chandler, Marilyn Lamm, King-Fu Leong, Stephen Iannaccone, Gregory Taborn, Philip Iannaccone, David Walterhouse. Targeted inhibition of Hedgehog (HH) signal transduction in rhabdomyosarcoma (RMS) reduces cell survival in vitro and tumor growth in vivo. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3245. doi:10.1158/1538-7445.AM2015-3245