Abstract 4128: PDGFR alpha is overexpressed in synovial sarcomas and can mediate rapamycin induced Akt activation
Authors: Ho Alan L. , Barbashina Violetta , Saito Tsuyoshi , Ladanyi Marc , Schwartz Gary K. 
Published: February 2011

Abstract
The clinical activity of rapamycin analogues (which inhibit mTOR complex 1 (TORC1)) in sarcomas has been modest to date. Published reports have demonstrated that Akt activation induced by these therapies is a clinically relevant mode of drug resistance that may be overcome with receptor tyrosine kinase (RTK) inhibitors. We screened the biochemical effects of rapamycin treatment in a panel of 14 sarcoma cell lines of diverse histologies. We found that 6 hours of exposure to 1 nM rapamycin induced Akt phosphorylation in 12 out of the 14 cell lines tested. In a subset of cell lines, including the synovial sarcoma cell line SYO-1, we found that rapamycin induced Akt phosphorylation was dependent upon the addition of exogenous serum. Hence, we hypothesized that growth factor mediated activation of an RTK is critical for rapamycin induced Akt activation in SYO-1 cells. In parallel, we examined our Affymetrix U133A expression profiling data from 137 patient tumors, corresponding to five different sarcoma subtypes (synovial sarcoma, desmoplastic small round cell tumor, alveolar rhabdomyosarcoma, alveolar soft part sarcoma and Ewing sarcoma). We observed that platelet-derived growth factor receptor alpha (PDGFRA) is the most highly overexpressed kinase gene in synovial sarcoma, with 14.5 fold higher levels of PDGFRA relative to the four other sarcomas analyzed (p=10−35). Immunohistochemistry revealed high expression of phosphorylated PDGFRA, PDGFRA, and PDGF-A ligand in synovial sarcoma tumor samples; no PDGFRA kinase (exons 12-16) and transmembrane (exons 18-21) domain mutations were identified in 25 tumors analyzed. We confirmed that SYO-1 cells possess high protein levels of PDGFRA by Western blot, and proceeded to examine the role of this RTK in rapamycin induced Akt activation. We found that exogenous addition of PDGF AB ligand to serum starved SYO-1 cells was sufficient for rapamycin induced Akt phosphorylation. Transfection of siRNA directed at PDGFRA also reduced Akt phosphorylation with rapamycin treatment. Imatinib mesylate, an inhibitor of PDGFR, in combination with rapamycin reduced PDGFRA and Akt phosphorylation and enhanced the anti-proliferative effect observed with either drug alone. The combination of imatinib and rapamycin failed to suppress Akt phosphorylation or enhance anti-tumor effects in other cell lines with lower or absent PDGFRA protein levels, including the synovial sarcoma cell line HS-SY-II. Antibody array analysis of 42 RTKs confirmed that SYO-1 cells possess higher baseline phosphorylated PDGFRA and no detectable phosphorylated IGF-1R relative to the HS-SY-II cell line. Taken together, these experiments suggest that PDGFRA activation is both sufficient and necessary for rapamycin induced Akt phosphorylation in some synovial sarcomas, and serve as a rationale to pursue clinical trials investigating combined TORC1 and PDGFRA inhibition in this disease.
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 4128.