Abstract LB-111: Comprehensive proteomic analysis of salivary gland cancer subtypes

Abstract

Abstract Background- Salivary gland cancer (SGC) is a highly heterogenous disease with distinct histological and pathological features. Although subtypes of salivary gland cancer have been extensively characterised based on histological features, very little is known at the proteomic level, as to how and what key proteins and signaling pathways are differentially activated. Currently, there are no approved targeted therapies for SGCs, although there appears to be benefit of androgen receptor (AR) and Her2 blockade in a small subset of molecularly defined patients (<5% overall). To discover other potential therapeutic targets, we performed an RPPA analysis using paired normal and tumor samples. Method- Reverse phase protein array (RPPA) analysis was performed to measure 195 total and/or phosphorylated proteins in matched normal and tumor samples from 75 SGC patients, including 16 salivary duct carcinomas (SDC); 14 mucoepidermoid carcinomas (MEC); 30 acinic cell carcinomas (ACC) and 15 adenocystic carcinomas (ADCC). Differences in protein expression between normal and tumor samples was assessed by paired t test. Results- RPPA analysis showed distinct protein expression and pathway activation between and within subtypes. Using a conservative cutoff to account for multiple testing (FDR <1%, corresponding to p≤0.005), several proteins were found to be differentially altered between normal and tumor tissue. In SDC, proteins regulating the glucose metabolic pathway were found to be upregulated. For example, PKM2 (a key player in the Warburg effect) and other glycolytic proteins (e.g. ACC1, malic enzyme and IDH1) were elevated in SDC tumors; indicating that these tumors may have increased dependency on glycolysis. Increased expression of other potentially druggable targets such as p-MEK, Akt, vascular endothelial growth factor (VEGFR), and AR were also observed. Consistent with other reports, AR was found at higher levels in tumors (vs. normal tissue) in 62.5% of SDC tumors (fold change = 2.33, p>0.001). Interestingly, AR overexpressing tumors also showed increased activation of EGFR (pY1173) compared to those with low AR expression. ACC tumor tissue expressed higher levels of proteins involved in cell cycle, DNA repair and metabolism. ADCC had higher expression of oncoproteins (c-kit and bcl-2); as well as IGF-binding protein 2 (IGFBP2) and its downstream effector proteins. MEC showed increased expression of EGFR. Conclusion- This study provides an insight into the molecular heterogeneity and enrichment of distinct signaling pathways in the different SGC subtypes. The observed changes in protein expression of EGFR, IGFR and PIK3CA could be driven by genetic aberrations, indeed copy number variations and mutations in several genes including EGFR, IGFR and PIK3CA have been observed in SDC. This study has identified proteins and signaling pathways that could potentially be targeted for the treatment of salivary gland cancer patients. Citation Format: Seema Mukherjee, Yoshitsugu Mitani, Robert Cardnell, You Hong Fan, Lixia Diao, Jing Wang, Adel K. El-Naggar, Lauren Averette Byers. Comprehensive proteomic analysis of salivary gland cancer subtypes. [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 LB-111. doi:10.1158/1538-7445.AM2015-LB-111