Abstract 562: HPRT surface localization on prostate cancer cells as a biomarker for immunotherapy
Authors: Townsend Michelle H. , Felsted Abigail M. , Cox Taylor P. , Ence Zachary E. , Piccolo Stephen R. , Robison Richard A. , O'Neill Kim 
Published: August 2018

The purpose of this study is to evaluate the expression and possible upregulation of the salvage pathway enzyme hypoxanthine guanine phosphoribosyltransferase (HPRT) in prostate cancer cells to determine if it could serve as a biomarker for prostate cancer diagnosis and treatment. Prostate cancer is the second most lethal cancer in men, and an estimated 26,730 men will die from the disease in 2017. We chose to evaluate the HPRT enzyme due to its involvement in nucleotide synthesis and cell cycle progression. Two prostate cancer cell lines were used for this analysis (PC3 and DU145) along with malignant tissue from 35 patients with prostate carcinoma. The surface localization of HPRT was determined utilizing flow cytometry and scanning electron microscopy, while upregulation within tissue was assessed using immunohistochemistry. Additionally, RNA-seq data from TCGA was used to evaluate general HPRT upregulation in patients with prostate cancer (n = 502) when compared to healthy individuals (n = 52). Throughout our investigation, we found a significant association between HPRT and the plasma membrane of DU145 cells, but found no presence on PC3 cells. Flow cytometry showed insignificant (p = 0.14) changes in fluorescence when PC3 cells were exposed to HPRT antibodies, while there was a significant increase in fluorescence when DU145 cells were treated with fluorescent HPRT antibody (p = 0.0004). To determine the distribution of HPRT across the membrane and ensure the observed expression was not due to cytoplasmic HPRT, gold conjugated antibodies were used for analysis with an electron microscope. The distribution of the gold on the cell surface showed random HPRT binding across the membrane with no patterns of localization. This analysis aided in confirming HPRT surface presence as the gold weight % of DU145 cells increased significantly when exposed to HPRT antibodies (p &lt; 0.0001). In addition to being presented on the surface of DU145 cells, tissue samples revealed variable HPRT expression as approximately 53% of prostate cancer patients evaluated had elevated levels of HPRT compared to normal controls, while 47% of patients had no upregulation. TCGA data revealed a significant (p = 1.53x10-4) increase in HPRT levels upon malignancy. While some patients had levels consistent with healthy controls, there was a significant number of patients with increased protein expression upon cancer development. The control of HPRT expression within these cancer cells has been linked preliminarily to p53 functionality. While PC3 cells are null for p53, DU145 cells have a gain of function (GOF) p53 mutation. GOF p53 mutations are known to influence salvage pathway enzyme expression and is an influential gene in HPRT expression in these cancer cells. These results strongly indicate a unique relationship between prostate cancer cells and HPRT and suggest HPRT as a possible biomarker for the detection and treatment of patients with prostate cancer.

Citation Format: Michelle H. Townsend, Abigail M. Felsted, Taylor P. Cox, Zachary E. Ence, Stephen R. Piccolo, Richard A. Robison, Kim O'Neill. HPRT surface localization on prostate cancer cells as a biomarker for immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 562.