Abstract 264: Differential expression of HPRT in prostate cancer leads to investigation of its ADCC effects

Abstract

Abstract As 1 in 41 American men will die of prostate cancer, this study proposes hypoxanthine- guanine phosphoribosyl transferase (HPRT) as another therapeutic target in the repertoire of prostate cancer immunotherapeutic targets. We evaluated the HPRT enzyme because of its role in the purine salvage pathway, nucleotide synthesis, and cell cycle progression. To evaluate the role of HPRT in prostate cancer, PC3 and DU145 prostate cancer cells were used because of their markedly different levels of HPRT expression. Flow cytometry and scanning electron microscopy showed the surface localization of HPRT while immunohistochemistry revealed HPRT upregulation within tissue. We consistently found a significant association between HPRT and the plasma membrane of DU145 cells, but found no HPRT presence on the surface of 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 on DU145 cells (p = 0.0004). To confirm this finding, we used gold conjugated antibodies to determine the distribution of HPRT across the membrane with an electron microscope. This analysis further supported the surface presence of HPRT on DU145 cells as the gold weight percentage of DU145 cells increased significantly when exposed to HPRT antibodies (p < 0.0001). In addition to its surface expression on DU145 cells, elevated levels of HPRT were found in 47% of prostate patient tissue samples compared to healthy controls (n = 25), while 53% of patients had no HPRT upregulation (n = 28). Additionally, RNA-seq data from The Cancer Genome Atlas (TCGA) was used to evaluate general HPRT levels in patients with prostate cancer (n = 502) and healthy individuals (n = 52). This data revealed a significant (p < 0.0001) increase in HPRT levels upon malignancy. While some patients’ levels were consistent with healthy control levels, there was a significant number of patients with increased protein expression upon cancer development. Because of the consistent and notable presence of HPRT on the surface of DU145 cells and in the tissue of prostate cancer patients, we began to investigate HPRT as a therapeutic target. Through MTS assays and live cell imaging, we assessed the potential of HPRT antibodies in an ADCC format. We observed increased DU145 cell death in wells treated with HPRT antibody when compared to wells treated with an isotype antibody and untreated wells (p < 0.01 and p < 0.0001, respectively). Furthermore, the amount of PC3 cell death in HPRT treated wells was insignificant when compared to isotype wells and untreated wells (p > 0.05). These results strongly indicate a unique relationship between prostate cancer cells and HPRT and supports the use of an HPRT antibody to harness the patient’s immune system in treating subsets of prostate cancer. Citation Format: Michelle H. Townsend, Kelsey A. Bennion, Zac E. Ence, Eliza E. Bitter, Abi M. Felsted, John E. Lattin, McKay D. Reese, Stephen R. Piccolo, Kim L. O'Neill. Differential expression of HPRT in prostate cancer leads to investigation of its ADCC effects [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 264.