Abstract B05: Immunoproteomic profiling in African American men with prostate cancer: Evidence for an autoimmune response to glycolytic enzymes

Prostate cancer (PCa) is the most frequently diagnosed cancer and the second leading cause of cancer-related male deaths in the U.S. This cancer is more aggressive in African American (AA) men, who are twice as likely to die from the disease when compared to other racial groups. Despite these racial disparities in PCa mortality, studies on PCa biology and biomarker discovery include mostly patients from European American (EA) backgrounds. There is a critical need to find minimally invasive PCa biomarkers that could aid in diagnosis and prognosis, especially in high risk populations like AA men. Immunoseroproteomics offers a minimally invasive approach to detect early malignant processes that trigger production of autoantibodies to tumor-associated antigens (TAA). These antibodies serve as “sensors” of tumorigenic events and can be assessed as potential biomarkers relevant to tumor biology. In this study we used this approach to interrogate anti-TAA autoantibody signatures in AA and EA men with PCa and explored racial differences in tumor immunobiology that could underlie PCa mortality disparities. We used serum samples from AA (n=59) and EA (n=50) men with PCa to probe one- and two-dimensional Western blots (WB) of protein lysates from aggressive PCa cell lines. AA PCa patient sera showed stronger immunoreactivity to proteins in PC3 cell lysates compared to the EA sera, with a 50 kD protein band frequently targeted by autoantibodies in several AA PCa sera. Serological proteomic analysis (SERPA) with mass spectrometry showed that these AA PCa autoantibodies recognized alpha-enolase (ENO1), known to be important in tumor formation, expansion, and glucose metabolism. In addition, several other glycolytic enzymes important for tumor proliferation were also identified as candidate prostate TAA, including GAPDH, phosphoglycerate kinase, and lactate dehydrogenase. These have been validated in other studies as overexpressed in PCa. The ENO1 autoantibody frequency, measured by ELISA in sera from 400 racially diverse men with and without PCa, was four-fold higher in PCa compared to non-PCa patients. Interestingly, although sera from AA men with PCa had stronger WB reactivity to ENO1 present in PC3 lysates compared to EA men, and the ENO1 autoantibodies were initially discovered in eight AA PCa sera but only in two EA sera, the anti-ENO1 autoantibody frequency measured by ELISA, which used E. coli-purified recombinant human ENO1 as a substrate, was higher in the EA PCa cohort. In addition, several anti-ENO1 positive sera from AA men with PCa showed increased immunoreactivity to this protein in the PCa cell lines LNCaP, MDA-PC2b, PC3, and DU145, but not against the purified ENO1 used in ELISA. The opposite was true in the EA PCa cohort, where strong reactivity against the purified ENO1 did not correlate with an equally strong immunoreactivity against the cellular ENO1. The EA PCa sera did not recognize the pattern of ENO1 expression observed with the AA sera in the same panel of prostate cell lines. These differences of immunoreactivity between AA and EA sera against PC3-ENO1 vs bacterially purified human ENO1 suggest that select AA sera may predominantly recognize ENO1 epitopes not displayed in the purified protein. A recent study found higher frequency of autoantibodies to phosphorylated ENO1 in pancreatic cancer sera compared to normal patient sera. Our own proteomic comparison of PC3-ENO1 and recombinant ENO1 yielded differences in phosphorylated, acetylated, and methylated amino acids. Taken together, these data suggest the anti-ENO1 autoantibodies from AA men with PCa target a modified form of ENO1 that may not be strongly recognized by EA men with PCa. These discrepancies point to racial differences in the immune response to prostate tumors. Citation Format: Tino Wilson Sanchez, Jitian Li, Liping Dai, Saied Mirshahidi, Guangyu Zhang, Nathan Wall, Colwick Wilson, Susanne Montgomery, Jianying Zhang, Carlos Casiano. Immunoproteomic profiling in African American men with prostate cancer: Evidence for an autoimmune response to glycolytic enzymes. [abstract]. In: Proceedings of the Eighth AACR Conference on The Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; Nov 13-16, 2015; Atlanta, GA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2016;25(3 Suppl):Abstract nr B05.