Background: In the U.S., the incidence and mortality rates of prostate cancer (CaP) for African American (AA) men are about 1.5 and 2.3 times higher compared to Caucasian American (CA) men, respectively. CaP is diagnosed at an earlier age and is more aggressive in AA compared to CA patients. The causes for these differences are multifactorial, but include genetic effects that contribute to CaP-associated health disparity. We previously showed that in androgen-dependent CaP cells, the androgen receptor (AR) suppresses levels of the receptor tyrosine kinase ErbB3, a molecule that is known to drive CaP progression, by stimulating the E3 ubiquitin ligase Nrdp1 (also called RNF41 or FLRF). The purpose of the current study was to investigate the role of the AR-Nrdp1-ErbB3 signaling axis in CaP development and progression in AA vs CA men with CaP.

Methods: All data were collected with approval from the University of California Davis (UCD) or VA Northern California Health Care System (VANCHCS) Institutional Review Board (IRB). Sections from prostate tumors of 157 patients who underwent radical retropubic prostatectomy at UCD (79) or VANCHCS (78) were analyzed for these studies. Tumor and nontumor areas were identified by a pathologist and 60-µm core samples were extracted from the specific areas of the donor blocks. The specimens were arranged in triplicate in a tissue microarray (TMA) using a Beecher Instruments Manual Tissue Arrayer. Hematoxylin-eosin staining was used as a reference for interpreting the additional sections of the TMA stained with antibodies to Nrdp1 and AR.

Results: Nrdp1 has two major proteins isoforms of 317 amino acids (36kDa) and 246 amino acids (28kDa)--both isoforms are expressed in CaP. Subcellular fractionation and immunofluorescence staining of various CaP cell lines showed that while the 36 kDa Nrdp1 was localized to both the cytoplasm and the nucleus, the 28 kDa was localized exclusively in the cytoplasm. Hence, we investigated the expression of Nrdp1 in primary prostate tissues from 157 individual patients, including 19 AA, 121 non-Hispanic CA, 5 Hispanic CA, and 11 others. Using a scoring system based on immunohistochemistry (IHC) scores from 0 to 3, where 0 represents no staining and 3 represents 100% staining, we observed that Nrdp1 was strongly expressed in the epithelial cells of the prostate and could be observed in both the nucleus and the cytoplasm. Comparison of cytoplasmic Nrdp1 levels showed no difference in expression between the different racial groups; however, nuclear Nrdp1 levels differed significantly between races (P = 0.008), with post-hoc testing showing significantly higher expression in CA patients than in AA patients (P = 0.002). Therefore, it is likely that the difference is in the 36-kDa fragment and not the 28-kDa fragment. Further, increased preoperative PSA is associated with significantly higher nuclear Nrdp1 levels (P = 0.030), with a Spearman correlation of 0.19. Finally, we demonstrated that nuclear Nrdp1 levels are significantly higher in subjects without acetylsalicylic acid (aspirin, ASA) use (P = 0.001).

Conclusions: Since PSA levels are considered to be a measure of AR transcriptional activity, the correlation between nuclear Nrdp1 and preoperative PSA is supportive of our previous report showing that Nrdp1 is a direct transcriptional target of the AR. Since preoperative PSA is indicative of advanced disease, these results suggest that one reason for higher incidence of CaP and higher CaP progression among AA patients compared to CA is due to the lower levels of nuclear Nrdp1 in the former. Further, since aspirin use increased the levels of nuclear Nrdp1, these data indicate that the use of aspirin may increase nuclear Nrdp1 levels, thereby decreasing the risk of advanced CaP and perhaps reducing CaP disparities between these groups.

Citation Format: Salma Siddiqui, Frank U. Melgoza, Blythe P. Durbin-Johnson, Ruth L. Vinall, Paramita M. Ghosh. Decreased nuclear expression of the E3 ubiquitin ligase Nrdp1 in African American men compared to Caucasian men with localized prostate cancer [abstract]. In: Proceedings of the Tenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2017 Sep 25-28; Atlanta, GA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2018;27(7 Suppl):Abstract nr A83.