N-glycosylation and microtubule integrity are involved in apical targeting of prostate-specific membrane antigen:: implications for immunotherapy

Prostate-specific membrane antigen (PSMA) is an important biomarker expressed in prostate cancer cells with levels proportional to tumor grade. The membrane association and correlation with disease stage portend a promising role for PSMA as an antigenic target for antibody-based therapies. Successful application of such modalities necessitates a detailed knowledge of the subcellular localization and trafficking of target antigen. In this study, we show that PSMA is expressed predominantly in the apical plasma membrane in epithelial cells of the prostate gland and in well-differentiated Madin-Darby canine kidney cells. We show that PSMA is targeted directly to the apical surface and that sorting into appropriate post-Golgi vesicles is dependent upon N-glycosylation of the protein. Integrity of the microtubule cytoskeleton is also essential for delivery and retention of PSMA at the apical plasma membrane domain, as destabilization of microtubules with nocodazole or commonly used chemotherapeutic Vinca alkaloids resulted in the basolateral expression of PSMA and increased the uptake of anti-PSMA antibody from the basolateral domain. These results may have important relevance to PSMA-based immunotherapy and imaging strategies, as prostate cancer cells can maintain a well-differentiated morphology even after metastasis to distal sites. In contrast to antigens on the basolateral surface, apical antigens are separated from the circulation by tight junctions that restrict transport of molecules across the epithelium. Thus, antigens expressed on the apical plasma membrane are not exposed to intravenously administered agents. The ability to reverse the polarity of PSMA from apical to basolateral could have significant implications for the use of PSMA as a therapeutic target.