Abstract
A new generation of antibodies against the prostate specific membrane antigen (PSMA) has been proven to bind specifically to PSMA molecules on the surface of living prostate cancer cells. To explore the potential of anti-PSMA antibodies as targeting moieties for macromolecular therapeutics for prostate cancer, fluorescently labeled HPMA (N-(2-hydroxypropyl)methacrylamide) copolymer-anti-PSMA antibody conjugates (P-anti-PSMA) were synthesized and the mechanisms of their endocytosis and subcellular trafficking in C4-2 prostate cancer cells were studied. Radioimmunoassays showed the dissociation constants of P-anti-PSMA for C4-2 prostate cancer cells in the low nanomolar range, close to values for free anti-PSMA. It indicated that conjugation of anti-PSMA to HPMA copolymers did not compromise their binding affinity. The rate of endocytosis of P-anti-PSMA was much faster than that of control HPMA copolymer conjugates containing nonspecific IgG. Selective pathway inhibitors of clathrin-mediated endocytosis and of macropinocytosis inhibited the internalization of P-anti-PSMA. Inhibition of clathrin-mediated endocytosis was further evidenced by down-regulation of clathrin heavy chain expression by siRNA. Using a dominant-negative mutant of dynamin (Dyn K44A) to abolish the clathrin-, caveolae-independent endocytic pathway, we found that some of P-anti-PSMA adopted this pathway to be endocytosed into C4-2 cells. Thus multiple receptor-mediated endocytic pathways, including clathrin-mediated endocytosis, macropinocytosis, and clathrin-, caveolae-independent endocytosis, were involved in the internalization of P-anti-PSMA. The extent of the participation of each pathway in P-anti-PSMA endocytosis was estimated. Membrane vesicles containing P-anti-PSMA rapidly colocalized with membrane vesicles overexpressing Rab7, a late endosome localized protein, demonstrating that a part of P-anti-PSMA was transported to late endosomes.