The gp160 human kidney differentiation antigen is identical to human aminopeptidase A (APA), a zinc-dependent cell-surface metallopeptidase which hydrolyzes peptides with N-terminal acidic residues. GP160/APA is constitutively expressed by proximal tubule cells, the normal cellular counterpart of most renal cancers (RCs). Immunohistochemical analysis of gp160/APA protein expression in 62 primary renal tumor specimens using monoclonal antibody S4 revealed heterogeneous or homogeneous expression of gp160/APA in 46/51 (90%) of clear cell carcinomas in contrast with 1/8 (13%) papillary renal tumors and 0/3 oncocytomas (p<0.001). Analysis of five primary clear cell carcinomas for gp160 protein expression immunohistochemically and associated APA catalytic activity revealed one tumor which expressed gp160/APA protein which was enzymatically inactive. Direct sequence analysis of DNA derived from this specimen could not detect mutations within the zinc-binding domain which would eliminate gp160/APA catalytic activity. These data indicate that the gp160/APA protein is expressed by primary clear cell but not papillary RCs or oncocytomas, and that alterations in gp160/APA protein including loss of protein expression or enzymatic activity occur in 20% of primary clear cell RCs.

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