Regulation of aminopeptidase N (EC 3.4.11.2; APN; CD13) by interferon-γ on the HL-60 cell line

Membrane-bound peptidases play important roles in the regulation of local concentrations of various signalling peptides such as the growth factors, hormones, chemokines and cytokines. That is accomplished by means of their enzyme activity. Recently, membrane-bound peptidases have also been shown to act as receptors, receiving signals from as yet undefined ligands and transducing them into the cell interior. By using either or both of these mechanisms, peptidases interact with fundamental cellular functions: growth, differentiation, activation and death. This study addressed the effects of a T-cell derived cytokine, interferon-gamma (IFN-γ) on the activity of aminopeptidase N (APN), an ectoenzyme processing several signal peptides. Cells of a myelo-monocytic cell line HL-60 were used as a model system, and APN was assayed at the levels of mRNA, its membrane marker CD13, and the enzyme activity. Regulation of CD13/APN by IFN-γ was found at all three levels. The direction of regulation was time-dependent: an initial down-regulation seen 24 and 48 hrs after the onset of treatment with IFN-γ was replaced by an up-regulation after 72 and/or 96 hrs. Up-regulation of CD13/APN observed after 96 hrs was preceded by an up-regulation of APN mRNA reaching its maximum after 72 hrs. The IFN-γ-induced regulation of APN was due to membrane aminopeptidase N, since it could be completely abrogated by an APN blocking antibody WM-15. The delayed up-regulation of CD13/APN (observed after 72 and/or 96 hrs), required de novo protein synthesis as it could be abrogated by cycloheximide, an inhibitor of protein synthesis. Possible role of endogenous (IFN-γ-induced) TGF-β in mediating CD13/APN up-regulation could be excluded, since no TGF-β was found in supernatants of IFN-γ treated HL-60 cells. Thus, our data show regulation of CD13/APN on cells of myelo-monocytic origin by a T-cell derived cytokine, IFN-γ. A similar mechanism might play a role in inflammation.