Inhibition of γ-glutamyl transpeptidase activity at the surface of human myeloid cells is correlated with macrophage maturation and transforming growth factor β production

The protease γ-glutamyl transpeptidase (γ-GT) activity was detected at the surface of human blood granulocytes and monocytes and myeloblastic HL-60 and monoblastic U937 leukemia cell lines using an enzymatic assay (cleavage of γ-glu-p-nitroanilide and inhibition by the specific irreversible inhibitor of γ-GT, i.e., acivicin). Flow cytometric analysis of γ-GT expression and detection of a 2.4-kb γ-GT mRNA species by Northern blot analysis confirmed the presence of γ-GT in cells of the monocytic- granulocytic lineage. Differentiation of HL-60, U937 cells, and blood monocytes along the macrophage pathway or granulocytic maturation of HL-60 cells was accompanied by an increase in γ-GT mRNA levels without modulation of cell surface γ-GT activity and protein. When added to leukemic cell cultures, acivicin produced a dose- and time-dependent inhibitory growth effect associated with the induction of morphological features characteristic of macrophage maturation and enhanced surface expression of phenotypic markers CD11b and CD71 characteristic of monocyte development. When cultured in the presence of acivicin, freshly isolated monocytes also underwent characteristic changes in morphology and antigenic phenotype (increase in CD71 and HLA-DR class II) consistent with their differentiation into macrophages. In parallel, a marked production of latent transforming growth factor (TGF)-β was observed in supernatants of cells cultured with acivicin, although TGF-β1 mRNA species were expressed in these cells at a level almost similar to that in unstimulated cell cultures. Moreover, acivicin-treated cells still differentiated into macrophages in the presence of a neutralizing antibody to TGF-β1/β2. Together, these data indicate that human myeloid precursor cells express a functional γ-GT; inhibition of proteolytic activity is associated with induction of macrophage maturation and TGF-β1 production.