Expression of the active form of MMP-2 on the surface of leukemic cells accounts for their in vitro invasion.

The enhanced expression of matrix metalloproteinases (MMP), especially gelatinases MMP-2 and MMP-9, has been associated with the invasive behavior of tumor cells. Previously we reported that primary acute myelogenous leukemia blasts and human leukemic cultured KG-1 cells but not HEL cells penetrate a reconstituted basement membrane (Matrigel) in an invasion assay. In this study, we investigated the role of MMP-2 and MMP-9 in in vitro invasion by leukemic cells. We found that both recombinant human tissue inhibitor of metalloproteinase-2 (rhTIMP-2) and anti-MMP-2 antibody inhibit the invasiveness of KG-1 cells in the Matrigel assay (by 76% and 51% respectively), while anti-MMP-9 antibody does not, indicating that MMP-2 but not MMP-9 in involved in the invasive process. KG-1 cells were found to secrete constitutively the latent (but not the activated) forms of both MMP-2 and MMP-9 and, after stimulation with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), higher levels of these pro-MMP. TPA stimulation, however, did not increase the in vitro invasiveness of these cells. Analysis by Western blot and flow cytometry revealed the presence of the activated form of MMP-2 (64 kDa) on the surface of KG-1 cells and primary AML blasts, as well as MT-MMP in the homogenates of these cells. This active form of MMP-2 was not detected on the surface of HEL cells, which were non-invasive in vitro, although these cells secreted pro-MMP-2. In conclusion, leukemic KG-1 and primary acute myelogenous leukemia cells, which secrete pro-MMP-2 and pro-MMP-9, were also shown to express the activated form of MMP-2 on their cell surface. We suggest that this active form is essential to the in vitro invasion of leukemic cells.