Identification of a novel 82 kDa proMMP-9 species associated with the surface of leukemic cells: (auto-)catalytic activation and resistance to inhibition by TIMP-1
Résumé
Matrix metalloproteinase 9 (MMP-9) plays a critical role in tumor progression. While the biochemical properties of the secreted form of proMMP-9 are well characterized, little is known about the function and activity of cell surface-associated proMMP-9. We purified a novel 82-kDa species of proMMP-9 from the plasma membrane of THP-1 leukemic cells, featuring substantial differences when compared to the secreted 94-kDa proMMP-9. The 82-kDa form was not detected in the medium even upon stimulation with a phorbol ester. It is truncated by 9 amino acid residues at its N-terminus, lacks O-linked oligosaccharides present in 94-kDa proMMP-9, but retains N-linked carbohydrates. Incubation of 94-kDa proMMP-9 with MMP-3 generated the well-known 82-kDa active form, but the 82-kDa proMMP-9 was converted to an active species of 35 kDa, which was also produced by autocatalytical processing in the absence of activating enzymes. The activated 35-kDa MMP-9 efficiently degraded gelatins, native collagen type IV, and fibronectin. The enzyme was less sensitive to TIMP-1 inhibition with IC 50} values of 82 nM compared with 1 nM of the 82-kDa active MMP-9. The synthetic MMP inhibitor GM6001 blocked both enzymes with a similar IC 50} value below 1 nM. The 82-kDa proMMP-9 is also produced in HL-60 and NB4 leukemic cell lines as well as ex vivo leukemic blast cells. It is, however, absent in neutrophils and mononuclear cells isolated from peripheral blood of healthy individuals. Thus, 82-kDa proMMP-9 expressed on the surface of malignant cells may escape inhibition by natural TIMP-1 facilitating cellular invasion in vivo.
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