Latent MMP-9 is bound to TIMP-1 before secretion

Expression patterns of matrix metalloproteinase-9 (MMP-9) and its specific inhibitor, tissue inhibitor of metalloproteinases-1 (TIMP-1), are closely correlated with physiological and pathological processes characterized by the degradation and accumulation of the extracellular matrix (ECM). Both, activated MMP-9 and pro-MMP-9 can bind to TIMP-1, and most cell types secrete MMP-9 in complex with TIMP-1. Utilizing immunofluorescence, we observed intracellular co-localization of MMP-9 and TIMP-1 in stimulated human fibrosarcoma cells. In the present study we searched for the origin of the complex formation between the latent enzyme and its specific inhibitor on a subcellular level. Fluorescence resonance energy transfer (FRET) between the fluorescently labeled enzyme and its inhibitor in co-transfected cells were measured. MMP-9 and TIMP-1 were fused to cyan (CFP) and yellow (YFP) variants of the green fluorescent protein and transiently expressed in human hepatoma cells. The intracellular distribution of fluorescently labeled TIMP-1 and MMP-9 was analyzed by confocal laser scanning microscopy. Intracellular complex formation in the Golgi apparatus was verified, demonstrating FRET between MMP-9-CFP and TIMP-1-YFP. Our data provide evidence that the proMMP-9-TIMP-1 complex is already present in the Golgi apparatus. This may be of significance for a number of intracellular and extracellular biochemical processes involving proMMP-9. However, the magnitude and functional relevance of this finding remain unknown.