Macrophages secrete matrix metalloproteinase 9 covalently linked to the core protein of chondroitin sulphate proteoglycans.

Matrix metalloproteinases (MMPs) secreted from the leukemic macrophage cell-line THP-1 have been investigated. Under serum-free conditions, this cell-line synthesizes and secretes proMMP-9, which was detected in the culture medium as a monomer of 92 kDa, and in dimeric forms, including a homodimer of approximately 225 kDa. In addition, a new heterodimer complex is described, in which proMMP-9 is covalently linked to the core protein of chondroitin sulphate proteoglycan (CSPG) through one or more disulphide bridges. After SDS-PAGE electrophoresis, at least two forms of this complex were detected, a large form in the stacking gel and a smaller form with an estimated size of 300 kDa. When the CS chains were removed by chondroitin ABC lyase treatment, heterodimers of proMMP-9/CSPG core protein of approximately 145, 127 and 109 kDa were found, based on zymography and Western blots. Since as much as 10-15 % of the total proMMP-9 secreted from THP-1 cells was covalently linked to CSPG, this association may have important implications for transport, targetting and regulation of the enzyme activity.