Expression of Metalloproteases by Cardiac Myocytes and Fibroblasts In Vitro

Regulation of the turnover of extracellular matrix (ECM) components has been attributed in part to matrix metalloproteases (MMP). Isolated cardiac myocytes and fibroblasts from different developmental stages express different patterns of MMPs in vitro. Zymography of media and cell extracts of fibroblasts and myocytes indicated several apparent molecular weights (Mr) with gelatinolytic activity with prominent bands at 92 and 72 kDa. No caseinolytic activity was detected. These MMPs were characteristic of known MMP-2 and MMP-9. Fibroblasts predominantly expressed the latent 72-kDa MMP, whereas myocytes expressed a latent 92-kDa MMP. Expression of these MMPs was not affected by density of culture or the type of ECM substrate on which the cells were grown. Sodium dodecyl sulfate (SDS)–activated MMP-2 showed specific cleavage patterns on collagen types I and III but not on fibronectin, collagen type IV, or laminin. The reaction of SDS-activated MMP-2 produced a 140-kDa fragment from collagen types I and III. No specific substrate patterns were observed with activated MMP-9. MMP-2 from fibroblasts could also be activated by mechanical tension developed by fibroblasts within collagen gels or by cyclically stretching Silastic membranes on which the fibroblasts were grown. When mechanical tension was inhibited in collagen gels by antibodies against the β1 integrin, the 72-kDa MMP, or cytochalasin D, the activated band at 62 kDa was not detected. Immunocytochemical localization with antibodies against MMP-2 showed a weak reaction on cardiac myocytes, but intense staining around the focal adhesions of migrating fibroblasts. In collagen gels, staining was localized to the leading pseudopodia of the fibroblasts. Together, these data indicate that the rat MMP-2 is a collagenase primarily associated with cardiac fibroblasts, activated by mechanical tension, and may be important in cellular ECM interactions.