ACTIVATION OF MATRIX METALLOPROTEINASE-2 FROM HEPATIC STELLATE CELLS REQUIRES INTERACTIONS WITH HEPATOCYTES

Activation of matrix metalloproteinase (MMP)-2, the 72-kd collagenase IV/gelatinase A, is involved in extracellular matrix remodeling It has been suggested that a membrane-type MMP (MT-MMP-1) and the tissue inh ibitor of metalloproteinase (TIMP)-2 are involved in MMP-2 processing, but the exact mechanism(s) of its activation remains unclear. We have investigated the role of cell-cell cooperation ill the activation of pro-MMP-2 in the liver, using pure cultures and cocultures of hepatocy tes and hepatic stellate cells (HSCs). Northern blot analysis and in s itu hybridization showed that, in both pure and cocultures, HSCs, but not hepatocytes, expressed MMP-2, TIMP-2, and MT-MMP-1 mRNA. Zymograph y analyses revealed the latent form of MMP-2 in medium from 2-day-old pure HSC cultures with higher amounts in medium from hepatocyte/HSC co -cultures. When hepatocytes were added to 10-day-old NSC cultures, the activated form of MMP-2 was detected, concomitantly with the depositi on of all abundant extracellular matrix. Incubation of plasma membrane -enriched fractions from hepatocytes with conditioned medium from pure NSC cultures generated the activated species of MMP-2 (62 and 59 kd). Activation of pro-MMP-2 by hepatocyte membranes was inhibited by EDTA , heat, and trypsin but not by serine proteinase inhibitors. These dat a show that the co-expression of TIMP-2, MMP-2, and MT-MMP-1 by HSCs d oes not lead to secretion of the activated form of MMP-2. Hepatocytes, which do not express MMP-2, TIMP-2, or MT-MMP-1, induce MMP-2 activat ion through a plasma membrane-dependent mechanism(s), thus suggesting that cell-cell interactions are involved in this process in vivo.