Monocyte-endothelial cell interactions play an important role in the early stages of atherosclerosis, and it is hypothesized that regulation of metalloproteinase production by these interactions contributes to this pathological process. The effects of monocytic cell-endothelial cell interactions on monocytic metalloproteinase production were investigated using an in vitro system, focusing on the role of endothelial cell secretions and physical contact as effectors in the regulation of monocytic metalloproteinase expression. Human umbilical vein endothelial cells (HUVECs) and the human monocytic cell line THP-1 were used, and changes in the levels of THP-1 metalloproteinase secretion and mRNA were measured. When THP-1 cells were incubated for 18 hours with HUVEC conditioned medium (CM), a four- to eightfold induction of the metalloproteinase MMP-9 was observed at both the mRNA and protein levels; however, levels of another metalloproteinase, MMP-2, were unaffected. The induction of MMP-9 by HUVEC CM was confirmed using freshly isolated human monocytes. A sevenfold increase in MMP-9 levels was observed with apically collected HUVEC CM but not with basally collected CM. THP-1 cells incubated with paraformaldehyde-fixed HUVECs and isolated HUVEC plasma membranes showed an eightfold increase in MMP-9 levels, and measurements of MMP-9 activity found in THP-1 conditioned medium due to either HUVEC contact or HUVEC CM showed a threefold increase. The molecular weight of the endothelial secreted effector molecule(s) was determined to be 30 +/- 6 kd. The data show that endothelial cells through the release of soluble factors and through direct contact with monocytic cells regulate monocytic metalloproteinase production, which has implications for the atherogenic process.