INTERACTIONS OF MONOCYTIC CELLS WITH HUMAN ENDOTHELIAL-CELLS STIMULATE MONOCYTIC METALLOPROTEINASE PRODUCTION

Monocyte-endothelial cell interactions play an important role in the e arly 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 c ell interactions on monocytic metalloproteinase production were invest igated using an in vitro system, focusing on the role of endothelial c ell secretions and physical contact as effecters in the regulation of monocytic metalloproteinase expression. Human umbilical vein endotheli al cells (HUVECs) and the human monocytic cell line THP-1 were used, a nd changes in the levels of THP-1 metalloproteinase secretion and mRNA were measured. When THP-1 cells were incubated for 18 hours with HUVE C conditioned medium (CM), a four- to eightfold induction of the metal loproteinase MMP-9 was observed at both the mRNA and protein levels; h owever, levels of another metalloproteinase, MMP-2, were unaffected. T he induction of MMP-9 by HUVEC CM was confirmed using freshly isolated human monocytes. A sevenfold increase in MMP-9 levels was observed wi th apically collected HUVEC CM but not with basally collected CM, THP- 1 cells incubated with paraformaldehyde-fixed HUVECs and isolated HUVE C plasma membranes showed an eightfold increase in MMP-9 levels, and m easurements of MMP-9 activity found in THP-1 conditioned medium due to either HUVEC contact or HUVEC CM showed a threefold increase, The mol ecular weight of the endothelial secreted effector molecule(s) was det ermined to be 30 +/- 6 kd, The data show that endothelial cells throug h the release of soluble factors and through direct contact with monoc ytic cells regulate monocytic metalloproteinase production, which has implications for the atherogenic process.