TETRASPANIN CD9 IS ASSOCIATED WITH VERY LATE-ACTING INTEGRINS IN HUMAN VASCULAR SMOOTH-MUSCLE CELLS AND MODULATES COLLAGEN MATRIX REORGANIZATION

CD9, a member of the tetraspanin family, and very late-acting (VLA) in tegrins are known to associate and form functional units on the surfac e of several cell types. We studied the changes in expression of CD9 a nd beta(1)-integrins (CD29, VLA) in human vascular smooth muscle cells (VSMCs) under in vitro culture conditions mimicking proliferative vas cular diseases. We also investigated possible interactions between CD9 and VLA integrins in VSMCs. We found that CD9 is highly expressed in VSMCs and is subject to modulation, depending on the proliferative/con tractile state of the cells. In the contractile phenotype, the levels of CD9, CD81, another tetraspanin, and CD29 are approximate to 50% of those found in the proliferative phenotype. Coimmunoprecipitation expe riments showed physical association between CD9 and CD29. CD9 was main ly associated with alpha(2) and alpha(3)-integrins (CD49b and c) and a lso with alpha(5)-integrin to a weaker extent. Functionally, the addit ion of anti-CD9 monoclonal antibodies (MoAbs) doubled the extent of co llagen gel contraction mediated by VSMCs, a model for the reorganizati on of the extracellular collagen matrix occurring in the vessel wall. Anti-CD29 MoAbs inhibited gel contraction, but anti-CD9 MoAbs countera cted this inhibitory effect of anti-CD29 MoAbs. Transfection of human CD9 into Chinese hamster ovary cells more than doubled the extent of C hinese hamster ovary cell-mediated collagen gel contraction (130% stim ulation), confirming a role for CD9 in extracellular matrix reorganiza tion. Thus, CD9 seems to be involved in the modulation of VLA integrin -mediated collagen matrix reorganization by VSMCs. These findings sugg est that high CD9 expression is associated with a proliferative state of VSMCs. The role of CD9 could be to modulate the function of VLA int egrins on the surface of VSMCs.