SELECTIVE LOCALIZATION OF MATRIX METALLOPROTEINASE-9, BETA(1) INTEGRINS, AND HUMAN LYMPHOCYTE ANTIGEN CLASS-I MOLECULES ON MEMBRANE-VESICLES SHED BY 8701-BC BREAST-CARCINOMA CELLS

The shedding of membrane vesicles from the cell surface is a vital pro cess considered to he involved in cell-cell and cell-matrix interactio ns and in tumor progression, By immunoelectron microscopic analysis of surface replicas of 8701-BC human breast carcinoma cells, we observed that membrane vesicles shed from plasma membranes contained densely c lustered gelatinase B [matrix metalloproteinase 9 (MMP-9)], beta(1) in tegrins, and human lymphocyte antigen class I molecules. By contrast, alpha-folate receptor was uniformly distributed on the smooth cell mem brane and shedding areas, Both cell surface clustering of selected mol ecules and membrane vesicle release were evident only when cells sere cultured in the presence of serum. Vesicle shedding occurred preferent ially at the edge or along narrow protrusions of the cell, Specific ac cumulation of proMMP-9 and active forms of MMP-9 in shed vesicles was also demonstrated by gelatin zymography, In addition, Western blotting analysis showed the presence of a large amount of proMMP-9/tissue inh ibitor of metalloproteinase 1 complex. The release of selected areas o f plasma membranes enriched with MMP-9 and beta(1) integrins indicates that membrane vesicle shedding from tumor cells plays an important ro le in the directional proteolysis of the extracellular matrix during c ellular migration. The presence of human lymphocyte antigen class I an tigens suggests a mechanism for tumor cells to escape from immune surv eillance.