AGGREGATION OF MONONUCLEATED PRECURSORS TRIGGERS CELL-SURFACE EXPRESSION OF ALPHA-V-BETA-3 INTEGRIN, ESSENTIAL TO FORMATION OF OSTEOCLAST-LIKE MULTINUCLEATED CELLS

alpha upsilon beta 3 is a key integrin mediating adhesion of multinucl eated osteoclasts during bone resorption. 1,25-dihydroxyvitamin D3 upr egulates alpha upsilon beta 3 integrin expression in mononucleated ost eoclast precursors and concomitantly stimulates their differentiation into osteoclasts. This suggests that this integrin could play a major role during osteoclast differentiation,We have developed an in vitro m odel, in which 1,25-dihydroxyvitamin D3 sequentially modifies the beha vior of macrophages: it first induces rounding up of these cells, then their subsequent aggregation and spreading, which finally leads to ce ll fusion and the formation of osteoclast-like multinucleated giant ce lls, We show that, while 1,25-dihydroxyvitamin D3 stimulates the de no vo synthesis of alpha upsilon beta 3 in macrophages early in this proc ess, its accumulation on the surface is triggered by cell aggregation, A high level of integrin alpha upsilon beta 3 cell surface expression correlates with macrophage spreading preceding fusion, This was confi rmed by means of novel cell permeable peptides containing the C-termin al sequence of the integrin beta 3 tail to specifically block alpha up silon beta 3 function. Although this peptide has no effect on the aggr egation step, it disrupts the spreading of osteoclast precursors and c onsequently inhibits their fusion, These findings suggest a novel role of the integrin alpha upsilon beta 3 in a discrete step of osteoclast differentiation.