Antibody-induced shedding of CD44 from adherent cells is linked to the assembly of the cytoskeleton

CD44 is a widely expressed integral membrane glycoprotein that serves as a specific adhesion receptor for the extracellular matrix glycosaminoglycan h yaluronan. CD44 participates in a variety of physiological and pathological processes through its role in cell adhesion. Under appropriate conditions, the ectodomain of CD44 is proteolytically removed from the cell surface. I n this study we show that excessive CD44 shedding can be induced in mouse f ibroblasts and monocytes upon exposure of these cells to a CD44-specific Ab immobilized on plastic, whereas treatment with phorbol ester induces signi ficantly enhanced CD44 release from the monocytes only. CD44 shedding proce eds normally in fibroblasts and monocytes deficient in TNF-a converting enz yme (TACE), a sheddase involved in the processing of several substrates. Co nversely, activation of the CD44 protease has no effect on the release of T NF-alpha from TACE-expressing cells, although the same metalloprotease inhi bitor effectively blocks both TACE and the CD44 sheddase. Concomitant with anti-CD44 Ab- or phorbol ester-induced CD44 shedding, dramatic changes are observed in cell morphology and the structure of the actin cytoskeleton. Di sruption of actin assembly with cytochalasin reduces CD44 shedding, but not the release of TNF-alpha. Moreover, pharmacological activation of Rho fami ly GTPases Rac1 and Cdc42, which regulate actin filament assembly into dist inct cytoskeletal structures, has a profound effect on CD44 release. We con clude that the CD44 sheddase and TACE are distinct enzymes, and that Ab- an d phorbol ester-enhanced cleavage of CD44 is controlled in a cell type-depe ndent fashion by Rho GTPases through the cytoskeleton.