CD44 is a
cell surface receptor for several extracellular matrix components and is implicated in
tumor cell invasion and
metastasis. Our previous studies have shown that CD44 expressed in
cancer cells is proteolytically cleaved at the extracellular domain through membrane-associated
metalloproteases and that CD44 cleavage plays a critical role in CD44-mediated
tumor cell migration (Okamoto, I., Kawano, Y., Tsuiki, H., Sasaki, J., Nakao, M., Matsumoto, M., Suga, M., Ando, M., Nakajima, M., and Saya, H. (1999) Oncogene 18, 1435-1446). In the present study, we first demonstrate rapid degradation of the membrane-tethered CD44 cleavage product through intracellular proteolytic pathways, and it occurs only after CD44 extracellular cleavage. To address the mechanisms regulating CD44 cleavage at the extracellular domain, we show that 12-O-tetradecanoylphorbol 13-acetate (TPA) and the
calcium ionophore ionomycin rapidly enhance
metalloprotease-mediated CD44 cleavage in U251MG cells via
protein kinase C-dependent and -independent pathways, respectively, suggesting the existence of multiple distinct pathways for regulation of CD44 cleavage. Concomitant with TPA-induced CD44 cleavage, TPA treatment induces redistribution of CD44 and ERM
proteins (
ezrin,
radixin, and
moesin) to newly generated membrane ruffling areas. Treatment with
lysophosphatidic acid, which is known to activate the Rho-dependent pathway, inhibits TPA-induced CD44 redistribution and CD44 cleavage. Furthermore, overexpression of Rac dominant active mutants results in the redistribution of CD44 to the Rac-induced ruffling areas and the enhancement of CD44 cleavage. These results suggest that the Rho family
proteins play a role in regulation of CD44 distribution and cleavage.