Melanoma chondroitin sulphate proteoglycan (MCSP) is a cell-surface antigen that has been implicated in the growth and invasion of melanoma tumours. Although this antigen is expressed early in melanoma progression, its biological function is unknown. MCSP can stimulate the integrin-α4β1-mediated adhesion and spreading of melanoma cells. Here we show that stimulated MCSP recruits tyrosine-phosphorylated p130cas, an adaptor protein important in tumour cell motility and invasion. MCSP stimulation also results in a pronounced activation and recruitment of the Rho-family GTPase Cdc42. MCSP-induced spreading of melanoma cells is dependent upon active Cdc42, a Cdc42-associated tyrosine kinase (Ack-1) and tyrosine phosphorylation of p130cas. Furthermore, vectors inhibiting Ack-1 or Cdc42 expression and/or function abrogate MCSP-induced tyrosine phosphorylation and recruitment of P130cas. Our findings indicate that MCSP may modify tumour growth or invasion by a unique signal-transduction pathway that links Cdc42 activation to downstream tyrosine phosphorylation and subsequent cytoskeletal reorganization.
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ACKNOWLEDGEMENTS We thank Y. Shimizu for valuable discussions. This work was supported by grant CA21463 from the NIH (to L.T.F.). L.T.F. is a recipient of an Allen-Pardee professorship in cancer biology. Correspondence and requests for materials should be addressed to J.B.M.