The ADAMs (a disintegrin and metalloprotease) comprise a large family of multidomain proteins with cellbinding and metalloprotease activities. The ADAM12 cysteine-rich domain (rADAM12-cys) supports cell attachment using syndecan-4 as a primary cell surface receptor that subsequently triggers ␤ 1 integrin-dependent cell spreading, stress fiber assembly, and focal adhesion formation. This process contrasts with cell adhesion on fibronectin, which is integrin-initiated but syndecan-4-dependent. In

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... the present study, we investigated ADAM12/syndecan-4 signaling leading to cell spreading and stress fiber formation. We demonstrate that syndecan-4, when present in significant amounts, promotes ␤ 1 integrin-dependent cell spreading and stress fiber formation in response to rADAM12-cys. A mutant form of syndecan-4 deficient in protein kinase C (PKC)␣ activation or a different member of the syndecan family, syndecan-2, was unable to promote cell spreading. GF109203X and Gö 6976, inhibitors of PKC, completely inhibited ADAM12/syndecan-4-induced cell spreading. Expression of syndecan-4, but not syn4⌬I, resulted in the accumulation of activated ␤ 1 integrins at the cell periphery in Chinese hamster ovary ␤1 cells as revealed by 12G10 staining. Further, expression of myristoylated, constitutively active PKC␣ resulted in ␤ 1 integrin-dependent cell spreading, but additional activation of RhoA was required to induce stress fiber formation. In summary, these data provide novel insights into syndecan-4 signaling. Syndecan-4 can promote cell spreading in a ␤ 1 integrin-dependent fashion through PKC␣ and RhoA, and PKC␣ and RhoA likely function in separate pathways. 1 The abbreviations used are: PKC, protein kinase C; mAb, monoclonal antibody; ADAM, a disintegrin and metalloprotease; GAG, glycosaminoglycan; EGFP, enhanced green fluorescent protein; FITC, fluorescein isothiocyanate; TRITC, tetramethylrhodamine isothiocyanate; CHO, chinese hamster ovary cells; CNF1, cytotoxic necrotizing factor 1; rADAM12-cys, recombinant ADAM12 cysteine-rich domain.