Recent studies have revealed an unexpected synergism between two seemingly unrelated protein families: CCN matricellular proteins and the tumor necrosis factor (TNF) family of cytokines. CCN proteins are dynamically expressed at sites of injury repair and inflammation, where TNF cytokines are also expressed. Although TNFalpha is an apoptotic inducer in some cancer cells, it activates NFkappaB to promote survival and proliferation in normal cells, and its cytotoxicity requires inhibition of de novo protein synthesis or NFkappaB signaling. The presence of CCN1, CCN2, or CCN3 overrides this requirement and unmasks the apoptotic potential of TNFalpha, thus converting TNFalpha from a proliferation-promoting protein into an apoptotic inducer. These CCN proteins also enhance the cytotoxicity of other TNF cytokines, including LTalpha, FasL, and TRAIL. Mechanistically, CCNs function through integrin alpha(6)beta(1) and the heparan sulfate proteoglycan (HSPG) syndecan-4 to induce reactive oxygen species (ROS) accumulation, which is essential for apoptotic synergism. Mutant CCN1 proteins defective for binding alpha(6)beta(1)-HSPGs are unable to induce ROS or apoptotic synergism with TNF cytokines. Further, knockin mice that express an alpha(6)beta(1)-HSPG-binding defective CCN1 are blunted in TNFalpha- and Fas-mediated apoptosis, indicating that CCN1 is a physiologic regulator of these processes. These findings implicate CCN proteins as contextual regulators of the inflammatory response by dictating or enhancing the cytotoxicity of TNFalpha and related cytokines.