Identification of integrin α_Mβ₂ as an adhesion receptor on peripheral blood monocytes for Cyr61 (CCN1) and connective tissue growth factor (CCN2): Immediate-early gene products expressed in atherosclerotic lesions

Cysteine-rich 61 (Cyr61, CCN1) and connective tissue growth factor (CTGF, CCN2) are growth factor-inducible immediate-early gene products found in blood vessel walls and healing cutaneous wounds. We previously reported that the adhesion of endothelial cells, platelets, and fibroblasts to these extracellular matrix-associated proteins is mediated through integrin receptors. In this study, we demonstrated that both Cyr61 and CTGF are expressed in advanced atherosclerotic lesions of apolipoprotein E-deficient mice. Because monocyte adhesion and transmigration are important for atherosclerosis, wound healing, and inflammation, we examined the interaction of THP-1 monocytic cells and isolated peripheral blood monocytes with Cyr61 and CTGF. THP-1 cells and monocytes adhered to Cyr61- or CTGFcoated wells in an activation-dependent manner and this process was mediated primarily through integrin α_Mβ₂. Additionally, expression of α_Mβ₂ on human embryonic kidney 293 cells resulted in enhanced cell adhesion to Cyr61. Consistent with these data, a GST-fusion protein containing the I domain of the integrin α_M subunit bound specifically to immobilized Cyr61 or CTGF. We have also investigated the requirement of cell surface heparan sulfate proteoglycans (HSPGs) as coreceptors for monocyte adhesion to Cyr61. Pretreatment of monocytes with heparin or heparinase I resulted in partial inhibition of cell adhesion to Cyr61. However, monocytes, but not fibroblasts, were capable of adhering to a Cyr61 mutant deficient in heparin binding activity. Collectively, these results show that activated monocytes adhere to Cyr61 and CTGF through integrin αMβ₂ and cell surface HSPGs. However, unlike fibroblast adhesion to Cyr61, cell surface HSPGs are not absolutely required for this adhesion process.