growth hormone, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), cAMP, vitamin D3, estrogen and tamoxifen, angiotensin II, hypoxia, UV light, and mechanical stretch. Protein structure and function Structural domains Full-length CYR61 protein contains 381 amino acids with an N-terminal secretory signal peptide followed by four structurally distinct domains. The four CYR61 domains are, from N- to C-termini, the insulin-like growth factor binding protein (IGFBP) domain, von Willebrand type C repeats (vWC) domain, thrombospondin type 1 repeat domain (TSR), and the C-terminal (CT) domain that contains a cysteine-knot motif. CCN1 has unusually high cysteine residue content (10% or 38 in total). The number and spacing of cysteine residues are completely conserved among CYR61 (CCN1), CTGF (CCN2), NOV (CCN3), and WISP-1 (CCN4), and are largely conserved with WISP-2 (CCN5), which lacks precisely the CT domain, and WISP3 (CCN6), which lacks 4 cysteines in the vWC domain. CYR61 is glycosylated, although the regulation and function of glycosylation are unknown. Integrin binding CYR61 binds directly to various integrin receptors in a cell type-dependent manner, including integrin αvβ3 in endothelial cells, α6β1 and heparan sulfate proteoglycans (HSPGs) in fibroblasts and smooth muscle cells αIIbβ3 in activated platelets, αMβ2 in monocytes and macrophages, and αDβ2 in macrophage foam cells. Where examined, syndecan-4 has been identified as the HSPG critical for CCN1 functions. The CYR61 binding sites for some of these integrins have been mapped (Figure 1). Due to the cell type specificity of integrin expression, CYR61 acts through distinct integrins to mediate specific functions in different types of cells. For example, CYR61 induces angiogenic functions in endothelial cells through αvβ3, and in fibroblasts promotes cellular senescence and enables TNFα to induce apoptosis through binding to α6β1-HSPGs. However, CYR61 supports cell adhesion through all of the integrins identified above. Cell signaling and function As a cell adhesive substrate, CYR61 induces the activation of focal adhesion kinase, paxillin, RAC, and sustained activation of MAPK/ERK1-2. In macrophages, CYR61 also activates the transcription factor NFκB and stimulates M1 polarization. CYR61 activates Akt signaling in thymic epithelial cells, promoting their proliferation and thus thymic size growth. CYR61 has potent angiogenic activity upon endothelial cells and induces neovascularization, first demonstrated in a corneal micropocket implant assay and subsequently confirmed in a rabbit ischemic hindlimb model. CYR61 also accelerates and promotes the chondrogenic differentiation of mouse limb bud mesenchymal cells, and stimulates osteoblast differentiation but inhibits osteoclastogenesis. Cyr61 is a strong inducer of reactive oxygen