Cysteine-rich angiogenic inducer 61 (CYR61) or CCN family member 1 (CCN1), is a
matricellular protein that in humans is encoded by the CYR61gene.[5]
CYR61 is a secreted,
extracellular matrix (ECM)-associated signaling protein of the CCN family (
CCN intercellular signaling protein).[6][7] CYR61 is capable of regulating a broad range of cellular activities, including cell adhesion, migration, proliferation, differentiation,
apoptosis, and
senescence through interaction with cell surface
integrin receptors and heparan sulfate proteoglycans. During embryonic development, CYR61 is critical for cardiac septal morphogenesis, blood vessel formation in
placenta, and vascular integrity. In adulthood CYR61 plays important roles in
inflammation and tissue repair, and is associated with diseases related to chronic inflammation, including
rheumatoid arthritis,
atherosclerosis,
diabetes-related
nephropathy and
retinopathy, and many different forms of cancers.
CCN protein family
CYR61 was first identified as a protein encoded by a serum-inducible gene in mouse
fibroblasts.[6][8] Other highly conserved homologs were later identified to comprise the CCN protein family (CCN intercellular signaling protein).[9][10][11] The CCN acronym is derived from the first three members of the family identified, namely CYR61 (CCN1),
CTGF (connective tissue growth factor, or CCN2), and
NOV (nephroblastoma overexpressed, or CCN3). These proteins, together with
WISP1 (CCN4),
WISP2 (CCN5), and
WISP3 (CCN6) comprise the six members of the family in vertebrates and have been renamed CCN1-6 in order of their discovery by international consensus.[12] CCN proteins function as
matricellular proteins, which are
extracellular matrix proteins that play regulatory roles, particularly in the context of
wound repair.[13]
Full-length CYR61 protein contains 381 amino acids with an
N-terminal secretory signal peptide followed by four structurally distinct domains.[17] 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,[18] α6β1 and
heparan sulfate proteoglycans (HSPGs) in fibroblasts and smooth muscle cells[19][20] αIIbβ3 in activated platelets,[21] αMβ2 in
monocytes and
macrophages,[22][23] and αDβ2 in macrophage foam cells.[24] Where examined, syndecan-4 has been identified as the HSPG critical for CCN1 functions.[25][26] 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,[27] and in fibroblasts promotes
cellular senescence and enables
TNFα to induce
apoptosis through binding to α6β1-HSPGs.[26][28] 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.[29] In macrophages, CYR61 also activates the transcription factor NFκB and stimulates M1 polarization.[23] CYR61 activates Akt signaling in thymic epithelial cells, promoting their proliferation and thus thymic size growth.[30] CYR61 has potent angiogenic activity upon endothelial cells and induces neovascularization, first demonstrated in a corneal micropocket implant assay[31] and subsequently confirmed in a rabbit ischemic hindlimb model.[32] CYR61 also accelerates and promotes the chondrogenic differentiation of mouse limb bud mesenchymal cells,[33] and stimulates osteoblast differentiation but inhibits osteoclastogenesis.[34][35][36] Cyr61 is a strong inducer of reactive oxygen species accumulation in fibroblastic cells, and this activity underlies many CYR61-induced apoptosis and senescence.[26][28] CYR61 is able to support
cell adhesion, stimulate
cell migration, promote growth factor-induced cell proliferation and differentiation in some cell types, promote apoptosis in synergy with TNF family cytokines, and induce cellular senescence in fibroblasts.
Embryonic development
During embryo development in mice, Cyr61 is highly expressed in the cardiovascular, skeletal, and neuronal systems.[37][38] Cyr61
knockout mice are embryonic lethal due to defects in cardiac septal morphogenesis, deficient blood vessel formation in placenta, and compromised vascular integrity.[39][40] In Xenopus laevis, Cyr61 is required for normal
gastrulation and modulation of
Wnt signaling.[41]
Clinical relevance
CYR61 is highly expressed at sites of inflammation and wound repair, and is associated with diseases involving chronic inflammation and tissue injury.[7]
Wound healing and fibrosis
In skin
wound healing, CYR61 is highly expressed in the
granulation tissue by
myofibroblasts, which proliferate and rapidly synthesize
ECM to maintain tissue integrity and to promote regeneration of parenchymal cells.[42][43] However, excessive matrix deposition can lead to fibrosis, scarring, and loss of tissue function. In skin wounds, CYR61 accumulates in the granulation tissue as myofibroblasts proliferate, and eventually reaches a sufficiently high level to drive the myofibroblasts themselves into senescence, whereupon these cells cease to proliferate and express matrix-degrading enzymes.[28] Thus, CYR61 limits synthesis and deposition of ECM by myofibroblasts, reducing the risk of fibrosis during wound healing.[44] In addition to skin wound healing, CYR61 expression is elevated in remodeling cardiomyocytes after
myocardial infarction,[45] in vascular injury,[20] and in the long bones during fracture repair.[46][47] Blockade of CYR61 by antibodies inhibits bone fracture healing in mice.[48] In the kidney, CYR61 is expressed in podocytes in normal adult and embryonic glomeruli, but expression is decreased in
IgA nephropathy,
diabetic nephropathy, and
membranous nephropathy, particularly in diseased kidneys with severe mesangial expansion.[49] CYR61 induction of
cellular senescence in the kidney is a potential therapy to limit fibrosis.[50]
Inflammation
CYR61 promotes the apoptotic functions of inflammatory cytokines such as
TNFα,
FasL, and
TRAIL.[26][51][52] It also reprograms macrophages towards M1 polarization through αMβ2-mediated activation of NF-κB.[23] CYR61 is upregulated in patients with
Crohn's disease and
ulcerative colitis.[53] CYR61 supports the patrolling behavior of murine resident Ly6Clow monocytes along the endothelial in the steady state and is required for their accumulation under viral-mimicking vascular inflammation.[54]
Arthritis
CYR61 is highly expressed in collagen-induced arthritis in rodents, and inhibition of CCN1 expression correlates with suppression of inflammatory arthritis.[55] CYR61 is also found in articular cartilage from patients with osteoarthritis and appears to suppress ADAMTS4 (aggrecanase) activity, possibly leading to cartilage cell (chondrocyte) cloning.[56]
Vascular diseases
CYR61 is overexpressed in vascular smooth muscle cells of
atherosclerotic lesions and in the
neointima of
restenosis after
balloon angioplasty, both in rodent models and in humans.[20][22][57][58] Suppression of CYR61 expression results in reduced
neointimal hyperplasia after balloon angioplasty, an effect that is reversed by delivery of CYR61 via gene transfer[59][60] In a mouse model of oxygen-induced retinopathy, expression of CYR61 in the vitreous humor produced significant beneficial effects in repairing damaged vasculature.[61]
However, CYR61 can also induce apoptosis and cellular senescence,[25][28][77] two well-established mechanisms of tumor suppression[78] Thus, whereas CYR61 can promote the proliferation of prostate cancer cells, it can also exacerbate apoptosis of these cells in the presence of the immune surveillance molecule TRAIL.[52][64][79] CYR61 has an inhibitory effect on some cancers, and suppresses tumor growth of
non-small-cell lung cancer (NSCLC) cells,[80]endometrial adenocarcinoma cells,[81] and in
melanoma cells.[82]
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