Articular adipose tissue is a ubiquitous component of human joints, and adiponectin is a protein hormone secreted predominantly by differentiated adipocytes and involved in energy homeostasis. We investigated the signaling pathway involved in IL-6 production caused by adiponectin in both rheumatoid arthritis synovial fibroblasts and osteoarthritis synovial fibroblasts. Rheumatoid arthritis synovial fibroblasts and osteoarthritis synovial fibroblasts expressed the AdipoR1 and AdipoR2 isoforms of the adiponectin receptor. Adiponectin caused concentration- and time-dependent increases in IL-6 production. Adiponectin-mediated IL-6 production was attenuated by AdipoR1 and 5′-AMP-activated protein kinase (AMPK)α1 small interference RNA. Pretreatment with AMPK inhibitor (araA and compound C), p38 inhibitor (SB203580), NF-κB inhibitor, IκB protease inhibitor, and NF-κB inhibitor peptide also inhibited the potentiating action of adiponectin. Adiponectin increased the kinase activity and phosphorylation of AMPK and p38. Stimulation of synovial fibroblasts with adiponectin activated IκB kinase α/β (IKK α/β), IκBα phosphorylation, IκBα degradation, p65 phosphorylation at Ser (276), p65 and p50 translocation from the cytosol to the nucleus, and κB-luciferase activity. Adiponectin-mediated an increase of IKK α/β activity, κB-luciferase activity, and p65 and p50 binding to the NF-κB element and was inhibited by compound C, SB203580 and AdipoR1 small interference RNA. Our results suggest that adiponectin increased IL-6 production in synovial fibroblasts via the AdipoR1 receptor/AMPK/p38/IKKαβ and NF-κB signaling pathway.

Angiogenesis, the growth of new blood vessels, is essential in the pathogenesis of joint inflammatory disorders such as rheumatoid arthritis (RA) and osteoarthritis (OA), facilitating the invasion of inflammatory cells and increase in local pain receptors that contribute to structural damage and pain. The angiogenic process is perpetuated by various mediators such as growth factors, primarily vascular endothelial growth factor (VEGF) and hypoxia-inducible factors (HIFs), as well as proinflammatory cytokines, various chemokines, matrix components, cell adhesion molecules, proteases, and others. Despite the development of potent, well-tolerated nonbiologic (conventional) and biologic disease-modifying agents that have greatly improved outcomes for patients with RA, many remain resistant to these therapies, are only partial responders, or cannot tolerate biologics. The only approved therapies for OA include symptom-modifying agents, such as analgesics, non-steroidal anti-inflammatory drugs (NSAIDs), steroids, and hyaluronic acid. None of the available treatments slow the disease progression, restore the original structure or enable a return to function of the damaged joint. Moreover, a number of safety concerns surround current therapies for RA and OA. New treatments are needed that not only target inflamed joints and control articular inflammation in RA and OA, but also selectively inhibit synovial angiogenesis, while preventing healthy tissue damage. This narrative review of the literature in PubMed focuses on the evidence illustrating the therapeutic benefits of modulating angiogenic activity in experimental RA and OA. This evidence points to new treatment targets in these diseases.

Connective tissue growth factor (CTGF, a.k.a. CCN2) is inflammatory mediator and abundantly expressed in osteoarthritis (OA). Angiogenesis is essential for OA progression. Here, we investigated the role of CTGF in vascular endothelial growth factor (VEGF) production and angiogenesis in OA synovial fibroblasts (OASFs). We showed that expression of CTGF and VEGF in synovial fluid were higher in OA patients than in controls. Directly applying CTGF to OASFs increased VEGF production then promoted endothelial progenitor cells tube formation and migration. CTGF induced VEGF by raising miR-210 expression via PI3K, AKT, ERK, and nuclear factor-κB (NF-κB)/ELK1 pathways. CTGF-mediating miR-210 upregulation repressed glycerol-3-phosphate dehydrogenase 1-like (GPD1L) expression and PHD activity and subsequently promoted hypoxia-inducible factor (HIF)-1α-dependent VEGF expression. Knockdown of CTGF decreased VEGF expression and abolished OASF-conditional medium-mediated angiogenesis in vitro as well as angiogenesis in chick chorioallantoic membrane and Matrigel-plug nude mice model in vivo. Taken together, our results suggest CTGF activates PI3K, AKT, ERK, and NF-κB/ELK1 pathway, leading to the upregulation of miR-210, contributing to inhibit GPD1L expression and prolyl hydroxylases 2 activity, promoting HIF-1α-dependent VEGF expression and angiogenesis in human synovial fibroblasts.

Chemokines modulate angiogenesis and metastasis that dictate cancer development in tumor microenvironment. Osteosarcoma is the most frequent bone tumor and is characterized by a high metastatic potential. Chemokine CCL5 (previously called RANTES) has been reported to facilitate tumor progression and metastasis. However, the crosstalk between chemokine CCL5 and vascular endothelial growth factor (VEGF) as well as tumor angiogenesis in human osteosarcoma microenvironment has not been well explored. In this study, we found that CCL5 increased VEGF expression and production in human osteosarcoma cells. The conditioned medium (CM) from CCL5-treated osteosarcoma cells significantly induced tube formation and migration of human endothelial progenitor cells. Pretreatment of cells with CCR5 antibody or transfection with CCR5 specific siRNA blocked CCL5-induced VEGF expression and angiogenesis. CCL5/CCR5 axis demonstrably activated protein kinase Cδ (PKCδ), c-Src and hypoxia-inducible factor-1 alpha (HIF-1α) signaling cascades to induce VEGF-dependent angiogenesis. Furthermore, knockdown of CCL5 suppressed VEGF expression and attenuated osteosarcoma CM-induced angiogenesis in vitro and in vivo. CCL5 knockdown dramatically abolished tumor growth and angiogenesis in the osteosarcoma xenograft animal model. Importantly, we demonstrated that the expression of CCL5 and VEGF were correlated with tumor stage according the immunohistochemistry analysis of human osteosarcoma tissues. Taken together, our findings provide evidence that CCL5/CCR5 axis promotes VEGF-dependent tumor angiogenesis in human osteosarcoma microenvironment through PKCδ/c-Src/HIF-1α signaling pathway. CCL5 may represent a potential therapeutic target against human osteosarcoma.