[26, 56, 57] Emerging evidence suggests that fluid and serum VEGF levels not only are elevated in RA patients with hypoxic conditions but also by pro-inflammatory cytokines IL-1 and TNF-α.[58] Currently, VEGF and its receptors are the best characterized system in the angiogenesis Talazoparib in vitro regulation of rheumatoid joints. The VEGFRs on EC membranes consist of the tyrosine kinases VEGFR-1 (Flt-1), VEGFR-2 (Flk-1/KDR) and VEGFR-3 (Flt-4). KDR is a main mediator of angiogenic,

mitogenic and permeability-enhancing effects of VEGF. Moreover, KDR is up-regulated in response to hypoxia, a main inducer of VEGF gene transcription.[59] It is demonstrated that hypoxia stimulated VEGF-A (the most important member of the VEGF family) and VEGFR-1 expression decrease VEGFR-2 levels in ECs. During hypoxic conditions, plasma membrane VEGFR-1 levels are elevated, while VEGFR-2 levels are depleted. One functional consequence of hypoxia is a decrease in VEGF-A-stimulated and VEGFR-2-regulated intracellular signaling along with lowered EC NOS activation. In addition,

the capillary, arterial and venous ECs subjected to hypoxia display a decreased cell migration in response to VEGF-A. A mechanistic elucidation is that VEGFR-1/VEGFR-2 ratio is substantially increased during hypoxia to obstruct VEGF-A-stimulated and VEGFR-2 regulated endothelial responses to magnify cell recovery and viability.[60] In another study, Eubank et al. in 2011 showed that hypoxia can selectively stimulate anti-angiogenic molecule expression in mononuclear

phagocytes in a granulocyte macrophage colony-stimulating factor (GM-CSF) GSK-3 beta phosphorylation enriched environment. The soluble VEGFR-1 (sVEGFR-1) is one of these molecules that act as a negative regulator for VEGF activity through VEGFR-2. Therefore, anti-angiogenic molecules can effect proliferation, migration and survival of ECs.[61] Placenta growth factor is another angiogenic factor and highly homologous with VEGF. PLGF can exert its angiogenic Alanine-glyoxylate transaminase effect by synergizing with VEGF. However, it does not have an effect on lymphatic vessel functionality.[62, 63] Furthermore, PLGF can promote the production of VEGF from monocytes and macrophages.[64] It has been recently reported that PLGF is highly expressed in synovial tissue and enhances the production of pro-inflammatory cytokines, including TNF-α and IL-6.[65] Oncostatin M (OSM) belongs to the IL-6 subfamily and is mostly produced by T lymphocytes. High levels of OSM are detected in the pannus of RA patients and it may rise the inflammatory responses in joints and eventually lead to bone erosion.[65] OSM promotes angiogenesis and EC migration, and potentiates the effects of IL-1β in promoting extracellular matrix turnover and human cartilage degradation.[66] It was also demonstrated that OSM increased messenger RNA (mRNA) and protein levels of PLGF in a time- and concentration-dependent manner in RA synovial fibroblasts.