Expression of angiopoietin-1 in osteoblasts and its inhibition by tumor necrosis factor-alpha and interferon-gamma

Angiogenesis is a crucial component of bone remodeling under both normal and pathophysiological conditions. Among the various mediators that regulate the angiogenic process is the angiopoietin (Ang) family of growth factors. Ang-1 stabilizes new blood vessels by recruiting surrounding mesenchymal cells and promoting their differentiation into vascular smooth muscle cells, whereas Ang-2 is a natural antagonist of Ang-1 and can inhibit angiogenesis. The expression of Ang-1 and Ang-2 in human osteoblasts (hOBs) isolated from rheumatoid arthritis (RA) and osteoarthritis (OA) patients and from healthy individuals has been examined. After incubation in the presence or absence of tumor necrosis factor-alpha (TNF-alpha) and/or interferon-gamma (IFN-gamma), the culture supernatants were assayed for Ang using an enzyme-linked immunosorbent assay. In addition, expression of Ang protein and mRNA was examined using immunohistochemical techniques and quantitative real-time polymerase chain reaction, respectively. It was found that hOBs expressed Ang-1 but not Ang-2 protein, and cultured hOBs from RA and OA patients and from healthy individuals all spontaneously secreted significant amounts of Ang-1 in the absence of any stimulation. Although stimulation with TNF-alpha or IFN-gamma had little or no effect on Ang-1 secretion, costimulation with IFN-gamma plus TNF-alpha dose- and time-dependently diminished secretion of Ang-1 from hOBs. This inhibitory effect was mediated in part by nuclear factor-kappa B via upregulated expression of inducible nitric oxide synthase and enhanced synthesis of nitric oxide. Taken together, these findings suggest that OBs are an important cellular source of Ang-1 and may modulate bone remodeling through regulation of angiogenesis.