Bacterial lipopolysaccharide directly induces angiogenesis through TRAF6-mediated activation of NF-kappaB and c-Jun N-terminal kinase

The intracellular pathways by which inflammatory mediators transmit their angiogenic signals is not well studied. The effects of a potent inflammatory mediator, bacterial lipopolysaccharide (LPS), are transmitted through Toll-like receptors (TLRs). A major, although not exclusive, LPS/TLR intracellular signaling pathway is routed through TNF (tumor necrosis factor) receptor associated factor 6 (TRAF6). In this report we demonstrate that LPS directly stimulates endothelial sprouting in vitro. By blocking TRAF6 activity using retroviral expression of a dominant-negative TRAF6 in endothelial cells, we show that TRAF6 is absolutely required for the LPS-initiated angiogenic response in vitro and in vivo. Inhibition of either c-Jun N-terminal kinase (JNK) activity or nuclear factor kappaB (NF-kappaB) activity, downstream of TRAF6, is sufficient to inhibit LPS-induced endothelial sprouting. In contrast, only inhibition of NF-kappaB, but not JNK, activity blocks basic fibroblast growth factor (bFGF)-induced angiogenesis. Our findings thus demonstrate a direct endothelial-stimulatory role of LPS in initiating angiogenesis through activation of TRAF6-dependent signaling pathways.