The transcription factor hypoxia-inducible factor 1 alpha (HIF-1 alpha) is regulated by oxygen availability as well as various inflammatory mediators, including tumor necrosis factor alpha (TNF alpha). Early work suggested that the phosphatidylinositol-3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways are involved in TNF alpha-mediated HIF-1 alpha accumulation and activation under normoxic conditions. Here, we provide evidence showing that I kappa B kinase beta (IKK beta) is required for HIF-1 alpha regulation by TNF alpha. We found that TNF alpha enhances HIF-1 alpha protein expression in various breast cancer cell lines under either normoxic or hypoxia-mimicking conditions, but has little effect on the HIF-1 alpha mRNA level. Increased HIF-1 alpha expression was found in IKK beta stable clones and transient transfectants, and depletion of IKK beta consistently reduced the amount of HIF-1 alpha protein. Treatment of cells with the IKK beta inhibitor Bay 11-7082 reduced the TNF alpha-induced HIF-1 alpha expression, suggesting that IKK beta is required in this signaling pathway. Decreased expression of vascular endothelial growth factor (VEGF), a direct target of HIF-1 alpha, was shown in IKK beta-knockout mouse embryonic fibroblast cells. We further demonstrated a positive correlation between IKK beta and VEGF expression in primary human breast cancer specimens. Our findings indicate that TNF alpha-induced HIF-1 alpha accumulation is IKK beta dependent, and may enable further understanding of the HIF-1 alpha regulation by inflammatory signals. (C) 2009 Elsevier Inc. All rights reserved.