Inflammatory mediators activate the transcriptional complex HIF-1 (hypoxia-inducible factor-1), the key regulator of hypoxia-induced gene expression. Here we report that bacterial LPS (lipopolysaccharide) induces HIF-1alpha mRNA expression and HIF-1alpha protein accumulation in human monocytes as well as in non-differentiated and differentiated cells of the human monocytic cell line THP-1 under normoxic conditions. LPS and hypoxia synergistically activated HIF-1. Whereas LPS increased HIF-1alpha mRNA expression through activation of a NF-kappaB (nuclear factor kappaB) site in the promoter of the HIF-1alpha gene, hypoxia post-translationally stabilized HIF-1alpha protein. HIF-1alpha activation was followed by increased expression of the HIF-1 target gene encoding ADM (adrenomedullin). Knocking down HIF-1alpha by RNA interference significantly decreased ADM expression, which underlines the importance of HIF-1 for the LPS-induced ADM expression in normoxia. Simultaneously with HIF-1 activation, an increase in p44/42 MAPK (mitogen-activated protein kinase) phosphorylation was observed after incubation with LPS. In cells pretreated with the p44/42 MAPK inhibitor PD 98059 or with RNAi (interfering RNA) directed against p44/42 MAPK, LPS-induced HIF-1alpha accumulation and ADM expression were significantly decreased. From these results we conclude that LPS critically involves the p44/42 MAPK and NF-kappaB pathway in the activation of HIF-1, which is an important transcription factor for LPS-induced ADM expression.