Lipopolysaccharide modulates cyclooxygenase-2 transcriptionally and posttranscriptionally in human macrophages independently from endogenous IL-1 beta and TNF-alpha.

The pathogenesis of septicemia can be triggered by LPS, a potent stimulus for PG synthesis. The enzyme cyclooxygenase (COX) is a rate-limiting step in PG production. COX exists as two isoforms: COX-1, which is constitutively expressed in most cell types, and COX-2, which is inducible by LPS and cytokines in a variety of cells. In this study we determined the role of the proinflammatory cytokines IL-1 beta and TNF-alpha released by LPS-stimulated U937 human macrophages in the regulation of COX-2. Macrophages exposed to LPS showed a rapid and sustained expression of COX-2 mRNA and protein for up to 48 h, whereas PGE2 production was notably enhanced only after 12 h. LPS increased COX-2 gene transcription and activation of the transcription factor NF-kappa B in a transient manner. LPS-treated macrophages produced high levels of TNF-alpha and moderate amounts of IL-1 beta protein. However, neutralizing Abs against these cytokines had no effect on COX-2 mRNA and protein expression, nor did they affect the stability of COX-2 mRNA. Interestingly, in the presence of LPS or exogenous IL-1 beta, COX-2 transcripts were stabilized, and actinomycin D inhibited their degradation. Only when LPS or IL-1 beta was removed did COX-2 mRNA decay with a t1/2 of >/=5 h. In contrast, dexamethasone promoted a faster decay of the LPS-induced COX-2 transcripts (t1/2 = 2.5 h). These results clearly demonstrate that LPS can regulate COX-2 at both transcriptional and posttranscriptional levels independently from endogenous IL-1 beta and TNF-alpha in human macrophages.