Inhibition of protein synthesis may result in superinduction of short-lived transcripts and has been attributed variably to stabilization of transcripts and/or increased gene transcription. Little is known about the kinetics of these processes and relevant transcriptional elements have not been identified. In this study, we describe superinduction of interleukin 8 (IL-8) mRNA, an important inflammatory mediator, in lung epithelial-like H292 cells and identify the underlying molecular mechanisms and their kinetics. Cycloheximide (CHI, 10 microg/ml), an inhibitor of protein synthesis, maximally increased IL-8 mRNA levels 30-fold in H292 cells. Tumour necrosis factor alpha (TNF-alpha), which induced IL-8 mRNA 3-fold, synergized with CHI causing a 150-fold increase at 6 h. CHI early on increased the stability of IL-8 mRNA (from 40 min in cells cultured with medium to more than 4 h with CHI). CHI also increased transcription as shown by transfection with IL-8 promoter constructs. Truncated and mutated constructs identified NF-kappaB and AP-1 binding sites as primary cis-acting elements in IL-8 gene transcription and IL-8 mRNA superinduction. Electrophoretic mobility shift assays indicated that CHI increased NF-kappaB and prolonged AP-1 DNA-binding activities and that the synergism of TNF-alpha and CHI on IL-8 mRNA expression was paralleled by a further increase of AP-1 DNA-binding activity. This synergism was still noticed when 4 h elapsed between the addition of CHI and that of TNF-alpha. Taken together, our results indicate that CHI interferes with both post-transcriptional and transcriptional repressive mechanisms of IL-8 mRNA expression.