Activation of both nuclear factor of activated T cells and inhibitor of nuclear factor-κB kinase β-subunit-/nuclear factor-κB is critical for cyclooxygenase-2 induction by benzo[a]pyrene in human bronchial epithelial cells

The carcinogenic effect of benzo[a]pyrene (B[a]P), presenting mainly in cigarette smoke and air pollution, has been well demonstrated both in vitro and in vivo. However, it is still not well understood how B[a]P facilitates pulmonary carcinogenesis. To explore this, we investigated the effect of B[a]P on the induction of cyclooxygenase-2 (COX-2), a critical enzyme implicated in inflammation and cancer development, as well as upstream signaling pathways leading to its expression in human bronchial epithelial cells (Beas-2B). We found that exposure of Beas-2B to B[a]P caused significant COX-2 induction at both the transcriptional and protein levels. B[a]P also switched on the nuclear factor of activated T cells (NFAT) and nuclear factor kappa B (NF-kappa B) signaling pathways. B[a]P-induced COX-2 expression was significantly blocked by inhibition of the NFAT pathway, and impairment of the NF-kappa B signaling pathway by ectopic expression of an inhibitor of nuclear factor-kappa B kinase beta-subunit (IKK beta) kinase inactive mutant (IKK beta-KM) also dramatically inhibited COX-2 induction. The IKK beta/NF-kappa B-dependent COX-2 induction was further confirmed in mouse embryonic fibroblasts with IKK beta deficiency (IKK beta(-/-)) and in those that expressed reconstituted IKK beta. However, activation of the NFAT and NF-kappa B signaling pathways by B[a]P were independent of each other, as blocking one signaling pathway didn't interrupt the activation of the other one. Mutation of either NFAT or NF-kappa B binding sites significantly blocked COX-2 promoter induction by B[a]P. Taken together, these data indicate that exposure of Beas-2B to B[a]P can upregulate COX-2 expression by increasing its transcription, which requires activation of both the NFAT and NF-kappa B signaling pathways.