TNF alpha induces expression of HIF-1 mRNA and protein but inhibits hypoxic stimulation of HIF-1 alpha transcriptional activity in airway smooth muscle cells

Abstract

Airway smooth muscle cells (ASMCs) participate in tissue remodeling characteristic of airway inflammatory diseases like asthma. Inflammation and hypoxia pathways are often interconnected and the regulatory subunit of the hypoxia inducible factor, HIF-1, has been recently shown to be induced by cytokines. Here we investigate the effect of individual or combined treatment of ASMCs with the inflammatory mediator TNF and/or hypoxia on the expression of HIF-1, HIF-1 targets and inflammation markers. TNF enhances HIF-1 protein and mRNA levels, under both normoxia and hypoxia. TNF-mediated induction of HIF-1 gene transcription is repressed by inhibition of the NF-B pathway. Despite the up-regulation of HIF-1 protein, the transcription of HIF-1 target genes remains low in the presence of TNF at normoxia and is even reduced at hypoxia. We show that the reduction in HIF-1 transcriptional activity by TNF is due to inhibition of the interaction of HIF-1 with ARNT and subsequent blocking of its binding to HREs. Comparison between hypoxia and TNF for their effects on the expression of inflammatory markers shows significant differences: hypoxia up-regulates the expression of IL-6, but not RANTES or ICAM, and reduces the induction of VCAM by TNF. Finally, ex vivo treatment of rabbit trachea strips with TNF increases HIF-1 protein levels, but reduces the expression of HIF-1 targets under hypoxia. Overall, TNF induces HIF-1 mRNA synthesis via an NF-B dependent pathway but inhibits binding of HIF-1 to ARNT and DNA, while hypoxia and TNF have distinct effects on ASMC inflammatory gene expression. J. Cell. Physiol. 228: 17451753, 2013. (c) 2013 Wiley Periodicals, Inc