Permissive Effects of Oxygen on Cyclic AMP and Interleukin-1 Stimulation of Surfactant Protein A Gene Expression Are Mediated by Epigenetic Mechanisms

Abstract

Surfactant protein A (SP-A) is important for immune defense within the alveolus. Cyclic AMP (cAMP) stimulation of SP-A expression in lung type II cells is O₂ dependent and mediated by increased phosphorylation and binding of thyroid transcription factor 1 (TTF-1) to an upstream response element (TTF-1-binding element [TBE]). Interleukin-1 (IL-1) stimulation of SP-A expression is mediated by NF-κB (p65/p50) activation and increased binding to the TBE. In this study, we found that O₂ also was permissive for IL-1 induction of SP-A expression and for cAMP and IL-1 stimulation of type II cell nuclear protein binding to the TBE. Using chromatin immunoprecipitation, we observed that when type II cells were cultured in 20% O₂, cAMP and IL-1 stimulated the recruitment of TTF-1, p65, CBP, and steroid receptor coactivator 1 to the TBE region of the SP-A promoter and increased local acetylation of histone H3; these effects were prevented by hypoxia. Hypoxia markedly reduced global levels of CBP and acetylated histone H3 and increased the expression of histone deacetylases. Furthermore, hypoxia caused a global increase in histone H3 dimethylated on Lys9 and increased the association of dimethyl histone H3 with the SP-A promoter. These results, together with findings that the histone deacetylase inhibitor trichostatin A and the methyltransferase inhibitor 5′-deoxy(5′-methylthio)adenosine markedly enhanced SP-A expression in lung type II cells, suggest that increased O₂ availability to type II cells late in gestation causes epigenetic changes that permit access of TTF-1 and NF-κB to the SP-A promoter. The binding of these transcription factors facilitates the recruitment of coactivators, resulting in the further opening of the chromatin structure and activation of SP-A transcription.

This research was supported in part by NIH grants 2-U01-HL-052647 and 5-R37-HL-050022. K.N.I. was supported by NIH grant 5-T32-HD-07190.

We are grateful to Meg Smith for her expert assistance in the preparation of primary cultures of fetal lung type II cells.