Ammonia Stimulates Heme Oxygenase‐1 Gene Expression in Human Endothelial Cells

Abstract

Ammonia is continuously generated during cell metabolism via the catabolism of purines, pyrimidines and polyamines, and deamination of amino acids. Although endothelial cells (ECs) produce substantial amounts of ammonia, the role of this gas in ECs is not known. Since heme oxygenase‐1 (HO‐1) is an important regulator of EC function, we investigated whether ammonia modulates the expression of HO‐1 in cultured human umbilical vein ECs. Treatment of ECs with ammonia, given as ammonium chloride or ammonium hydroxide, stimulated a concentration‐ and time‐dependent increase in HO‐1 protein. A significant rise in HO‐1 protein was first detected after 4 hours and levels progressively increased over 24 hours of ammonia exposure. Ammonia‐mediated increases in HO‐1 protein expression were paralleled by significant elevations in HO‐1 mRNA. The induction of HO‐1 by ammonia was dependent on de novo RNA synthesis since it was blocked by the transcriptional inhibitor actinomycin D. Ammonia also stimulated a rapid and sustained increase in the production of reactive oxygen species (ROS) beginning one hour after ammonia administration. However, incubation of ECs with the antioxidant N‐acetyl‐L‐cysteine prevented the formation of ROS and blocked the induction of HO‐1 by ammonia. In conclusion, this study found that ammonia stimulates HO‐1 gene expression in vascular endothelium via a transcriptional‐ and redox‐sensitive pathway. The ability of ammonia to induce HO‐1 may provide a potential mechanism by which this gas regulates EC function. Supported by NIH grant HL59976.