Modulation of the heme oxygenase HO-1 expression by hyperosmolarity and betaine in primary rat hepatocytes

The influence of hyperosmotic shrinkage and the osmolyte betaine on heme ox ygenase HO-1 expression was studied in cultured rat hepatocytes, Hyperosmol arity transiently suppressed HO-1 induction in response to hemin or medium addition at the levels of mRNA and protein expression. Pretreatment of the cells with betaine largely restored induction of both HO-1 mRNA and protein under hyperosmotic conditions. Exposure of HO-1-expressing hepatocytes to cycloheximide unraveled a hyperosmotic acceleration of HO-1 degradation whi ch was counteracted by betaine and the proteolysis inhibitor MG-132. The HO -1 mRNA stability remained unaffected by hyperosmolarity and betaine as sho wn by application of the transcription inhibitor actinomycin D. The data su ggest a modulation of HO-1 expression by hyperosmolarity and betaine at the transcriptional level and at the level of proteasomal degradation. Hyperos motic suppression of HO-1 expression was accompanied by a moderate but sign ificant loss of hepatocyte viability, which was prevented by betaine, The h yperosmotic impairment of hepatocyte viability was insensitive to betaine i n presence of the heme oxygenase inhibitor zinc protoporphyrin IX. However, treatment of the hepatocytes with bilirubin or 8-Br-cGMP improved hepatocy te viability under hyperosmotic conditions to the control niveau, Thus, sta bilizing HO-1 expression may contribute to hepatocyte protection against hy perosmotic stress by organic osmolytes. (C) 2001 Academic Press.