Submitted on February 24, 2006
Accepted on December 28, 2006
Osmotic regulation of betaine homocysteine-S-methyltransferase expression in H4IIE rat hepatoma cells
Christine Schafer1, Lars Hoffmann2, Katrin Heldt2, Mohammad Reza Lornejad-Schafer1, Gernot Brauers2, Thor Gehrmann1, Timothy A Garrow3, Dieter Haussinger1, Ertan Mayatepek2, Bernd Schwahn2, and Freimut Schliess1*
1 Clinic for Gastroenterology, Hepatology, and Infectiology, Heinrich-Heine-University, Dusseldorf, Germany
2 Clinic for General Paediatrics, Heinrich-Heine-University, Dusseldorf, Germany
3 Department of Food Science and Human Nutrition, University of Illinois at Urbana, Urbana, Illinois, United States
* To whom correspondence should be addressed. E-mail: schliess{at}med.uni-duesseldorf.de.
Cell hydration changes critically affect liver metabolism and gene expression. In the course of gene expression studies using Nylon cDNA-arrays we found that hyperosmolarity (405mosmol/l) suppressed the betaine-homocysteine methyltransferase (BHMT) mRNA expression in H4IIE rat hepatoma cells. This was confirmed by Northern blot and real time quantitative RT-PCR analysis, which in addition unravelled a pronounced induction of BHMT mRNA expression by hypoosmotic (205mosmol/l) swelling. Osmotic regulation of BHMT mRNA expression was largely paralleled at the levels of BHMT protein and enzymatic activity. Like hyperosmotic NaCl, hyperosmotic raffinose but not hyperosmotic urea suppressed BHMT mRNA expression, suggesting that cell shrinkage rather than increased ionic strength or hyperosmolarity per se is the trigger. Hypoosmolarity increased the expression of a reporter gene driven by the entire human BHMT promoter, whereas destabilization of BHMT mRNA was observed under hyperosmotic conditions. Osmosensitivity of BHMT mRNA expression was impaired by inhibitors of tyrosine kinases and cyclic nucleotide-dependent kinases. The osmotic regulation of BHMT may be part of a cell volume-regulatory response and additionally lead to metabolic alterations which depend on the availability of betaine-derived methyl groups.
