Downregulation of hepatic betaine:homocysteine methyltransferase (BHMT) expression in taurine-deficient mice is reversed by taurine supplementation in vivo

Abstract

The cysteine dioxygenase (Cdo1)-null and the cysteine sulfinic acid decarboxylase (Csad)-null mouse are not able to synthesize hypotaurine/taurine by the cysteine/cysteine sulfinate pathway and have very low tissue taurine levels. These mice provide excellent models for studying the effects of taurine on biological processes. Using these mouse models, we identified betaine:homocysteine methyltransferase (BHMT) as a protein whose in vivo expression is robustly regulated by taurine. BHMT levels are low in liver of both Cdo1-null and Csad-null mice, but are restored to wild-type levels by dietary taurine supplementation. A lack of BHMT activity was indicated by an increase in the hepatic betaine level. In contrast to observations in liver of Cdo1-null and Csad-null mice, BHMT was not affected by taurine supplementation of primary hepatocytes from these mice. Likewise, CSAD abundance was not affected by taurine supplementation of primary hepatocytes, although it was robustly upregulated in liver of Cdo1-null and Csad-null mice and lowered to wild-type levels by dietary taurine supplementation. The mechanism by which taurine status affects hepatic CSAD and BHMT expression appears to be complex and to require factors outside of hepatocytes. Within the liver, mRNA abundance for both CSAD and BHMT was upregulated in parallel with protein levels, indicating regulation of BHMT and CSAD mRNA synthesis or degradation.

Keywords: Betaine; Betaine:homocysteine methyltransferase; Cysteine dioxygenase; Cysteine sulfinic acid decarboxylase; Hepatocytes; Liver; Taurine.

Fig. 1

BHMT protein abundance (a), BHMT mRNA abundance (b), taurine level (c), CSAD protein abundance (d) and CSAD mRNA abundance (e) in liver from Cdo1-null and wild-type mice fed either a basal, high-fat, or high-fat + taurine diet. Values shown in bar graphs are mean ± SEM for six mice and are expressed as fold the value for wild-type mice of the same sex fed the basal diet; asterisk indicates the value for the Cdo1-null mice is significantly different (p < 0.05) than that for wild-type mice of the same sex and dietary treatment group based on post hoc Tukey’s comparison tests. Representative western blots are shown below the bar graphs for protein abundances

Fig. 2

Taurine (a) and hypotaurine (b) levels, BHMT protein abundance (c), CSAD protein abundance (d), and representative western blots for BHMT and CSAD (e) in primary hepatocytes from Cdo-null (−/−) and wild-type (+/+) mice after culture in basal taurine-free medium or in medium supplemented with 0.05 or 0.5 mM taurine. Values shown in bar graphs are mean ± SEM for three separate hepatocyte preparations for cells cultured for 24 h with treatment medium; values are expressed as fold the value for hepatocytes from wild-type mice cultured in basal medium. Asterisk indicates the value for hepatocytes from Cdo1-null mice is significantly different (p < 0.05) than that for hepatocytes from wild-type mice based on a post hoc Tukey’s comparison tests. Bars not denoted by the same letter are significantly different (p < 0.05) by post hoc Tukey’s comparison. The representative western blots are shown for hepatocytes cultured for 24 h and for 48 h in treatment medium

Fig. 3

BHMT protein abundance (a), BHMT mRNA abundance (b), taurine level (c), CDO protein abundance (d) and CDO mRNA abundance (e) in liver from Csad-null mice fed either a taurine-free basal diet and supplemented without taurine or with taurine (1 g taurine per 100 mL drinking water). Values shown in bar graphs are mean ± SEM for three mice and are expressed as fold the value for taurine-supplemented mice of the same sex; asterisk indicates the value for the taurine-deficient Csad-null mice is significantly different (p < 0.05) than that for taurine-supplemented Csad-null mice of the same sex based on Student’s t test. Representative western blots are shown below the bar graphs for protein abundances

Fig. 4

Overview of methionine and cysteine metabolism with the effects of taurine deficiency secondary to deletion of the Cdo1 gene on CSDC and BHMT abundances and betaine levels shown by arrows