The GABA-betaine transporter (BGT1): protection against alcoholic liver disease

Alkoholmisbruk og høy fettinntak medfører et antall helseproblemer inkludert leversykdom (alkoholindusert leversykdom, ALD; non-alkoholisk leversykdom, NALD). Metioninmetabolismen er forstyrret i ALD/NALD, og både alkohol og mye fett øker behovet for betain. Leveren kan skaffe seg betain enten ved å absorbere betain fra maten (ved hjelp av transportør-proteiner) eller ved å omdanne kolin til betain (ved hjelp av kolindehydrogenase, CHDH). På tross av at tilførsel av betain via kosten motvirker ALD/NALD, har de ansvarlige transportører for betain ikke blitt identifisert. Et overordnet mål for dette prosjektet er å finne ut hvor viktig den såkalte "betain-GABA transportøren (BGT1; slc6a12) er for å beskytte leveren mot de skadelige effektene av etanol og høyt fettinntak. For å nå dette målet vil vi først bestemme den cellulære distribusjon av BGT1 i leveren. Deretter vil vi måle BGT1's andel av den totale opptaksaktivitet for kolin og betain for å kunne avgjøre om og i hvilken grad andre transportører bidrar til opptaket av betain. Parallelt med dette vil vi studere konsekvensene av BGT1/CHDH-mangel på metabolismen av kolin, betain, cystein og metionin. Vi vil også fore normalmus og mus som mangler BGT1 eller CHDH eller begge med fettrik mat for å se om disse musene er mer sårbare enn normalmus. Basert på våre erfaringer med musene, antar vi at mennesker med defekt betainopptak vil kunne leve normale liv og få barn, men de vil muligens tåle alkohol dårlig. Dette kan kanskje være en av forklaringene på hvorfor noen mennesker får leverskade av alkohol i relativt ung alder mens andre lignende livstil ikke utvikler alvorlig leversykdom. Hvis defekt betainopptak medfører redusert beskyttelse, vil det være mulig å utvikle tester for å identifisere personer med høy risiko for leversykdom.

Alcoholic and non-alcoholic fatty liver diseases (AFLD and NAFLD) represent significant health problems globally. Betaine appears to play important a protective role, but betaine supplements are not approved for treatment of fatty liver disease. At present, betaine is approved by the EU only for treatment of a rare inborn error of metabolism (homocystinuria), and as a novel food to meet additional requirements for intense muscular effort. Betaine is classified as a non-essential nutrient, while choline is an essential one. A reason for the reluctance to approve betaine for more widespread use is that we know too little about what it is doing and how it is handled in the body. The primary aim of the present project was to clarify the roles of the betaine-GABA transporter (BGT1; Slc6a12) with respect to liver betaine uptake. The results from the project fall into three main categories: Firstly, information on BGT1 with respect to localization in the liver, and information on the transporters involved in betaine uptake by the liver and the small intestine. Secondly, information on how betaine supplements affects the liver lipid content in preventing or attenuating the development of fatty liver. Thirdly, information of redundancy with respect to mechanisms that regenerate methionine and degrade homocysteine. Further, the project strengthens national and international competence, and supported the career development of young, highly promising researchers and doctors. One significant impact of the project is to provide scientific basis for nutrient recommendations that though betaine is a non-essential nutrient, the populations with heavy alcohol and high fat consumption may consider to courage the intake of choline- or betaine-rich foods. The potential impact for society (health and economy) of a decision to approve or not approve betaine is huge.

Alcohol abuse causes numerous health problems. One of these is alcoholic liver disease (ALD), which partly results from perturbations of hepatocyte methionine metabolism: Ethanol inhibits the methionine synthase pathway, increasing the demand for betaine. Hepatic betaine is supplied via the diet or by conversion of choline to betaine. Ethanol inhibits the latter. Dietary betaine supplements attenuate ALD, but this requires that the hepatocyte can absorb the supplied betaine from the blood. The hepatic uptake is catalyzed by transporter proteins of which the betaine-GABA transporter (BGT1) is believed to be the responsible transporter. We have created BGT1 deficient mice, observed that these mice survive, and shown BGT1 to be predominantly a liver protein. Importantly, we have just found (unpublished) that under non-challenging conditions BGT1-deficient mice have a mild reduction in hepatic choline. Compensatory increased production of betaine from choline may explain how these mice avoid liver disease, but they may be vulnerable to increases in betaine demand induced by ethanol and high fat diets. If the mice models reflect human status, it is important to identify subjects with defective betaine uptake as these may be at particular risk of developing ALD. We propose to: 1. Test whether BGT1 expression is restricted to hepatocytes (WP#1). 2. Determine the contribution of BGT1 to the total betaine transport (WP#2), and test whether the SIT1, OCTN2 and OAT9 transporters contribute to the betaine uptake. 3. Test if defect betaine uptake increases choline requirements for choline using BGT1/choline dehydrogenase-double knockouts (WP#3&4). 4. Examine if BGT1 deficiency increases ethanol or high fat hepatotoxicity (WP#4). 5. Identify putative gender and mouse strain differences in choline and methionine pathways (WP#5). This project will determine the importance of betaine and possibly contribute to new strategies for prevention and treatment of ALD in humans.