The Abcc2-Lith2 locus induces hypersensitivity to the biliary lithogenic effect of oestrogens by precipitating the onset of lithogenic bile

Abstract

Background and Aim: We recently established Abcc2 to be the molecular basis underlying Lith2, one of the major cholesterol gallstone susceptibility loci identified in inbred mice fed a lithogenic diet. Based on the direct role of ABCC2 in the hepatohiliary transport of different drug and hormone conjugates, many of which are well-recognised to constitute high risk factors for human cholelithiasis, we hypotbetised that the Lith2/Abcc2 locus impacts not only the fithogenic response to dietary intervention but also to drugs and hormones. We tested our hypothesis here, using estrogen as a model compound. Methods: AKi.L/th2 s congenic mice (Lith2) and AKR controls were injected i.p. with ethinyl estradiol (EE) at the dose of 1X or 3X (5 or 15mg/kg-day) for 3 days. Bile flow and biliary lipids were assessed from hepatic bile with standard procedures. Hepatic bile acids were assayed by LC-MS-MS. Results: Body weights were identical between groups, establishing that all mice received an equivalent amount of EE Without EE-treatment, biliary lipid secretion rates, biliary lipid coupling and CSI were undifferentiable between groups. After EE 1X, however, while the controls increased their bile salts (BS) secretion rate and modestly but significantly decreased their biliary secretion of phospholipids (PL) and cholesterol (Ch) (most likely due to the uncoupling effect of EE-conjugates), the Lith2 group exhibited a declining BS secretion rate, a drop 2 times more pronounced in PL secretion rates and a sharp rise in biliary Ch, culminating in a marked elevation of Ch/BS or PL ratios and CSI. Tripling the dose of EE had no additional effect in lath2 mice but led the controls to now develop an hepatobiliary and lithogenic profile undistinguishable from the congenic animals. Percent of hepatic dihydroxylated BS were elevated in all Lith2 mice treated with EE but only in the controls receiving EE 3X, thereby being in direct relationship with biliary lithogenicity. Conclusions: The Lith2 locus, and most likely Abcc2, is not solely involved in the high-lithogeinc risk following a lithogenic diet, but also induces an hypersensitivity to the hthogenic effect of estrogens. Dihydroxylated hepatic bile acids appears central to the development of lithogenic bile, possibly through activation of nuclear receptor/ABCG5-8 expression. We speculate that such gene-estrogen interaction plays a role in the lithogenic risk associated with gender, pregnancy and estrogen-based therapies.