Liver and kidney cross talk: liver-derived bile acids and kidney water retention.

Edema formation and ascites are common features in liver disease. The excessive accumulation of body water is caused by increased kidney water retention; yet the mechanisms linking liver and kidney function remain incompletely understood. This review explores the emerging concept of a liver-kidney axis, with bile acids (BAs) as potential mediators for kidney water handling. In cholestatic liver disease, as seen in liver cirrhosis, impaired hepatic BA flow to the duodenum elevates systemic BA concentrations, which can modulate signaling pathways through the BA receptors Farnesoid X receptor (FXR, NR1H4) and G-protein-coupled bile acid receptor 1 (TGR5, GPBAR1), thereby increasing kidney Na+ and water retention in experimental models. Moreover, BAs can directly activate the sodium channel ENaC in isolated mouse collecting ducts and in mouse distal colon epithelial cells. Thus, BAs can promote sodium and water retention in parallel with the classical sodium- and water-retaining systems, such as the renin-angiotensin-aldosterone system. This effect may potentially contribute to water retention in pregnancy-associated conditions and in proteinuric kidney diseases, where liver function is affected, e.g., by loss of plasma proteins, altering hepatic BA metabolism. In this review, we will describe the synthesis of BAs, their modifications, and kidney BA metabolism in health and disease, and highlight a potential dynamic interplay between liver and kidney function, mediated in part by BAs, with implications for understanding and treating water retention disorders.