Uremic solutes modulate hepatic bile acid handling and induce mitochondrial toxicity

Chronic kidney disease (CKD) is accompanied by accumulating levels of uremic solutes in the circulation. Changes in the size and composition of the bile acid pool have also been observed. We investigated via which mechanisms uremic solutes may interfere with hepatocyte function and thus contribute to altered bile acid handling. We studied interference on the level of bile acid synthesis by cytochrome P450 7A1 (CYP7A1), explored effects on hepatic bile acid transporters, and investigated effects on mitochondrial function. In HEK293 cells overexpressing bile salt transporters, we observed that p-cresyl sulfate inhibited Na⁺-taurocholate cotransporting polypeptide (NTCP)-mediated uptake of taurocholic acid (TCA), whereas organic anion-transporting polypeptide 1B1 (OATP1B1)-mediated TCA uptake was increased. Assays in transporter-overexpressing membrane vesicles revealed that kynurenic acid inhibited TCA transport via the bile salt efflux pump (BSEP), whereas p-cresyl glucuronide and hippuric acid increased TCA efflux via multidrug resistance-associated protein 3 (MRP3). Moreover, indoxyl sulfate decreased mRNA expression of NTCP, OATP1B3 and CYP7A1 in primary human hepatocytes. Transport studies confirmed a decreased TCA uptake in indoxyl sulfate-exposed hepatocytes. Decreased hepatocyte viability was found for all seven uremic solutes tested, whereas five out of seven also decreased intracellular ATP levels and mitochondrial membrane potential. In conclusion, uremic solutes affect hepatic bile acid transport and mitochondrial function. This can contribute to the altered bile acid homeostasis observed in CKD patients.

Na(+),K(+)-ATPase isoform selectivity for digitalis-like compounds is determined by two amino acids in the first extracellular loop

Digitalis-like compounds (DLCs) comprise a diverse group of molecules characterized by a cis-trans-cis ring-fused steroid core linked to a lactone. They have been used in the treatment of different medical problems including heart failure, where their inotropic effect on heart muscle is attributed to potent Na(+),K(+)-ATPase inhibition. Their application as drugs, however, has declined in recent past years due to their small safety margin. Since human Na(+),K(+)-ATPase is represented by four different isoforms expressed in a tissue-specific manner, one of the possibilities to improve the therapeutic index of DLCs is to exploit and amend their isoform selectivity. Here, we aimed to reveal the determinants of selectivity of the ubiquitously expressed α1 isoform and the more restricted α2 isoform toward several well-known DLCs and their hydrogenated forms. Using baculovirus to express various mutants of the α2 isoform, we were able to link residues Met(119) and Ser(124) to differences in affinity between the α1 and α2 isoforms to ouabain, dihydro-ouabain, digoxin, and dihydro-digoxin. We speculate that the interactions between these amino acids and DLCs affect the initial binding of these DLCs. Also, we observed isoform selectivity for DLCs containing no sugar groups.

Na,K-ATPase activity modulates Src activation: a role for ATP/ADP ratio

Digitalis-like compounds (DLCs), specific inhibitors of Na,K-ATPase, are implicated in cellular signaling. Exposure of cell cultures to ouabain, a well-known DLC, leads to up- or down regulation of various processes and involves activation of Src kinase. Since Na,K-ATPase is the only known target for DLC binding an in vitro experimental setup using highly purified Na,K-ATPase from pig kidney and commercially available recombinant Src was used to investigate the mechanism of coupling between the Na,K-ATPase and Src. Digoxin was used as a representative DLC for inhibition of Na,K-ATPase. The activation of Src kinase was measured as the degree of its autophosphorylation. It was observed that in addition to digoxin, Src activation was dependent on concentrations of other specific ligands of Na,K-ATPase: Na(+), K(+), vanadate, ATP and ADP. The magnitude of the steady-state ATPase activity therefore seemed to affect Src activation. Further experiments with an ATP regenerating system showed that the ATP/ADP ratio determined the extent of Src activation. Thus, our model system which represents the proposed very proximal part of the Na,K-ATPase-Src signaling cascade, shows that Src kinase activity is regulated by both ATP and ADP concentrations and provides no evidence for a direct interaction between Na,K-ATPase and Src.