Electrophysiological Characterization of Human and Mouse Sodium-Dependent Citrate Transporters (NaCT/SLC13A5) Reveal Species Differences with Respect to Substrate Sensitivity and Cation Dependence

Abstract

The citric acid cycle intermediate citrate plays a crucial role in metabolic processes such as fatty acid synthesis, glucose metabolism, and b-oxidation. Citrate is imported from the circulation across the plasma membrane into liver cells mainly by the sodium-dependent citrate transporter (NaCT; SLC13A5). Deletion of NaCT from mice led to metabolic changes similar to caloric restriction; therefore, NaCT has been proposed as an attractive therapeutic target for the treatment of obesity and type 2 diabetes. In this study, we expressed mouse and human NaCT into Xenopus oocytes and examined some basic functional properties of those transporters. Interestingly, striking differences were found between mouse and human NaCT with respect to their sensitivities to citric acid cycle intermediates as substrates for these transporters. Mouse NaCT had at least 20-to 800-fold higher affinity for these intermediates than human NaCT. Mouse NaCT is fully active at physiologic plasma levels of citrate, but its human counterpart is not. Replacement of extracellular sodium by other monovalent cations revealed that human NaCT was markedly less dependent on extracellular sodium than mouse NaCT. The low sensitivity of human NaCT for citrate raises questions about the translatability of this target from the mouse to the human situation and raises doubts about the validity of this transporter as a therapeutic target for the treatment of metabolic diseases in humans.

A dynamic anchor domain in slc13 transporters controls metabolite transport

“…In the mouse liver, knockout of the slc13 transporter, NaCT, inhibits lipogenesis by lowering the cytoplasmic concentrations of the fatty acid biogenesis intermediatecitrate (11,21). However, functional human NaCT measurements in Xenopus occytes show significantly lower affinity to citrate compared to the reported mouse isoform, as we report here and previously reported by Zwart et al (22). When monitored in mammalian cells, human NaCT Km was ~600 µM (23).…”

The human longevity gene homolog INDY and interleukin‐6 interact in hepatic lipid metabolism

“…Finally, a recent publication raised questions about the translatability of mINDY as a target for the treatment of metabolic disease from the mouse to the human situation due to differences in uptake properties between species. (33) Therefore, we directly measured 14 C-citrate uptake into mouse, rat, and human primary hepatocytes incubated with low physiological concentrations of citrate. In this setting, citrate uptake was highest in human primary hepatocytes, followed by mouse and rats (Fig.…”

Model‐Based Assessment of Plasma Citrate Flux Into the Liver: Implications for NaCT as a Therapeutic Target

“…1 Whether the effects of NaCT knockout in mice can be translated into humans is unclear, because of speciesspecific differences with respect to substrate-sensitivity 8,9 and cation-dependency. 10 Cross-species translation is fur-ther complicated by the fact that there are at least two other transporters that play a role in citrate homeostasis: low-affinity sodium-dependent dicarboxylate cotransporter (NaDC1) encoded by SLC13A2, and high-affinity sodiumdependent dicarboxylate cotransporter (NaDC3) encoded by SLC13A3. Although NaCT is located primarily in the liver of both humans and mice, its relative importance to hepatic citrate uptake appears to be different across these species.…”

“…Based on the hepatic clearance and plasma concentration of citrate, the hepatic influx of plasma citrate was calculated maximally to be 17 g/day with a 95% CI of [9][10][11][12][13][14][15][16][17][18][19][20] (Eq. 5).…”