Abstract
The sodium dependent SLC13 family transporters comprise of the five genes SLC13A1, SLC13A2 (NaDC1), SLC13A3 (NaDC3), SLC13A4 and SLC13A5 (NaCT). Among them the three NaDC1, NaDC3 and NaCT are sodium dependent transporters such as di-carboxylates (succinate, malate, α-ketoglutarate) and tricarboxylates (citrate). The mouse and the human NaCT structures have still not been crystallized, the information to the structures is taken from the related bacterial transporter of VcINDY. Citrate in the cytosol works as precursor for the fatty acid synthesis, cholesterol, and low-density lipoproteins. The excess citrate from the matrix is translocated to the cytosol for fatty acid synthesis through these receptors and thus controls the energy balance by downregulating the glycolysis, tricarboxylic acid (TCA), and fatty acid breakdown. These transporters play an important role in regulating various metabolic diseases including cancer, diabetes, obesity, fatty liver diseases and CNS disorders. These di and tricarboxylate transporters are emerging as new targets for metabolic disorders such as obesity and diabetes. The mutation in the function of the NaCT causes several neurological diseases including neonatal epilepsy and impaired brain development whereas mutation of the citrate present in the liver may provide positive effect. Therefore, continued efforts from the earlier work on citrate transporter are required for the development of citrate inhibitors. In this review the structure, function, and regulation of the NaCT receptors are discussed. The review also highlights citrate role in diagnosing diseases such as cancer, diabetes, fatty liver, and diabetes. The therapeutic perspective of synthetic inhibitors against NaCT receptors are succinctly summarized.
Keywords: Citrate, dicarboxylates, NaCT, ATP citrate lyase, citrate transporter
Introduction
Citrate, malate, succinate, and oxaloacetate are formed in steps of Citric acid cycle. These intermediates are precursors of anabolic pathways for fatty acids, cholesterol, glucose, pyruvate, amino acids, and nucleotides. These also help in the regulation of important biological processes including fatty acid synthesis and glycolysis [1]. They act as a source for the cytosolic acetyl CoA for the synthesis of fatty acids, cholesterol and isoprenoids [2]. Citrate metabolite involved in tricarboxylic acid (TCA) cycle is generated in the matrix of the mitochondria [3]. The steps involve carboxylation of the glucose derived pyruvate and then acetylation with Acetyl CoA. The citrate metabolite is involved in the TCA cycle to yield ATPvia generated NADH and FADH2 entering the electron transport chain (ETC). The excess citrate in the matrix is transported out to the cytoplasm through mitochondrial citrate carrier. Citrate inhibits the glucose catabolism by allosterically inhibiting phosphofructokinase-1 (rate-limiting enzyme in glycolysis) and stimulate gluconeogenesis by activating fructose 1,6-biphosphatase. It also acts as a carbon source for the lipid biosynthesis (fatty acids and cholesterol) with the generation of acetyl CoA. The acetyl CoA is the allosteric activator of the acetyl CoA carboxylase (rate limiting enzyme for fatty acid synthesis) and generated malonyl CoA. The malonyl CoA is the intermediate involved in the fatty acid synthesis in the cytoplasm. Thus, citrate acts as a building bridge between the fatty acid and carbohydrate metabolism [4] (Figure 1) . It was confirmed inMucor circinelloides where citrate produced from the TCA cycle was translocated (mitochondrial citrate transporter) across mitochondrial inner membrane and cleaved to oxaloacetate (OAA) and acetyl-CoA by ATP citrate lyase. The acetyl CoA along with NADPH are used to produce fatty acid biosynthesis in the cytosol. The two genes responsible for coding of citrate transporters i.e., citrate transporter and tricarboxylate transporter are overexpressed, cause increased lipid accumulation whereas decrease in extracellular citrate concentration [5]. The blockade of two plasma membrane malate transporter 2-oxoglutarate: malate antiporters (SoDIT-a and SoDIT-b) in the fungus results in increased malate available for the lipid biosynthesis [6]. The citrate transport is promoted by ΔpNa+(chemical gradient of sodium ions) and ΔpH (pH gradient across the membrane) and not by ∆ψ (electrical potential across membrane) showing that citrate transport is electroneutral process. The sodium-ion-dependent citrate carrier also causes sodium counterflow in the absence of citrate [7]. The cytosolic concentration of the citrate also determines the rate of the fatty acid synthesis. The NaCT (sodium-dependent citrate transporter) in liver cell plays an important role in transporting citrate and is used as a target for anti-obesity drugs. The extracellular citrate enters the plasma membrane of specific cells through Na+-coupled citrate transporter (NaCT)i.e., SLC13A5 [8]. The inner mitochondrial membrane channel is blocked by the externally bound high affinity Mg2+and may be activated by the citrate or other Mg2+chelators. The presence of exogenous MgCl2 strongly inhibits the citrate-induced depolarization and is independent of the presence of sodium, potassium, or chlorine ions. The exogenous citrate in the Mg2+ free medium has been shown to suppress the reactive oxygen species (ROS) and uncoupling of the mitochondria. It also suppresses the H2O2 and the stimulation of respiration of mitochondria (suppress the depolarized mitochondria and amount of the citrate transporter)[9]. The sodium dependent SLC13 family transporters carry these intermediates from plasma membranes into the cells and is comprises of five genes. The three sodium dependent transporters of the human SLC13 family includes hNaDC1 (SLC13A2), hNaDC3 (SLC13A3), and hNaCT (SLC13A5). These receptors are emerging as drug targets for various metabolic disorders [1]. Some of the recent patents on sodium citrate transporters are listed inTable 1 . The other two genes include SLC13 co-transporters SLC13A1 and SLC13A4 which are involved in transportation of renal sodium dependent anions such as selenite, sulphate, and thiosulphate [10].
Figure 1: Citrate-pyruvate shuttle and linkage of citrate as fatty acids and carbohydrate metabolism.
Table 1: Some of the patents on sodium citrate transporters are listed below: