The Mitochondrial Citrate Carrier SLC25A1/CIC and the Fundamental Role of Citrate in Cancer, Inflammation and Beyond

Abstract

The mitochondrial citrate/isocitrate carrier, CIC, has been shown to play an important role in a growing list of human diseases. CIC belongs to a large family of nuclear-encoded mitochondrial transporters that serve the fundamental function of allowing the transit of ions and metabolites through the impermeable mitochondrial membrane. Citrate is central to mitochondrial metabolism and respiration and plays fundamental activities in the cytosol, serving as a metabolic substrate, an allosteric enzymatic regulator and, as the source of Acetyl-Coenzyme A, also as an epigenetic modifier. In this review, we highlight the complexity of the mechanisms of action of this transporter, describing its involvement in human diseases and the therapeutic opportunities for targeting its activity in several pathological conditions.

Keywords: 22.q11.2; CIC; CTP; NAFLD/NASH; SLC25A1; cancer; citrate; diabetes; inflammation; metabolism; mitochondria.

Figure 1

Preview of the activities of citrate/isocitrate carrier (CIC) (also see text for explanation).

Figure 2

Comparison of the structure of CIC inhibitors. The in vivo IC50 is indicated for each of the compounds.

Figure 3

Schematic representation of the CIC promoter and transcription factors’ binding sites (TFBSs) identified in various studies. The positions of these TFBSs are representative and do not reflect the actual position in the promoter.

Figure 4

Pathways to the generation of cytosolic and mitochondrial citrate and its utilization. Glucose-derived citrate is obtained through the conversion of glucose to glucose-6-phosphate, which, with a series of enzymatic reactions, is then converted to pyruvate. Pyruvate is reduced to lactate via lactate dehydrogenase (LDH) or, alternatively, transported into mitochondria to produce Acetyl-Coenzyme A (Ac-CoA) via pyruvate dehydrogenase (PDH). Citrate synthase then catalyzes the condensation of acetyl-CoA with oxaloacetate to yield citrate that is exported in the cytosol by CIC. Lactate can also enter the mitochondria and be converted to pyruvate by a mitochondrial lactate dehydrogenase (mtLDH) regenerating citrate. Mitochondrial citrate and, to a lesser extent, lactate fuel the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC). Citrate can also be uptaken from the extracellular space and transported to the cytosol via SLC13A5. In the cytosol, citrate provides Ac-CoA via ATP citrate lyase (ACLY) for protein acetylation and can enter the mevalonate pathway for cholesterol biosynthesis mediated by hydroxymethylglutaryl-CoA synthase (HMGCS) and hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and the fatty acid synthetic pathway via acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN). Cytosolic Ac-CoA can be also generated by acetyl-CoA synthetase 2 (ACSS2) which converts acetate derived from deacetylation reactions to acetyl-CoA. Cytosolic citrate inhibits phosphofructokinase 1 (PFK1) and pyruvate kinase (PK), thus playing an active role in controlling glycolytic flux. An alternative source of mitochondrial or cytosolic citrate is supplied by reductive carboxylation of alpha-ketoglutarate to isocitrate, mediated in the cytosol by isocitrate dehydrogenase 1 (IDH1) and in the mitochondria by IDH2. Additional abbreviations: HK—hexokinase; G6PD—glucose-6-phosphate dehydrogenase; 6PGL—6-phosphogluconolactonase; 6PGD—6-phosphogluconate dehydrogenase; Rpi—ribose-5-phosphate isomerase; PGI—phosphoglucose isomerase; ME—malic enzyme; MDH—malate dehydrogenase; CS—citrate synthase; ACO2—aconitase 2; IDH—isocitrate dehydrogenase; α-KGDH—α-Ketoglutarate dehydrogenase; SCS—succinyl coenzyme A synthetase; SDH—succinate dehydrogenase; FH—fumarase; ACO1—aconitase 1; GHD—glutamate dehydrogenase; GLS—glutaminase.

Figure 5

Schematic representation of some of the salient effects of CTPI-2 in the non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD/NASH) model.

Figure 6

Involvement of CIC-dependent mitochondrial oxidative metabolism in adaptation to stress (see text for explanation).

Figure 7

CIC induces a pro-inflammatory program in macrophages (see text for explanation). The transcription rate of the SLC25A1 gene is induced in monocytes by lipopolysaccharides (LPS), tumor necrosis factor alpha (TNFα) and interferon gamma (IFNγ) via recruitment to the promoter of nuclear factor kappa B (NFkB) and signal transducer and activator of transcription 1 (STAT1). CIC induction in these situations leads not only to the expected increase in Ac-CoA but also to enhanced synthesis of prostaglandin E2 (PGE2) and inducible nitric oxide synthetase (iNOS). At the bottom of the figure, there is a simplistic representation of the macrophage populations depicted in the two opposite phenotypes, M1 and M2. In the NAFLD/NASH liver, CIC inhibition represses markers of the pro-inflammatory macrophage phenotype. Abbreviations that are not in the main text: reactive oxygen species, ROS; iNOS, inducible nitric oxide synthase; FN1, fibronectin 1; MRC1, Mannose Receptor C-Type 1.

Figure 8

Mitochondrial and cytosolic pathways leading to D2-L2 hydroxyglutaric acid (D-L-2HG) accumulation (see text for explanation). The proposed model envisions that as a consequence of disruption of CIC-mediated citrate export activity, cytosolic citrate is reduced, leading to loss of the feedback loop on PFK. This leads to enhanced glycolysis and production of pyruvate, which, on one side, is converted to lactate, resulting in lactic acidosis. Excess pyruvate also enters mitochondria, where it is converted to citrate/isocitrate. Due to a lack of the export activity of CIC, this excess citrate is converted to TCA cycle intermediates downstream of citrate, leading to accumulation of α-KG and also of succinate, fumarate and malate (not shown in the figure), which are then secreted in urine. In red are the potential steps for conversion of α-KG to L-2HG or D-2HG, by either IDH1 or IDH2; in blue are the enzymes involved in the degradation pathway. IDH1/IDH2m or wt: mutant or wild-type forms of these enzymes. See text for additional abbreviations.