Krebs Cycle Reborn in Macrophage Immunometabolism.

A striking change has happened in the field of immunology whereby specific metabolic processes have been shown to be a critical determinant of immune cell activation. Multiple immune receptor types rewire metabolic pathways as a key part of how they promote effector functions. Perhaps surprisingly for immunologists, the Krebs cycle has emerged as the central immunometabolic hub of the macrophage. During proinflammatory macrophage activation, there is an accumulation of the Krebs cycle intermediates succinate and citrate, and the Krebs cycle-derived metabolite itaconate. These metabolites have distinct nonmetabolic signaling roles that influence inflammatory gene expression. A key bioenergetic target for the Krebs cycle, the electron transport chain, also becomes altered, generating reactive oxygen species from Complexes I and III. Similarly, alternatively activated macrophages require α-ketoglutarate-dependent epigenetic reprogramming to elicit anti-inflammatory gene expression. In this review, we discuss these advances and speculate on the possibility of targeting these events therapeutically for inflammatory diseases.

Sex Differences in Intestinal Carbohydrate Metabolism Promote Food Intake and Sperm Maturation.

Physiology and metabolism are often sexually dimorphic, but the underlying mechanisms remain incompletely understood. Here, we use the intestine of Drosophila melanogaster to investigate how gut-derived signals contribute to sex differences in whole-body physiology. We find that carbohydrate handling is male-biased in a specific portion of the intestine. In contrast to known sexual dimorphisms in invertebrates, the sex differences in intestinal carbohydrate metabolism are extrinsically controlled by the adjacent male gonad, which activates JAK-STAT signaling in enterocytes within this intestinal portion. Sex reversal experiments establish roles for this male-biased intestinal metabolic state in controlling food intake and sperm production through gut-derived citrate. Our work uncovers a male gonad-gut axis coupling diet and sperm production, revealing that metabolic communication across organs is physiologically important. The instructive role of citrate in inter-organ communication might be significant in more biological contexts than previously recognized.

Toll-like Receptor Signaling Rewires Macrophage Metabolism and Promotes Histone Acetylation via ATP-Citrate Lyase.

Toll-like receptor (TLR) activation induces inflammatory responses in macrophages by activating temporally defined transcriptional cascades. Whether concurrent changes in the cellular metabolism that occur upon TLR activation influence the quality of the transcriptional responses remains unknown. Here, we investigated how macrophages adopt their metabolism early after activation to regulate TLR-inducible gene induction. Shortly after TLR4 activation, macrophages increased glycolysis and tricarboxylic acid (TCA) cycle volume. Metabolic tracing studies revealed that TLR signaling redirected metabolic fluxes to generate acetyl-Coenzyme A (CoA) from glucose resulting in augmented histone acetylation. Signaling through the adaptor proteins MyD88 and TRIF resulted in activation of ATP-citrate lyase, which in turn facilitated the induction of distinct LPS-inducible gene sets. We postulate that metabolic licensing of histone acetylation provides another layer of control that serves to fine-tune transcriptional responses downstream of TLR activation. Our work highlights the potential of targeting the metabolic-epigenetic axis in inflammatory settings.

O-GlcNAcylation of ATP-citrate lyase couples glucose supply to lipogenesis for rapid tumor cell proliferation.

Elevated lipid synthesis is one of the best-characterized metabolic alterations in cancer and crucial for membrane expansion. As a key rate-limiting enzyme in de novo fatty acid synthesis, ATP-citrate lyase (ACLY) is frequently up-regulated in tumors and regulated by posttranslational modifications (PTMs). Despite emerging evidence showing O-GlcNAcylation on ACLY, its biological function still remains unknown. Here, we observed a significant upregulation of ACLY O-GlcNAcylation in various types of human tumor cells and tissues and identified S979 as a major O-GlcNAcylation site. Importantly, S979 O-GlcNAcylation is required for substrate CoA binding and crucial for ACLY enzymatic activity. Moreover, it is sensitive to glucose fluctuation and decisive for fatty acid synthesis as well as tumor cell proliferation. In response to EGF stimulation, both S979 O-GlcNAcylation and previously characterized S455 phosphorylation played indispensable role in the regulation of ACLY activity and cell proliferation; however, they functioned independently from each other. In vivo, streptozocin treatment- and EGFR overexpression-induced growth of xenograft tumors was mitigated once S979 was mutated. Collectively, this work helps comprehend how cells interrogate the nutrient enrichment for proliferation and suggests that although mammalian cell proliferation is controlled by mitogen signaling, the ancient nutrition-sensing mechanism is conserved and still efficacious in the cells of multicellular organisms.

TFEB agonist clomiphene citrate activates the autophagy-lysosomal pathway and ameliorates Alzheimer's disease symptoms in mice.

Autophagy is a conserved eukaryotic cellular clearance and recycling process through the lysosome-mediated degradation of damaged organelles and protein aggregates to maintain homeostasis. Impairment of the autophagy-lysosomal pathway is implicated in the pathogenesis of Alzheimer's disease (AD). Transcription factor EB (TFEB) is a master regulator of autophagy and lysosomal biogenesis. Therefore, activating TFEB and autophagy provides a novel strategy for AD treatment. We previously described that clomiphene citrate (CC) promotes nuclear translocation of TFEB and increases autophagy and lysosomal biogenesis. In this study, 7 and 3-month-old APP/PS1 mice were treated with TFEB agonist CC and assessed. The behavioral tests were performed using Morris water maze and open field test. Additional changes in Aß pathology, autophagy and inflammatory response were determined. We found that CC activated TFEB and the autophagy-lysosomal pathway in neuronal cells. Moreover, using mouse model of Alzheimer's disease, CC treatment promoted clearance of Aß plaques and ameliorated cognitive function in both 7 and 3-month-old APP/PS1 mice. The CC-induced activation of TFEB occurs by promoting acetylation of TFEB for nuclear translocation. These findings provide a molecular mechanism for the TFEB-mediated activation of the autophagy-lysosome pathway by CC, which has the potential to be repurposed and applied in the treatment or prevention of AD.

ACLY alternative splicing correlates with cancer phenotypes.

ATP-citrate lyase (ACLY) links carbohydrate and lipid metabolism and provides nucleocytosolic acetyl-CoA for protein acetylation. ACLY has two major splice isoforms: the full-length canonical "long" isoform and an uncharacterized "short" isoform in which exon 14 is spliced out. Exon 14 encodes 10 amino acids within an intrinsically disordered region and includes at least one dynamically phosphorylated residue. Both isoforms are expressed in healthy tissues to varying degrees. Analysis of human transcriptomic data revealed that the percent spliced in (PSI) of exon 14 is increased in several cancers and correlated with poorer overall survival in a pan-cancer analysis, though not in individual tumor types. This prompted us to explore potential biochemical and functional differences between ACLY isoforms. Here, we show that there are no discernible differences in enzymatic activity or stability between isoforms or phosphomutants of ACLY in vitro. Similarly, both isoforms and phosphomutants were able to rescue ACLY functions, including fatty acid synthesis and bulk histone acetylation, when re-expressed in Acly knockout cells. Deletion of Acly exon 14 in mice did not overtly impact development or metabolic physiology nor did it attenuate tumor burden in a genetic model of intestinal cancer. Notably, expression of epithelial splicing regulatory protein 1 (ESRP1) is highly correlated with ACLY PSI. We report that ACLY splicing is regulated by ESRP1. In turn, both ESRP1 expression and ACLY PSI are correlated with specific immune signatures in tumors. Despite these intriguing patterns of ACLY splicing in healthy and cancer tissues, functional differences between the isoforms remain elusive.

Two enzymes contribute to citrate production in the mitochondrion of Toxoplasma gondii.

Citrate synthase catalyzes the first and the rate-limiting reaction of the tricarboxylic acid (TCA) cycle, producing citrate from the condensation of oxaloacetate and acetyl-coenzyme A. The parasitic protozoan Toxoplasma gondii has full TCA cycle activity, but its physiological roles remain poorly understood. In this study, we identified three proteins with predicted citrate synthase (CS) activities two of which were localized in the mitochondrion, including the 2-methylcitrate synthase (PrpC) that was thought to be involved in the 2-methylcitrate cycle, an alternative pathway for propionyl-CoA detoxification. Further analyses of the two mitochondrial enzymes showed that both had citrate synthase activity, but the catalytic efficiency of CS1 was much higher than that of PrpC. Consistently, the deletion of CS1 resulted in a significantly reduced flux of glucose-derived carbons into TCA cycle intermediates, leading to decreased parasite growth. In contrast, disruption of PrpC had little effect. On the other hand, simultaneous disruption of both CS1 and PrpC resulted in more severe metabolic changes and growth defects than a single deletion of either gene, suggesting that PrpC does contribute to citrate production under physiological conditions. Interestingly, deleting Δcs1 and Δprpc individually or in combination only mildly or negligibly affected the virulence of parasites in mice, suggesting that both enzymes are dispensable in vivo. The dispensability of CS1 and PrpC suggests that either the TCA cycle is not essential for the asexual reproduction of tachyzoites or there are other routes of citrate supply in the parasite mitochondrion.

Randomized, Multicenter Study to Assess the Effects of Different Doses of Sildenafil on Mortality in Adults With Pulmonary Arterial Hypertension.

BACKGROUND: Sildenafil, approved for pulmonary arterial hypertension (PAH), has a recommended adult dose of 20 mg TID, with a previously approved 5-mg TID dose by the US Food and Drug Administration. Safety concerns arose because of common off-label use of higher doses, particularly after pediatric data linked higher doses to increased mortality. To assess this, the Food and Drug Administration mandated a study evaluating the effects of various sildenafil doses on mortality in adults with PAH. METHODS: This randomized, double-blind study compared sildenafil at doses of 5, 20, or 80 mg TID in adults with PAH. The primary objective was noninferiority of 80 mg of sildenafil versus 5 mg for all-cause mortality. Secondary end points included time to clinical worsening and change in 6-minute walk distance at 6 months. Interim analyses were planned at 50% and 75% of the anticipated mortality events. Safety and tolerability were assessed in the intention-to-treat population. RESULTS: The study was halted after the first interim analysis, demonstrating noninferiority for 80 mg of sildenafil versus 5 mg. Of 385 patients enrolled across all dose groups, 78 died. The primary analysis showed a hazard ratio of 0.51 (99.7% CI, 0.22-1.21; P<0.001 for noninferiority) for overall survival comparing 80 mg of sildenafil with 5 mg. Time to clinical worsening favored 80 mg of sildenafil compared with 5 mg (hazard ratio, 0.44 [99.7% CI, 0.22-0.89]; P<0.001). Sildenafil at 80 mg improved 6-minute walk distance from baseline at 6 months compared with 5 mg (least square mean change, 18.9 m [95% CI, 2.99-34.86]; P=0.0201). No significant differences were found between 80 mg of sildenafil and 20 mg in mortality, clinical worsening, and 6-minute walk distance. Adverse event-related drug discontinuations were numerically higher with 80 mg of sildenafil. CONCLUSIONS: Sildenafil at 80 mg was noninferior to sildenafil at 5 mg when examining all-cause mortality in adults with PAH. Secondary efficacy end points favored 80 mg of sildenafil over 5 mg. On the basis of these findings, the Food and Drug Administration recently revoked the approval of 5 mg of sildenafil for adults with PAH, reinforced 20 mg TID as the recommended dose, and now allows dose titration up to 80 mg TID, if needed. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT02060487.

A dual-function RNA balances carbon uptake and central metabolism in Vibrio cholerae.

Bacterial small RNAs (sRNAs) are well known to modulate gene expression by base pairing with trans-encoded transcripts and are typically non-coding. However, several sRNAs have been reported to also contain an open reading frame and thus are considered dual-function RNAs. In this study, we discovered a dual-function RNA from Vibrio cholerae, called VcdRP, harboring a 29 amino acid small protein (VcdP), as well as a base-pairing sequence. Using a forward genetic screen, we identified VcdRP as a repressor of cholera toxin production and link this phenotype to the inhibition of carbon transport by the base-pairing segment of the regulator. By contrast, we demonstrate that the VcdP small protein acts downstream of carbon transport by binding to citrate synthase (GltA), the first enzyme of the citric acid cycle. Interaction of VcdP with GltA results in increased enzyme activity and together VcdR and VcdP reroute carbon metabolism. We further show that transcription of vcdRP is repressed by CRP allowing us to provide a model in which VcdRP employs two different molecular mechanisms to synchronize central metabolism in V. cholerae.

Stimulation of neutral lipid synthesis via viral growth factor signaling and ATP citrate lyase during vaccinia virus infection.

Fatty acid metabolism can provide various products essential for viral infections. How vaccinia virus (VACV), the prototype of poxviruses, modulates fatty acid metabolism is not well understood. Here, we show that VACV infection results in increased neutral lipid droplet synthesis, the organelles that play a crucial role in storing and mobilizing fatty acids for energy production via ß-oxidation. Citrate is the first tricarboxylic acid (TCA) cycle intermediate that can be transported to the cytosol to be converted to acetyl-CoA for de novo fatty acid biosynthesis. We found that VACV infection stimulates the S455 phosphorylation of ATP citrate lyase (ACLY), a pivotal enzyme that links citrate metabolism with lipid metabolism. We demonstrate that the inhibition of neutral lipid droplet synthesis and ACLY severely suppresses VACV replication. Remarkably, we found that virus growth factor (VGF)-induced signaling is essential for the VACV-mediated upregulation of ACLY phosphorylation and neutral lipid droplets. Finally, we report that VGF-induced EGFR-Akt pathway and ACLY phosphorylation are important for VACV stimulation of neutral lipid synthesis. These findings identified a new way of rewiring cell metabolism by a virus and a novel function for VGF in the governance of virus-host interactions through the induction of a key enzyme at the crossroads of the TCA cycle and fatty acid metabolism. Our study also provides a mechanism for the role played by VGF and its downstream signaling cascades in the modulation of lipid metabolism in VACV-infected cells.IMPORTANCENeutral lipid droplets are vital players in cellular metabolism. Here, we showed that VACV induces neutral lipid droplet synthesis in infected primary human foreskin fibroblasts and identified the cellular and viral factors needed. We identified VACV encoded growth factor (VGF) as an essential viral factor that induces cellular EGFR-Akt signaling to increase lipid droplets. Interestingly, VACV increases the S455 phosphorylation of ACLY, a key metabolic enzyme that sits at the crossroads of carbohydrate and lipid metabolism in a VGF-EGFR-Akt-dependent manner. We also found that ACLY is vital for VACV-induced lipid droplet synthesis. Our findings identified the modulation of ACLY by a virus and identified it as a potential target for antiviral development against pathogenic poxviruses. Our study also expands the role of growth factor signaling in boosting VACV replication by targeting fatty acid metabolism.

Vitamin D receptor alleviates lipid peroxidation in diabetic nephropathy by regulating ACLY/Nrf2/Keap1 pathway.

The membrane lipid damage caused by reactive oxygen species(ROS) and various peroxides, namely lipid peroxidation, plays an important role in the progression of diabetic nephropathy (DN).We previously reported that vitamin D receptor(VDR) plays an active role in DN mice by modulating autophagy disorders. However, it is unclear whether the ATP-citrate lyase (ACLY)/NF-E2-related factor-2 (Nrf2)/Kelch-like ECH-associated protein 1 (Keap1) pathway is associated with the reduction of lipid peroxidation by VDR in the DN model. We found that in the DN mouse model, VDR knockout significantly aggravated mitochondrial morphological damage caused by DN, increased the expression of ACLY, promoted the accumulation of ROS, lipid peroxidation products Malondialdehyde(MDA) and 4-hydroxy-2-nonenal (4-HNE),consumed the Nrf2/Keap1 system, thus increasing lipid peroxidation. However, the overexpression of VDR and intervention with the VDR agonist paricalcitol (Pari) can reduce the above damage. On the other hand, cellular experiments have shown that Pari can significantly reduce the elevated expression of ACLY and ROS induced by advanced glycation end products (AGE). However, ACLY overexpression partially eliminated the positive effects of the VDR agonist. Next, we verified the transcriptional regulation of ACLY by VDR through chromatin immunoprecipitation (ChIP)-qPCR and dual luciferase experiments. Moreover, in AGE models, knockdown of ACLY decreased lipid peroxidation and ROS production, while intervention with Nrf2 inhibitor ML385 partially weakened the protective effect of ACLY downregulation. In summary, VDR negatively regulates the expression of ACLY through transcription, thereby affecting the state of Nrf2/Keap1 system and regulating lipid peroxidation, thereby inhibiting kidney injury induced by DN.

Nuclear Acetyl-CoA Production by ACLY Promotes Homologous Recombination.

While maintaining the integrity of the genome and sustaining bioenergetics are both fundamental functions of the cell, potential crosstalk between metabolic and DNA repair pathways is poorly understood. Since histone acetylation plays important roles in DNA repair and is sensitive to the availability of acetyl coenzyme A (acetyl-CoA), we investigated a role for metabolic regulation of histone acetylation during the DNA damage response. In this study, we report that nuclear ATP-citrate lyase (ACLY) is phosphorylated at S455 downstream of ataxia telangiectasia mutated (ATM) and AKT following DNA damage. ACLY facilitates histone acetylation at double-strand break (DSB) sites, impairing 53BP1 localization and enabling BRCA1 recruitment and DNA repair by homologous recombination. ACLY phosphorylation and nuclear localization are necessary for its role in promoting BRCA1 recruitment. Upon PARP inhibition, ACLY silencing promotes genomic instability and cell death. Thus, the spatial and temporal control of acetyl-CoA production by ACLY participates in the mechanism of DNA repair pathway choice.

A specialized metabolic pathway partitions citrate in hydroxyapatite to impact mineralization of bones and teeth.

Citrate is a critical metabolic substrate and key regulator of energy metabolism in mammalian cells. It has been known for decades that the skeleton contains most (>85%) of the body's citrate, but the question of why and how this metabolite should be partitioned in bone has received singularly little attention. Here, we show that osteoblasts use a specialized metabolic pathway to regulate uptake, endogenous production, and the deposition of citrate into bone. Osteoblasts express high levels of the membranous Na+-dependent citrate transporter solute carrier family 13 member 5 (Slc13a5) gene. Inhibition or genetic disruption of Slc13a5 reduced osteogenic citrate uptake and disrupted mineral nodule formation. Bones from mice lacking Slc13a5 globally, or selectively in osteoblasts, showed equivalent reductions in cortical thickness, with similarly compromised mechanical strength. Surprisingly, citrate content in mineral from Slc13a5-/- osteoblasts was increased fourfold relative to controls, suggesting the engagement of compensatory mechanisms to augment endogenous citrate production. Indeed, through the coordinated functioning of the apical membrane citrate transporter SLC13A5 and a mitochondrial zinc transporter protein (ZIP1; encoded by Slc39a1), a mediator of citrate efflux from the tricarboxylic acid cycle, SLC13A5 mediates citrate entry from blood and its activity exerts homeostatic control of cytoplasmic citrate. Intriguingly, Slc13a5-deficient mice also exhibited defective tooth enamel and dentin formation, a clinical feature, which we show is recapitulated in primary teeth from children with SLC13A5 mutations. Together, our results reveal the components of an osteoblast metabolic pathway, which affects bone strength by regulating citrate deposition into mineral hydroxyapatite.

Carbon sources and pathways for citrate secreted by human prostate cancer cells determined by NMR tracing and metabolic modeling.

Prostate epithelial cells have the unique capacity to secrete large amounts of citrate, but the carbon sources and metabolic pathways that maintain this production are not well known. We mapped potential pathways for citrate carbons in the human prostate cancer metastasis cell lines LNCaP and VCaP, for which we first established that they secrete citrate (For LNCaP 5.6 ± 0.9 nmol/h per 106 cells). Using 13C-labeled substrates, we traced the incorporation of 13C into citrate by NMR of extracellular fluid. Our results provide direct evidence that glucose is a main carbon source for secreted citrate. We also demonstrate that carbons from supplied glutamine flow via oxidative Krebs cycle and reductive carboxylation routes to positions in secreted citrate but likely do not contribute to its net synthesis. The potential anaplerotic carbon sources aspartate and asparagine did not contribute to citrate carbons. We developed a quantitative metabolic model employing the 13C distribution in extracellular citrate after 13C glucose and pyruvate application to assess intracellular pathways of carbons for secreted citrate. From this model, it was estimated that in LNCaP about 21% of pyruvate entering the Krebs cycle is converted via pyruvate carboxylase as an anaplerotic route at a rate more than sufficient to compensate carbon loss of this cycle by citrate secretion. This model provides an estimation of the fraction of molecules, including citrate, leaving the Krebs cycle at every turn. The measured ratios of 13C atoms at different positions in extracellular citrate may serve as biomarkers for (malignant) epithelial cell metabolism.

Carrier-Free Nanoagent Interfering with Cancer-Associated Fibroblasts' Metabolism to Promote Tumor Penetration for Boosted Chemotherapy.

Elevated production of extracellular matrix (ECM) in tumor stroma is a critical obstacle for drug penetration. Here we demonstrate that ATP-citrate lyase (ACLY) is significantly upregulated in cancer-associated fibroblasts (CAFs) to produce tumor ECM. Using a self-assembling nanoparticle-design approach, a carrier-free nanoagent (CFNA) is fabricated by simply assembling NDI-091143, a specific ACLY inhibitor, and doxorubicin (DOX) or paclitaxel (PTX), the first-line chemotherapeutic drug, via multiple noncovalent interactions. After arriving at the CAFs-rich tumor site, NDI-091143-mediated ACLY inhibition in CAFs can block the de novo synthesis of fatty acid, thereby dampening the fatty acid-involved energy metabolic process. As the lack of enough energy, the energetic CAFs will be in a dispirited state that is unable to produce abundant ECM, thereby significantly improving drug perfusion in tumors and enhancing the efficacy of chemotherapy. Such a simple drug assembling strategy aimed at CAFs' ACLY-mediated metabolism pathway presents the feasibility of stromal matrix reduction to potentiate chemotherapy.

Inhibition of mitochondrial citrate shuttle alleviates metabolic syndromes induced by high-fat diet.

The mitochondrial citrate shuttle, which relies on the solute carrier family 25 member 1 (SLC25A1), plays a pivotal role in transporting citrate from the mitochondria to the cytoplasm. This shuttle supports glycolysis, lipid biosynthesis, and protein acetylation. Previous research has primarily focused on SLC25A1 in pathological models, particularly high-fat diet (HFD)-induced obesity. However, the impact of SLC25A1 inhibition on nutrient metabolism under HFD remains unclear. To address this gap, we used zebrafish (Danio rerio) and Nile tilapia (Oreochromis niloticus) to evaluate the effects of inhibiting Slc25a1. In zebrafish, we administered Slc25a1-specific inhibitors (CTPI-2) for 4 wk, whereas Nile tilapia received intraperitoneal injections of dsRNA to knock down slc25a1b for 7 days. Inhibition of the mitochondrial citrate shuttle effectively protected zebrafish from HFD-induced obesity, hepatic steatosis, and insulin resistance. Of note, glucose tolerance was unaffected. Inhibition of Slc25a1 altered hepatic protein acetylation patterns, with decreased cytoplasmic acetylation and increased mitochondrial acetylation. Under HFD conditions, Slc25a1 inhibition promoted fatty acid oxidation and reduced hepatic triglyceride (TAG) accumulation by deacetylating carnitine palmitoyltransferase 1a (Cpt1a). In addition, Slc25a1 inhibition triggered acetylation-induced inactivation of Pdhe1α, leading to a reduction in glucose oxidative catabolism. This was accompanied by enhanced glucose uptake and storage in zebrafish livers. Furthermore, Slc25a1 inhibition under HFD conditions activated the SIRT1/PGC1α pathway, promoting mitochondrial proliferation and enhancing oxidative phosphorylation for energy production. Our findings provide new insights into the role of nonhistone protein acetylation via the mitochondrial citrate shuttle in the development of hepatic lipid deposition and hyperglycemia caused by HFD.NEW & NOTEWORTHY The mitochondrial citrate shuttle is a crucial physiological process for maintaining metabolic homeostasis. In the present study, we found that inhibition of mitochondrial citrate shuttle (Slc25a1) could alleviate metabolic syndromes induced by high-fat diet (HFD) through remodeling hepatic protein acetylation modification. Briefly, Slc25a1 inhibition reduces hepatic triglyceride deposition by deacetylating Cpt1a and reduces glucose oxidative catabolism by acetylating Pdhe1α. Our study provides new insights into the treatment of diet-induced metabolic syndromes.

SIRT2-mediated deacetylation of ACLY promotes the progression of oesophageal squamous cell carcinoma.

ATP citrate lyase (ACLY), as a key enzyme in lipid metabolism, plays an important role in energy metabolism and lipid biosynthesis of a variety of tumours. Many studies have shown that ACLY is highly expressed in various tumours, and its pharmacological or gene inhibition significantly inhibits tumour growth and progression. However, the roles of ACLY in oesophageal squamous cell carcinoma (ESCC) remain unclear. Here, our data showed that ACLY inhibitor significantly attenuated cell proliferation, migration, invasion and lipid synthesis in different ESCC cell lines, whereas the proliferation, migration, invasion and lipid synthesis of ESCC cells were enhanced after ACLY overexpression. Furthermore, ACLY inhibitor dramatically suppressed tumour growth and lipid metabolism in ESCC cells xenografted tumour model, whereas ACLY overexpression displayed the opposite effect. Mechanistically, ACLY protein harboured acetylated modification and interacted with SIRT2 protein in ESCC cells. The SIRT2 inhibitor AGK2 significantly increased the acetylation level of ACLY protein and inhibited the proliferation and migration of ESCC cells, while overexpression of ACLY partially reversed the inhibitory effect of AGK2 on ESCC cells. Overall, these results suggest that targeting the SIRT2/ACLY signalling axis may be a potential therapeutic strategy for ESCC patients.

Differential modulation of Target of Rapamycin activity under single and combined iron and sulfur deficiency in tomato plants.

Over the past few decades, a close relationship between sulfur (S) and iron (Fe) in terms of functionality and nutrition was demonstrated in the tomato. However, very little is known about the regulatory mechanisms underlying S/Fe interactions. Recently, the potential role of citrate in plant adaptation to Fe deficiency and combined S and Fe deficiency has been described. It is known that an impaired organic acid metabolism may stimulate a retrograde signal, which has been proven to be linked to the Target of Rapamycin (TOR) signaling in yeast and animal cells. Recent reports provided evidence of TOR involvement in S nutrient sensing in plants. This suggestion prompted us to investigate whether TOR may play a role in the cross-talk of signaling pathway occurring during plant adaptation to combined nutrient deficiency of Fe and S. Our results revealed that Fe deficiency elicited an increase of TOR activity associated with enhanced accumulation of citrate. In contrast, S deficiency resulted in decreased TOR activity and citrate accumulation. Interestingly, citrate accumulated in shoots of plants exposed to combined S/Fe deficiency to values between those found in Fe- and S-deficient plants, again correlated with TOR activity level. Our results suggest that citrate might be involved in establishing a link between plant response to combined S/Fe deficiency and the TOR network.

A citrate efflux transporter important for manganese distribution and phosphorus uptake in rice.

The plant citrate transporters, functional in mineral nutrient uptake and homeostasis, usually belong to the multidrug and toxic compound extrusion transporter family. We identified and functionally characterized a rice (Oryza sativa) citrate transporter, OsCT1, which differs from known plant citrate transporters and is structurally close to rice silicon transporters. Domain analysis depicted that OsCT1 carries a bacterial citrate-metal transporter domain, CitMHS. OsCT1 showed citrate efflux activity when expressed in Xenopus laevis oocytes and is localized to the cell plasma membrane. It is highly expressed in the shoot and reproductive tissues of rice, and its promoter activity was visible in cells surrounding the vasculature. The OsCT1 knockout (KO) lines showed a reduced citrate content in the shoots and the root exudates, whereas overexpression (OE) line showed higher citrate exudation from their roots. Further, the KO and OE lines showed variations in the manganese (Mn) distribution leading to changes in their agronomical traits. Under deficient conditions (Mn-sufficient conditions followed by 8 days of 0 µm MnCl2 · 4H2 O treatment), the supply of manganese towards the newer leaf was found to be obstructed in the KO line. There were no significant differences in phosphorus (P) distribution; however, P uptake was reduced in the KO and increased in OE lines at the vegetative stage. Further, experiments in Xenopus oocytes revealed that OsCT1 could efflux citrate with Mn. In this way, we provide insights into a mechanism of citrate-metal transport in plants and its role in mineral homeostasis, which remains conserved with their bacterial counterparts.

Metabolic acidosis in chronic kidney disease: mere consequence or also culprit?

Metabolic acidosis is a frequent complication in non-transplant chronic kidney disease (CKD) and after kidney transplantation. It occurs when net endogenous acid production exceeds net acid excretion. While nephron loss with reduced ammoniagenesis is the main cause of acid retention in non-transplant CKD patients, additional pathophysiological mechanisms are likely inflicted in kidney transplant recipients. Functional tubular damage by calcineurin inhibitors seems to play a key role causing renal tubular acidosis. Notably, experimental and clinical studies over the past decades have provided evidence that metabolic acidosis may not only be a consequence of CKD but also a driver of disease. In metabolic acidosis, activation of hormonal systems and the complement system resulting in fibrosis have been described. Further studies of changes in renal metabolism will likely contribute to a deeper understanding of the pathophysiology of metabolic acidosis in CKD. While alkali supplementation in case of reduced serum bicarbonate < 22 mmol/l has been endorsed by CKD guidelines for many years to slow renal functional decline, among other considerations, beneficial effects and thresholds for treatment have lately been under intense debate. This review article discusses this topic in light of the most recent results of trials assessing the efficacy of dietary and pharmacological interventions in CKD and kidney transplant patients.