Acetyl coenzyme A (AcCoA) is the central biosynthetic precursor for fatty acid synthesis and protein acetylation. In the conventional view of mammalian cell metabolism, AcCoA is primarily generated from glucose-derived pyruvate through the citrate shuttle and adenosine triphosphate citrate lyase (ACL) in the cytosol1-3. However, proliferating cells that exhibit aerobic glycolysis and those exposed to hypoxia convert glucose to lactate at near stoichiometric levels, directing glucose carbon away from the tricarboxylic acid cycle (TCA) and fatty acid synthesis4. Although glutamine is consumed at levels exceeding that required for nitrogen biosynthesis5, the regulation and utilization of glutamine metabolism in hypoxic cells is not well understood. Here we show that human cells employ reductive metabolism of alpha-ketoglutarate (αKG) to synthesize AcCoA for lipid synthesis. This isocitrate dehydrogenase 1 (IDH1) dependent pathway is active in most cell lines under normal culture conditions, but cells grown under hypoxia rely almost exclusively on the reductive carboxylation of glutamine-derived αKG for de novo lipogenesis. Furthermore, renal cell lines deficient in the von Hippel-Lindau (VHL) tumor suppressor protein preferentially utilize reductive glutamine metabolism for lipid biosynthesis even at normal oxygen levels. These results identify a critical role for oxygen in regulating carbon utilization in order to produce AcCoA and support lipid synthesis in mammalian cells.

SUMMARY Hypoxic and VHL-deficient cells use glutamine to generate citrate and lipids through reductive carboxylation (RC) of α-ketoglutarate. To gain insights into the role of HIF and the molecular mechanisms underlying RC, we took advantage of a panel of disease-associated VHL mutants and showed that HIF expression is necessary and sufficient for the induction of RC in human renal cell carcinoma (RCC) cells. HIF expression drastically reduced intracellular citrate levels. Feeding VHL-deficient RCC cells with acetate or citrate or knocking down PDK-1 and ACLY restored citrate levels and suppressed RC. These data suggest that HIF-induced low intracellular citrate levels promote the reductive flux by mass action to maintain lipogenesis. Using [1–13C] glutamine, we demonstrated in vivo RC activity in VHL-deficient tumors growing as xenografts in mice. Lastly, HIF rendered VHL-deficient cells sensitive to glutamine deprivation in vitro, and systemic administration of glutaminase inhibitors suppressed the growth of RCC cells as mice xenografts.

Immunotherapy has revolutionized cancer treatment, but its efficacy is severely hindered by the lack of effective predictors. Herein, we developed a homogeneous, low‐volume, efficient, and sensitive exosomal programmed death‐ligand 1 (PD‐L1, a type of transmembrane protein) quantitation method for cancer diagnosis and immunotherapy response prediction (HOLMES‐ExoPD‐L1). The method combines a newly evolved aptamer that efficiently binds to PD‐L1 with less hindrance by antigen glycosylation than antibody, and homogeneous thermophoresis with a rapid binding kinetic. As a result, HOLMES‐ExoPD‐L1 is higher in sensitivity, more rapid in reaction time, and easier to operate than existing enzyme‐linked immunosorbent assay (ELISA)‐based methods. As a consequence of an outstanding improvement of sensitivity, the level of circulating exosomal PD‐L1 detected by HOLMES‐ExoPD‐L1 can effectively distinguish cancer patients from healthy volunteers, and for the first time was found to correlate positively with the metastasis of adenocarcinoma. Overall, HOLMES‐ExoPD‐L1 brings a fresh approach to exosomal PD‐L1 quantitation, offering unprecedented potential for early cancer diagnosis and immunotherapy response prediction.

Antimicrobial peptides (AMPs) play pivotal roles in the innate defense of vertebrates. A novel AMP (cathelicidin-PY) has been identified from the skin secretions of the frog Paa yunnanensis . Cathelicidin-PY has an amino acid sequence of RKCNFLCKLKEKLRTVITSHIDKVLRPQG. Nuclear magnetic resonance (NMR) spectroscopy analysis revealed that cathelicidin-PY adopts a tertiary structure with a mostly positively charged surface containing a helix (Thr15-Ser19). It possesses strong antimicrobial activity, low hemolytic activity, low cytotoxicity against RAW 264.7 cells, and strong anti-inflammatory activity. The action of antimicrobial activity of cathelicidin-PY is through the destruction of the cell membrane. Moreover, cathelicidin-PY exerts anti-inflammatory activity by inhibiting the production of nitric oxide (NO) and inflammatory cytokines such as tumor necrosis factor (TNF-α), interleukin-6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1). Cathelicidin-PY inhibits the activation of Toll-like receptor 4 (TLR4) inflammatory response pathways induced by lipopolysaccharide (LPS). The NMR titration experiments indicated that cathelicidin-PY can bind to LPS. In conclusion, we have identified a novel potent peptide antibiotic with both antimicrobial and anti-inflammatory activities and laid the groundwork for future research and development.

Biogenesis of iron-sulfur clusters requires a concerted delivery of iron and sulfur to target proteins. It is now clear that sulfur in iron-sulfur clusters is derived from L-cysteine via cysteine desulfurases. However, the specific iron donor for the iron-sulfur cluster assembly still remains elusive. Previous studies showed that IscA, a member of the iron-sulfur cluster assembly machinery in Escherichia coli, is a novel iron-binding protein, and that the iron-bound IscA can provide iron for the iron-sulfur cluster assembly in a proposed scaffold IscU in vitro. However, genetic studies have indicated that IscA is not essential for the cell growth of E. coli. In the present paper, we report that SufA, an IscA paralogue in E. coli, may represent the redundant activity of IscA. Although deletion of IscA or SufA has only a mild effect on cell growth, deletion of both IscA and SufA in E. coli results in a severe growth phenotype in minimal medium under aerobic growth conditions. Cell growth is restored when either IscA or SufA is re-introduced into the iscA-/sufA- double mutant, demonstrating further that either IscA or SufA is sufficient for their functions in vivo. Purified SufA, like IscA, is an iron-binding protein that can provide iron for the iron-sulfur cluster assembly in IscU in the presence of a thioredoxin reductase system which emulates the intracellular redox potential. Site-directed mutagenesis studies show that the SufA/IscA variants that lose the specific iron-binding activity fail to restore the cell growth of the iscA-/sufA- double mutant. The results suggest that SufA and IscA may constitute the redundant cellular activities to recruit intracellular iron and deliver iron for the iron-sulfur cluster assembly in E. coli.

Increasing evidence suggests that sulfur in ubiquitous ironsulfur clusters is derived from L-cysteine via cysteine desulfurases. In Escherichia coli, the major cysteine desulfurase activity for biogenesis of iron-sulfur clusters has been attributed to IscS. The gene that encodes IscS is a member of an operon isc-SUA, which also encodes two highly conserved proteins: IscU and IscA. Previous studies suggested that both IscU and IscA may act as the iron-sulfur cluster assembly scaffold proteins. However, recent evidence indicated that IscA is an iron-binding protein that can provide iron for the iron-sulfur cluster assembly in IscU (Ding, H., Harrison, K., and Lu, J. Iron-sulfur clusters are one of the most ancient and ubiquitous redox centers in almost all living organisms (1-3). Throughout evolution, iron-sulfur clusters have become integral parts of diverse biological processes including energy conversion and the regulation of gene expression. It is now clear that biogenesis of iron-sulfur clusters is not a spontaneous process. The pioneering work by Dean's group (4) revealed that sulfur in iron-sulfur clusters is derived from L-cysteine via cysteine desulfurases, a group of pyridoxal 5-phosphate-dependent enzymes that are conserved from bacteria to humans (5-7). In Escherichia coli, there are at least three cysteine desulfurases: IscS 2 (5), SufS (8), and CSD (cysteine sulfinate desulfinase) (9, 10). Deletion of gene iscS greatly diminishes the specific activities of iron-sulfur proteins in E. coli (11,12), suggesting that IscS is the major cysteine desulfurase for biogenesis of ironsulfur clusters. Gene iscS is a member of an operon iscSUA, which also encodes two highly conserved proteins: IscU and IscA (13,14). Biochemical studies indicated that IscS catalyzes desulfurization of L-cysteine and transfers sulfane sulfur for the iron-sulfur cluster assembly in IscU via specific protein-protein interactions (15-18). Accordingly, IscU was characterized as an iron-sulfur cluster assembly scaffold protein (15-21).The function of IscA, however, still remains elusive. Previous studies suggested that IscA is an alternative iron-sulfur cluster assembly scaffold protein (22-29), because IscA, like IscU, can bind iron-sulfur clusters in the presence of ferrous iron and sulfide in vitro. On the other hand, recent studies indicated that IscA is a novel iron binding protein with an iron association constant of 2.0 -3.0 ϫ 10 19 M Ϫ1 in the presence of the thioredoxin reductase system (30) or dithiothreitol (31) and that the iron-loaded IscA can provide iron for the iron-sulfur cluster assembly in IscU (32, 33). To reconcile the two models proposed for the function of IscA, here we re-evaluated the ironsulfur cluster binding activity of IscA and IscU under physiologically relevant conditions and found that in the presence of ferrous iron, L-cysteine, and cysteine desulfurase IscS, IscU is a preferred iron-sulfur cluster assembly scaffold protein. On the other hand, when L-cysteine is not present in the incubation solution, Isc...

SummaryThe nitric oxide (NO) cytotoxicity has been well documented in bacteria and mammalian cells. However, the underlying mechanism is still not fully understood. Here we report that transient NO exposure effectively inhibits cell growth of Escherichia coli in minimal medium under anaerobic growth conditions and that cell growth is restored when the NO-exposed cells are either supplemented with the branched-chain amino acids (BCAA) anaerobically or returned to aerobic growth conditions. The enzyme activity measurements show that dihydroxyacid dehydratase (

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