Myeloid-derived suppressor cells (MDSCs) possess immunosuppressive activities, which allow cancers to escape immune surveillance and become non-responsive to immune checkpoints blockade. Here we report hypoxia as a cause of MDSC accumulation. Using hepatocellular carcinoma (HCC) as a cancer model, we show that hypoxia, through stabilization of hypoxia-inducible factor-1 (HIF-1), induces ectoenzyme, ectonucleoside triphosphate diphosphohydrolase 2 (ENTPD2/CD39L1), in cancer cells, causing its overexpression in HCC clinical specimens. Overexpression of ENTPD2 is found as a poor prognostic indicator for HCC. Mechanistically, we demonstrate that ENTPD2 converts extracellular ATP to 5′-AMP, which prevents the differentiation of MDSCs and therefore promotes the maintenance of MDSCs. We further find that ENTPD2 inhibition is able to mitigate cancer growth and enhance the efficiency and efficacy of immune checkpoint inhibitors. Our data suggest that ENTPD2 may be a good prognostic marker and therapeutic target for cancer patients, especially those receiving immune therapy.

Cancer cells experience an increase in oxidative stress. The pentose phosphate pathway (PPP) is a major biochemical pathway that generates antioxidant NADPH. Here, we show that transketolase (TKT), an enzyme in the PPP, is required for cancer growth because of its ability to affect the production of NAPDH to counteract oxidative stress. We show that TKT expression is tightly regulated by the Nuclear Factor, Erythroid 2-Like 2 (NRF2)/Kelch-Like ECHAssociated Protein 1 (KEAP1)/BTB and CNC Homolog 1 (BACH1) oxidative stress sensor pathway in cancers. Disturbing the redox homeostasis of cancer cells by genetic knockdown or pharmacologic inhibition of TKT sensitizes cancer cells to existing targeted therapy (Sorafenib). Our study strengthens the notion that antioxidants are beneficial to cancer growth and highlights the therapeutic benefits of targeting pathways that generate antioxidants.M etabolic reprogramming has recently been recognized as a hallmark of cancer (1). Cancer cells preferentially use glycolysis instead of oxidative phosphorylation to generate energy even in the presence of oxygen (O 2 ). This metabolic shift, named the Warburg Effect, channels glucose intermediates for macromolecule and antioxidant synthesis. A very important metabolic pathway that connects with glycolysis is the pentose phosphate pathway (PPP). The major goal of the PPP is the production of ribose-5-phosphate (R5P) and NADPH. R5P is the major backbone of RNA and is critical to nucleotide synthesis. NADPH is the major antioxidant that maintains the two major redox molecules, glutathione and thioredoxin, in the reduced state. NADPH therefore counteracts reactive oxygen species (ROS), enabling cancer cells to survive oxidative stress.The PPP is composed of the oxidative and nonoxidative arms. The oxidative arm of the PPP produces NADPH and ribose by three irreversible steps. First, glucose-6-phosphate dehydrogenase (G6PD) converts glucose-6-phosphate (G6P) to 6-phospho-gluconolactone and NAPDH. Second, phosphogluconolactonase converts 6-phospho-gluconolactone to 6-phosphogluconate. Third, 6-phosphogluconate dehydrogenase converts 6-phosphogluconate to ribulose-5-phosphate (Ru5P) and NAPDH. Ru5P then enters the nonoxidative arm of the PPP. Ru5P is converted to xylulose-5-phosphate (X5P) and Ru5P by epimerase and isomerase, respectively. The transketolase (TKT) family [transketolase-like 1 (TKTL1) and TKTL2] transfers two-carbon groups from X5P to R5P to generate sedoheptulose-7-phosphate (S7P) to glyceraldehyde-3-phosphate (G3P). Transaldolase (TALDO) transfers three-carbon groups from S7P to G3P to generate erythrose-4-phosphate (E4P) and fructose-6-phosphate (F6P). Finally, TKT transfers two-carbon groups from X5P to E4P to generate G3P and F6P, which reenter glycolysis. All enzymes in the nonoxidative arm of the PPP are reversible, allowing cells to adapt to the dynamic metabolic demands. When cells experience high oxidative stress, metabolites from the nonoxidative arm are rechanneled into glycolysis to refill the oxidative arm for...

The roles of caveolin-1 (cav-1) in regulating blood-brain barrier (BBB) permeability are unclear yet. We previously reported that cav-1 was down-regulated and the production of nitric oxide (NO) induced the loss of cav-1 in focal cerebral ischemia and reperfusion injury. The present study aims to address whether the loss of cav-1 impacts on BBB permeability and matrix metalloproteinases (MMPs) activity during cerebral ischemia-reperfusion injury. We found that focal cerebral ischemia-reperfusion down-regulated the expression of cav-1 in isolated cortex microvessels, hippocampus, and cortex of ischemic brain. The down-regulation of cav-1 was correlated with the increased MMP-2 and -9 activities, decreased tight junction (TJ) protein zonula occludens (ZO)-1 expression and enhanced BBB permeability. Treatment of N Gnitro-L-arginine methyl ester [L-NAME, a non-selective nitric oxide synthase (NOS) inhibitor] reserved the expression of cav-1, inhibited MMPs activity, and reduced BBB permeability.To elucidate the roles of cav-1 in regulating MMPs and BBB permeability, we used two approaches including cav-1 knockdown in cultured brain microvascular endothelial cells (BMECs) in vitro and cav-1 knockout (KO) mice in vivo. Cav-1 knockdown remarkably increased MMPs activity in BMECs. Meanwhile, with focal cerebral ischemia-reperfusion, cav-1 deficiency mice displayed higher MMPs activities and BBB permeability than wild-type mice. Interestingly, the effects of L-NAME on MMPs activity and BBB permeability was partly reversed in cav-1 deficiency mice. These results, when taken together, suggest that cav-1 plays important roles in regulating MMPs activity and BBB permeability in focal cerebral ischemia and reperfusion injury. The effects of L-NAME on MMPs activity and BBB permeability are partly mediated by preservation of cav-1. Keywords: blood-brain barrier, caveolin-1, ischemia, nitric oxide synthase.