Cancer SLC6A6-mediated taurine uptake transactivates immune checkpoint genes and induces exhaustion in CD8+ T cells.

Taurine is used to bolster immunity, but its effects on antitumor immunity are unclear. Here, we report that cancer-related taurine consumption causes T cell exhaustion and tumor progression. The taurine transporter SLC6A6 is correlated with aggressiveness and poor outcomes in multiple cancers. SLC6A6-mediated taurine uptake promotes the malignant behaviors of tumor cells but also increases the survival and effector function of CD8+ T cells. Tumor cells outcompete CD8+ T cells for taurine by overexpressing SLC6A6, which induces T cell death and malfunction, thereby fueling tumor progression. Mechanistically, taurine deficiency in CD8+ T cells increases ER stress, promoting ATF4 transcription in a PERK-JAK1-STAT3 signaling-dependent manner. Increased ATF4 transactivates multiple immune checkpoint genes and induces T cell exhaustion. In gastric cancer, we identify a chemotherapy-induced SP1-SLC6A6 regulatory axis. Our findings suggest that tumoral-SLC6A6-mediated taurine deficiency promotes immune evasion and that taurine supplementation reinvigorates exhausted CD8+ T cells and increases the efficacy of cancer therapies.

Type 2 Diabetes Variants Disrupt Function of SLC16A11 through Two Distinct Mechanisms.

Type 2 diabetes (T2D) affects Latinos at twice the rate seen in populations of European descent. We recently identified a risk haplotype spanning SLC16A11 that explains ∼20% of the increased T2D prevalence in Mexico. Here, through genetic fine-mapping, we define a set of tightly linked variants likely to contain the causal allele(s). We show that variants on the T2D-associated haplotype have two distinct effects: (1) decreasing SLC16A11 expression in liver and (2) disrupting a key interaction with basigin, thereby reducing cell-surface localization. Both independent mechanisms reduce SLC16A11 function and suggest SLC16A11 is the causal gene at this locus. To gain insight into how SLC16A11 disruption impacts T2D risk, we demonstrate that SLC16A11 is a proton-coupled monocarboxylate transporter and that genetic perturbation of SLC16A11 induces changes in fatty acid and lipid metabolism that are associated with increased T2D risk. Our findings suggest that increasing SLC16A11 function could be therapeutically beneficial for T2D. VIDEO ABSTRACT.

The solute carrier SPNS2 recruits PI(4,5)P2 to synergistically regulate transport of sphingosine-1-phosphate.

Solute carrier spinster homolog 2 (SPNS2), one of only four known major facilitator superfamily (MFS) lysolipid transporters in humans, exports sphingosine-1-phosphate (S1P) across cell membranes. Here, we explore the synergistic effects of lipid binding and conformational dynamics on SPNS2's transport mechanism. Using mass spectrometry, we discovered that SPNS2 interacts preferentially with PI(4,5)P2. Together with functional studies and molecular dynamics (MD) simulations, we identified potential PI(4,5)P2 binding sites. Mutagenesis of proposed lipid binding sites and inhibition of PI(4,5)P2 synthesis reduce S1P transport, whereas the absence of the N terminus renders the transporter essentially inactive. Probing the conformational dynamics of SPNS2, we show how synergistic binding of PI(4,5)P2 and S1P facilitates transport, increases dynamics of the extracellular gate, and stabilizes the intracellular gate. Given that SPNS2 transports a key signaling lipid, our results have implications for therapeutic targeting and also illustrate a regulatory mechanism for MFS transporters.

Amino Acids and Their Transporters in T Cell Immunity and Cancer Therapy.

Metabolism reprogramming is critical for both cancer progression and effective immune responses in the tumor microenvironment. Amino acid metabolism in different cells and their cross-talk shape tumor immunity and therapy efficacy in patients with cancer. In this review, we focus on multiple amino acids and their transporters, solute carrier (SLC) members. We discuss their involvement in regulation of immune responses in the tumor microenvironment and assess their associations with cancer immunotherapy, chemotherapy, and radiation therapy, and we review their potential as targets for cancer therapy. We stress the necessity to understand individual amino acids and their transporters in different cell subsets, the molecular intersection between amino acid metabolism, and effective T cell immunity and its relevance in cancer therapies.

Two-pore channels regulate endomembrane tension to enable remodeling and resolution of phagolysosomes.

Phagocytes promptly resolve ingested targets to replenish lysosomes and maintain their responsiveness. The resolution process requires that degradative hydrolases, solute transporters, and proteins involved in lipid traffic are delivered and made active in phagolysosomes. It also involves extensive membrane remodeling. We report that cation channels that localize to phagolysosomes were essential for resolution. Specifically, the conductance of Na+ by two-pore channels (TPCs) and the presence of a Na+ gradient between the phagolysosome lumen and the cytosol were critical for the controlled release of membrane tension that permits deformation of the limiting phagolysosome membrane. In turn, membrane deformation was a necessary step to efficiently transport the cholesterol extracted from cellular targets, permeabilizing them to hydrolases. These results place TPCs as regulators of endomembrane remodeling events that precede target degradation in cases when the target is bound by a cholesterol-containing membrane. The findings may help to explain lipid metabolism dysfunction and autophagic flux impairment reported in TPC KO mice and establish stepwise regulation to the resolution process that begins with lysis of the target.

Ammonia transporter RhBG initiates downstream signaling and functional responses by activating NFκB.

Transceptors, solute transporters that facilitate intracellular entry of molecules and also initiate intracellular signaling events, have been primarily studied in lower-order species. Ammonia, a cytotoxic endogenous metabolite, is converted to urea in hepatocytes for urinary excretion in mammals. During hyperammonemia, when hepatic metabolism is impaired, nonureagenic ammonia disposal occurs primarily in skeletal muscle. Increased ammonia uptake in skeletal muscle is mediated by a membrane-bound, 12 transmembrane domain solute transporter, Rhesus blood group-associated B glycoprotein (RhBG). We show that in addition to its transport function, RhBG interacts with myeloid differentiation primary response-88 (MyD88) to initiate an intracellular signaling cascade that culminates in activation of NFκB. We also show that ammonia-induced MyD88 signaling is independent of the canonical toll-like receptor-initiated mechanism of MyD88-dependent NFκB activation. In silico, in vitro, and in situ experiments show that the conserved cytosolic J-domain of the RhBG protein interacts with the Toll-interleukin-1 receptor (TIR) domain of MyD88. In skeletal muscle from human patients, human-induced pluripotent stem cell-derived myotubes, and myobundles show an interaction of RhBG-MyD88 during hyperammonemia. Using complementary experimental and multiomics analyses in murine myotubes and mice with muscle-specific RhBG or MyD88 deletion, we show that the RhBG-MyD88 interaction is essential for the activation of NFkB but not ammonia transport. Our studies show a paradigm of substrate-dependent regulation of transceptor function with the potential for modulation of cellular responses in mammalian systems by decoupling transport and signaling functions of transceptors.

The ion-coupling mechanism of human excitatory amino acid transporters.

Excitatory amino acid transporters (EAATs) maintain glutamate gradients in the brain essential for neurotransmission and to prevent neuronal death. They use ionic gradients as energy source and co-transport transmitter into the cytoplasm with Na+ and H+ , while counter-transporting K+ to re-initiate the transport cycle. However, the molecular mechanisms underlying ion-coupled transport remain incompletely understood. Here, we present 3D X-ray crystallographic and cryo-EM structures, as well as thermodynamic analysis of human EAAT1 in different ion bound conformations, including elusive counter-transport ion bound states. Binding energies of Na+ and H+ , and unexpectedly Ca2+ , are coupled to neurotransmitter binding. Ca2+ competes for a conserved Na+ site, suggesting a regulatory role for Ca2+ in glutamate transport at the synapse, while H+ binds to a conserved glutamate residue stabilizing substrate occlusion. The counter-transported ion binding site overlaps with that of glutamate, revealing the K+ -based mechanism to exclude the transmitter during the transport cycle and to prevent its neurotoxic release on the extracellular side.

SLC16 Family: From Atomic Structure to Human Disease.

The solute carrier 16 (SLC16) family represents a diverse group of membrane proteins mediating the transport of monocarboxylates across biological membranes. Family members show a variety of functional roles ranging from nutrient transport and intracellular pH regulation to thyroid hormone homeostasis. Changes in the expression levels and transport function of certain SLC16 transporters are manifested in severe health disorders including cancer, diabetes, and neurological disorders. L-Lactate-transporting SLC16 family members play essential roles in the metabolism of certain tumors and became validated drug targets. This review illuminates the SLC16 family under a new light using structural information obtained from a SLC16 homolog. Furthermore, the role of these transporters in cancer metabolism and how their inhibition can contribute to anticancer therapy are discussed.

N6-methyladenosine modification of SLC38A7 promotes cell migration, invasion, oxidative phosphorylation, and mitochondrial function in gastric cancer.

Solute carrier (SLC) 38 family responsible for trans-membrane transport of neutral amino acids, plays a role in the proliferation, invasion, and metastasis of cancer cells, but its role in gastric cancer (GC) progression remains unclear. This study aimed to explore the biological effects of SLC38A7 and its regulatory mechanisms in GC. RNA expression data, tumor tissue specimens, and GC cell lines were used for bioinformatics and experimental analyses. Cell Counting Kit-8 assay, wound healing assay, and Transwell invasion assay were used to evaluate cell viability, migration, and invasion, respectively. Oxidative phosphorylation, mitochondrial membrane potential and expression of the critical proteins in the mitochondrial respiratory chain were assayed using extracellular flux analysis, flow cytometry, and Western blot, respectively. RNA immunoprecipitation assay was used to explore the mechanisms of N6-methyladenosine (m6A) methylation. SLC38A7 was upregulated in GC tissue and cell lines. SLC38A7 silencing suppressed cell viability, migration, invasion, oxidative phosphorylation, and mitochondrial function in cancer cells. SLC38A7 overexpression had the opposite biological effects. Interactions between SLC38A7 and methyltransferase like 3 (METTL3) or insulin-like growth factor 2 mRNA-binding protein 2 (IGF2BP2) were detected. SLC38A7 mRNA stability was maintained by METTL3/IGF2BP2 axis in an m6A-dependent manner. Our results suggest that SLC38A7, stabilized by METTL3 and IGF2BP2-mediated m6A methylation, enhances cell viability, migration, invasion, oxidative phosphorylation, and mitochondrial function in GC, highlighting its role as a potential therapeutic target for GC.

Identification of SLC25A46 interaction interfaces with mitochondrial membrane fusogens Opa1 and Mfn2.

Mitochondrial fusion requires the sequential merger of four bilayers to two. The outer-membrane solute carrier family 25 member (SLC25A46) interacts with both the outer and inner membrane dynamin family GTPases mitofusin 1/2 and optic atrophy 1 (Opa1). While SLC25A46 levels are known to affect mitochondrial morphology, how SLC25A46 interacts with mitofusin 1/2 and Opa1 to regulate membrane fusion is not understood. In this study, we use crosslinking mass spectrometry and AlphaFold 2 modeling to identify interfaces mediating an SLC25A46 interaction with Opa1 and Mfn2. We reveal that the bundle signaling element of Opa1 interacts with SLC25A46, and present evidence of an Mfn2 interaction involving the SLC25A46 cytosolic face. We validate these newly identified interaction interfaces and show that they play a role in mitochondrial network maintenance.