SLC16 Family: From Atomic Structure to Human Disease

Deciphering metabolic heterogeneity in retinoblastoma unravels the role of monocarboxylate transporter 1 in tumor progression

BACKGROUND

Tumors exhibit metabolic heterogeneity, influencing cancer progression. However, understanding metabolic diversity in retinoblastoma (RB), the primary intraocular malignancy in children, remains limited.

METHODS

The metabolic landscape of RB was constructed based on single-cell transcriptomic sequencing from 11 RB and 5 retina samples. Various analyses were conducted, including assessing overall metabolic activity, metabolic heterogeneity, and the correlation between hypoxia and metabolic pathways. Additionally, the expression pattern of the monocarboxylate transporter (MCT) family in different cell clusters was examined. Validation assays of MCT1 expression and function in RB cell lines were performed. The therapeutic potential of targeting MCT1 was evaluated using an orthotopic xenograft model. A cohort of 47 RB patients was analyzed to evaluate the relationship between MCT1 expression and tumor invasion.

RESULTS

Distinct metabolic patterns in RB cells, notably increased glycolysis, were identified. This metabolic heterogeneity correlated closely with hypoxia. MCT1 emerged as the primary monocarboxylate transporter in RB cells. Disrupting MCT1 altered cell viability and energy metabolism. In vivo studies using the MCT1 inhibitor AZD3965 effectively suppressed RB tumor growth. Additionally, a correlation between MCT1 expression and optic nerve invasion in RB samples suggested prognostic implications.

CONCLUSIONS

This study enhances our understanding of RB metabolic characteristics at the single-cell level, highlighting the significance of MCT1 in RB pathogenesis. Targeting MCT1 holds promise as a therapeutic strategy for combating RB, with potential prognostic implications.

The crosstalk between glucose metabolism and telomerase regulation in cancer

Accumulated alterations in metabolic control provide energy and anabolic demands for enhanced cancer cell proliferation. Exemplified by the Warburg effect, changes in glucose metabolism during cancer progression are widely recognized as a characteristic of metabolic disorders. Since telomerases are a vital factor in maintaining DNA integrity and stability, any damage threatening telomerases could have a severe impact on DNA and, subsequently, whole-cell homeostasis. However, it remains unclear whether the regulation of glucose metabolism in cancer is connected to the regulation of telomerase. In this review, we present the latest insights into the crosstalk between telomerase function and glucose metabolism in cancer cells. However, at this moment this subject is not well investigated that the association is mostly indirectly regulations and few explicit regulating pathways were identified between telomerase and glucose metabolism. Therefore, the information presented in this review can provide a scientific basis for further research on the detail mechanism and the clinical application of cancer therapy, which could be valuable in improving the effectiveness of chemotherapy.

Lower GLUT1 and unchanged MCT1 in Alzheimer's disease cerebrovasculature

The brain is a highly demanding organ, utilizing mainly glucose but also ketone bodies as sources of energy. Glucose transporter-1 (GLUT1) and monocarboxylates transporter-1 (MCT1) respectively transport glucose and ketone bodies across the blood-brain barrier. While reduced glucose uptake by the brain is one of the earliest signs of Alzheimer's disease (AD), no change in the uptake of ketone bodies has been evidenced yet. To probe for changes in GLUT1 and MCT1, we performed Western immunoblotting in microvessel extracts from the parietal cortex of 60 participants of the Religious Orders Study. Participants clinically diagnosed with AD had lower cerebrovascular levels of GLUT1, whereas MCT1 remained unchanged. GLUT1 reduction was associated with lower cognitive scores. No such association was found for MCT1. GLUT1 was inversely correlated with neuritic plaques and cerebrovascular β-secretase-derived fragment levels. No other significant associations were found between both transporters, markers of Aβ and tau pathologies, sex, age at death or apolipoprotein-ε4 genotype. These results suggest that, while a deficit of GLUT1 may underlie the reduced transport of glucose to the brain in AD, no such impairment occurs for MCT1. This study thus supports the exploration of ketone bodies as an alternative energy source for the aging brain.

pH regulators and their inhibitors in tumor microenvironment

As an important characteristic of tumor, acidic tumor microenvironment (TME) is closely related to immune escape, invasion, migration and drug resistance of tumor. The acidity of the TME mainly comes from the acidic products produced by the high level of tumor metabolism, such as lactic acid and carbon dioxide. pH regulators such as monocarboxylate transporters (MCTs), carbonic anhydrase IX (CA IX), and Na+/H+ exchange 1 (NHE1) expel protons directly or indirectly from the tumor to maintain the pH balance of tumor cells and create an acidic TME. We review the functions of several pH regulators involved in the construction of acidic TME, the structure and structure-activity relationship of pH regulator inhibitors, and provide strategies for the development of small-molecule antitumor inhibitors based on these targets.

Molecular evidence for the adaptive evolution in euryhaline bivalves

Marine bivalves inhabiting intertidal and estuarine areas are frequently exposed to salinity stress due to persistent rainfall and drought. Through prolonged adaptive evolution, numerous bivalves have developed eurysalinity, which are capable of tolerating a wide range of salinity fluctuations through the sophisticated regulation of physiological metabolism. Current research has predominantly focused on investigating the physiological responses of bivalves to salinity stress, leaving a significant gap in our understanding of the adaptive evolutionary characteristics in euryhaline bivalves. Here, comparative genomics analyses were performed in two groups of bivalve species, including 7 euryhaline species and 5 stenohaline species. We identified 24 significantly expanded gene families and 659 positively selected genes in euryhaline bivalves. A significant co-expansion of solute carrier family 23 (SLC23) facilitates the transmembrane transport of ascorbic acids in euryhaline bivalves. Positive selection of antioxidant genes, such as GST and TXNRD, augments the capacity of active oxygen species (ROS) scavenging under salinity stress. Additionally, we found that the positively selected genes were significantly enriched in KEGG pathways associated with carbohydrates, lipids and amino acids metabolism (ALDH, ADH, and GLS), as well as GO terms related to transmembrane transport and inorganic anion transport (SLC22, CLCND, and VDCC). Positive selection of MCT might contribute to prevent excessive accumulation of intracellular lactic acids during anaerobic metabolism. Positive selection of PLA2 potentially promote the removal of damaged membranes lipids under salinity stress. Our findings suggest that adaptive evolution has occurred in osmoregulation, ROS scavenging, energy metabolism, and membrane lipids adjustments in euryhaline bivalves. This study enhances our understanding of the molecular mechanisms underlying the remarkable salinity adaption of euryhaline bivalves.

Paralog-dependent isogenic cell assay cascade generates highly selective SLC16A3 inhibitors

Despite being considered druggable and attractive therapeutic targets, most of the solute carrier (SLC) membrane transporters remain pharmacologically underexploited. One of the reasons for this is a lack of reliable chemical screening assays, made difficult by functional redundancies among SLCs. In this study we leveraged synthetic lethality between the lactate transporters SLC16A1 and SLC16A3 in a screening strategy that we call paralog-dependent isogenic cell assay (PARADISO). The system involves five isogenic cell lines, each dependent on various paralog genes for survival/fitness, arranged in a screening cascade tuned for the identification of SLC16A3 inhibitors. We screened a diversity-oriented library of ∼90,000 compounds and further developed our hits into slCeMM1, a paralog-selective and potent SLC16A3 inhibitor. By implementing chemoproteomics, we showed that slCeMM1 is selective also at the proteome-wide level, thus fulfilling an important criterion for chemical probes. This study represents a framework for the development of specific cell-based drug discovery assays.

Surfaceome Profiling Identifies Basigin-Chaperoned Protein Clients

The surface proteome or "surfaceome" is a critical mediator of cellular biology, facilitating cell-to-cell interactions and communication with extracellular biomolecules. Constituents of the surfaceome can serve as biomarkers for changing cell states and as targets for pharmacological intervention. While some pathways of cell surface trafficking are well characterized to allow prediction of surface localization, some non-canonical trafficking mechanisms do not. Basigin (Bsg), a cell surface glycoprotein, has been shown to chaperone protein clients to the cell surface. However, understanding which proteins are served by Bsg is not always straightforward. To accelerate such identification, we applied a surfaceome proximity labeling method that is integrated with quantitative mass spectrometry-based proteomics to discern changes in the surfaceome of hepatic stellate cells that occur in response to the genetic loss of Bsg. Using this strategy, we observed that the loss of Bsg leads to corresponding reductions in the cell surface expression of monocarboxylate transporters MCT1 and MCT4. We also found that these relationships were unique to Bsg and not found in neuroplastin (Nptn), a related family member. These results establish the utility of the surfaceome proximity labeling method to determine clients of cell surface chaperone proteins.

Targeting monocarboxylate transporters (MCTs) in cancer: How close are we to the clinics?

As a result of metabolic reprogramming, cancer cells display high rates of glycolysis, causing an excess production of lactate along with an increase in extracellular acidity. Proton-linked monocarboxylate transporters (MCTs) are crucial in the maintenance of this metabolic phenotype, by mediating the proton-coupled lactate flux across cell membranes, also contributing to cancer cell pH regulation. Among the proteins codified by the SLC16 gene family, MCT1 and MCT4 isoforms are the most explored in cancers, being overexpressed in many cancer types, from solid tumours to haematological malignancies. Similarly to what occurs in particular physiological settings, MCT1 and MCT4 are able to mediate lactate shuttles among cancer cells, and also between cancer and stromal cells in the tumour microenvironment. This form of metabolic cooperation is responsible for important cancer aggressiveness features, such as cell proliferation, survival, angiogenesis, migration, invasion, metastasis, immune tolerance and therapy resistance. The growing understanding of MCT functions and regulation is offering a new path to the design of novel inhibitors that can be foreseen in clinical practices. This review provides an overview of the role of MCT isoforms in cancer and summarizes the recent advances in their pharmacological targeting, highlighting the potential of new potent and selective MCT1 and/or MCT4 inhibitors in cancer therapeutics, and anticipating its inclusion in clinical practice.

MCT1-dependent energetic failure and neuroinflammation underlie optic nerve degeneration in Wolfram syndrome mice

Wolfram syndrome 1 (WS1) is a rare genetic disorder caused by mutations in the WFS1 gene leading to a wide spectrum of clinical dysfunctions, among which blindness, diabetes, and neurological deficits are the most prominent. WFS1 encodes for the endoplasmic reticulum (ER) resident transmembrane protein wolframin with multiple functions in ER processes. However, the WFS1-dependent etiopathology in retinal cells is unknown. Herein, we showed that Wfs1 mutant mice developed early retinal electrophysiological impairments followed by marked visual loss. Interestingly, axons and myelin disruption in the optic nerve preceded the degeneration of the retinal ganglion cell bodies in the retina. Transcriptomics at pre-degenerative stage revealed the STAT3-dependent activation of proinflammatory glial markers with reduction of the homeostatic and pro-survival factors glutamine synthetase and BDNF. Furthermore, label-free comparative proteomics identified a significant reduction of the monocarboxylate transport isoform 1 (MCT1) and its partner basigin that are highly enriched on retinal glia and myelin-forming oligodendrocytes in optic nerve together with wolframin. Loss of MCT1 caused a failure in lactate transfer from glial to neuronal cell bodies and axons leading to a chronic hypometabolic state. Thus, this bioenergetic impairment is occurring concurrently both within the axonal regions and cell bodies of the retinal ganglion cells, selectively endangering their survival while impacting less on other retinal cells. This metabolic dysfunction occurs months before the frank RGC degeneration suggesting an extended time-window for intervening with new therapeutic strategies focused on boosting retinal and optic nerve bioenergetics in WS1.

Single-Cell Transcriptomics Reveals the Expression of Aging- and Senescence-Associated Genes in Distinct Cancer Cell Populations

The human aging process is associated with molecular changes and cellular degeneration, resulting in a significant increase in cancer incidence with age. Despite their potential correlation, the relationship between cancer- and ageing-related transcriptional changes is largely unknown. In this study, we aimed to analyze aging-associated transcriptional patterns in publicly available bulk mRNA-seq and single-cell RNA-seq (scRNA-seq) datasets for chronic myelogenous leukemia (CML), colorectal cancer (CRC), hepatocellular carcinoma (HCC), lung cancer (LC), and pancreatic ductal adenocarcinoma (PDAC). Indeed, we detected that various aging/senescence-induced genes (ASIGs) were upregulated in malignant diseases compared to healthy control samples. To elucidate the importance of ASIGs during cell development, pseudotime analyses were performed, which revealed a late enrichment of distinct cancer-specific ASIG signatures. Notably, we were able to demonstrate that all cancer entities analyzed in this study comprised cell populations expressing ASIGs. While only minor correlations were detected between ASIGs and transcriptome-wide changes in PDAC, a high proportion of ASIGs was induced in CML, CRC, HCC, and LC samples. These unique cellular subpopulations could serve as a basis for future studies on the role of aging and senescence in human malignancies.

High Expression of SLC16A1 as a Biomarker to Predict Poor Prognosis of Urological Cancers

Objective

Tumor metabolism has always been the focus of cancer research. SLC16A1, as a key factor in catalysis of monocarboxylate transport across the plasma membrane, has been found to be associated with the occurrence and metastasis of a variety of cancers, but its prognostic significance and mechanism in different tumors are still unclear.

Methods

Based on the gene expression matrix and clinical information of human cancer tissues acquired from TCGA and GTEX databases, the differential expression of SLC16A1 in different tumors and normal tissues was analyzed. To confirm the association between its expression, the mutation of MMRS gene, and the expression level of DNMTs. Univariate Cox regression was applied to analyze the association between SLC16A1 expression and patient prognosis. The effect of SLC16A1 expression on patient survival was examined by Kaplan Meier analysis. GSEA was used to identify related signaling pathways.

Results

The expression of SLC16A1 was differentially expressed in most tumors, especially in the urinary tract where it is commonly highly expressed, and differential expression of SLC16A1 in different clinical stages. SLC16A1 expression was significantly positively correlated with MMRS gene mutation and DNMTS expression. Moreover, high SLC16A1 expression was associated with poorer overall survival (OS) and progression-free survival (PFS) in urological cancers. In particular, the results of the enrichment analysis showed that SLC16A1 was associated with processes such as cell adhesion and many signaling pathways affecting cell cycle were significantly enriched in the group with high-expressed SLC16A1.

Conclusion

SLC16A1 expression was upregulated in urological cancer. SLC16A1 may promote tumor development by regulating the epigenetic process of urological cancer and demonstrated a great potential as a prognostic biomarker of urological cancer patients.

The making of a potent L-lactate transport inhibitor

L-lactate is an important metabolite, energy source, and signaling molecule in health and disease. In mammals, its transport across biological membranes is mediated by monocarboxylate transporters (MCTs) of the solute carrier 16 (SLC16) family. Malfunction, overexpression or absence of transporters of this family are associated with diseases such as cancer and type 2 diabetes. Moreover, lactate acts as a signaling molecule and virulence factor in certain bacterial infections. Here, we report the rational, structure-guided identification of potent, nanomolar affinity inhibitors acting on an L-lactate-specific SLC16 homologue from the bacterium Syntrophobacter fumaroxidans (SfMCT). High-resolution crystal structures of SfMCT with bound inhibitors uncovered their interaction mechanism on an atomic level and the role of water molecules in inhibitor binding. The presented systematic approach is a valuable procedure for the identification of L-lactate transport inhibitors. Furthermore, identified inhibitors represent potential tool compounds to interfere with monocarboxylate transport across biological membranes mediated by MCTs.

Cysteine 159 delineates a hinge region of the alternating access monocarboxylate transporter 1 and is targeted by cysteine-modifying inhibitors

Monocarboxylate transporter isoforms 1-4, MCT, of the solute carrier SLC16A family facilitate proton-coupled transport of l-lactate. Growth of tumors that exhibit the Warburg effect, that is, high rates of anaerobic glycolysis despite availability of oxygen, relies on swift l-lactate export, whereas oxygenic cancer cells import circulating l-lactate as a fuel. Currently, MCTs are viewed as promising anticancer targets. Small-molecule inhibitors have been found, and, recently, high-resolution protein structures have been obtained. Key questions, however, regarding the exact binding sites of cysteine-modifying inhibitors and the substrate translocation cycle lack a conclusive experimental basis. Here, we report Cys159 of the ubiquitous human MCT1 to reside in a critical hinge region of the alternating access-type transporter. We identified Cys159 as the binding site of the organomercurial pCMBS. The inhibitory effect of pCMBS was proposed to be indirect via modification of the chaperone basigin. We provide evidence that pCMBS locks MCT1 in its outward open conformation in a wedge-like fashion. We corroborated this finding using smaller cysteine-modifying reagents that size-dependently inhibited l-lactate transport. The smallest modifiers targeted additional cysteines as shown by a C159S mutant. We found a Cys399/Cys400 pair to constitute the second hinge of the transporter that tolerated only individual replacement by serine. The hinge cysteines, in particular the selectively addressable Cys159, provide natural anchors for placing probes into MCTs to report, for instance, on the electrostatics or hydration upon binding of the transported l-lactate substrate and the proton cosubstrate.

MCT2 overexpression promotes recovery of cognitive function by increasing mitochondrial biogenesis in a rat model of stroke

Monocarboxylate transporter 2 (MCT2) is the predominant monocarboxylate transporter expressed by neurons. MCT2 plays an important role in brain energy metabolism. Stroke survivors are at high risk of cognitive impairment. We reported previously that stroke-induced cognitive impairment was related to impaired energy metabolism. In the present study, we report that cognitive function was impaired after stroke in rats. We found that MCT2 expression, but not that of MCT1 or MCT4, was markedly decreased in the rat hippocampus at 7 and 28 days after transient middle cerebral artery occlusion (tMCAO). Moreover, MCT2 overexpression promoted recovery of cognitive function after stroke. The molecular mechanism underlying these effects may be related to an increase in adenosine monophosphate-activated protein kinase-mediated mitochondrial biogenesis induced by overexpression of MCT2. Our findings suggest that MCT2 activation ameliorates cognitive impairment after stroke.

From Proteomic Mapping to Invasion-Metastasis-Cascade Systemic Biomarkering and Targeted Drugging of Mutant BRAF-Dependent Human Cutaneous Melanomagenesis

Simple Summary

Despite the recent advances in human malignancy therapy, metastasis and chemoresistance remain the principal causes of cancer-derived deaths. Given the fatal forms of cutaneous metastatic melanoma, we herein employed primary (WM115) and metastatic (WM266-4) melanoma cells, both obtained from the same patient, to identify novel biomarkers and therapeutic agents. Through state-of-the-art technologies including deep proteome landscaping, immunofluorescence phenotyping, and drug toxicity screening, we were able to describe new molecular programs, oncogenic drivers, and drug regimens, controlling the invasion-metastasis cascade during BRAF^V600D-dependent melanomagenesis. It proved that proteomic navigation could foster the development of systemic biomarkering and targeted drugging for successful treatment of advanced disease.

Abstract

Melanoma is classified among the most notoriously aggressive human cancers. Despite the recent progress, due to its propensity for metastasis and resistance to therapy, novel biomarkers and oncogenic molecular drivers need to be promptly identified for metastatic melanoma. Hence, by employing nano liquid chromatography-tandem mass spectrometry deep proteomics technology, advanced bioinformatics algorithms, immunofluorescence, western blotting, wound healing protocols, molecular modeling programs, and MTT assays, we comparatively examined the respective proteomic contents of WM115 primary (n = 3955 proteins) and WM266-4 metastatic (n = 6681 proteins) melanoma cells. It proved that WM115 and WM266-4 cells have engaged hybrid epithelial-to-mesenchymal transition/mesenchymal-to-epithelial transition states, with TGF-β controlling their motility in vitro. They are characterized by different signatures of SOX-dependent neural crest-like stemness and distinct architectures of the cytoskeleton network. Multiple signaling pathways have already been activated from the primary melanoma stage, whereas HIF1α, the major hypoxia-inducible factor, can be exclusively observed in metastatic melanoma cells. Invasion-metastasis cascade-specific sub-routines of activated Caspase-3-triggered apoptosis and LC3B-II-dependent constitutive autophagy were also unveiled. Importantly, WM115 and WM266-4 cells exhibited diverse drug response profiles, with epirubicin holding considerable promise as a beneficial drug for metastatic melanoma clinical management. It is the proteome navigation that enables systemic biomarkering and targeted drugging to open new therapeutic windows for advanced disease.