Monocarboxylate Transporters 1 and 2 Are Responsible for L-Lactate Uptake in Differentiated Human Neuroblastoma SH-SY5Y Cells.

L-Lactate transport via monocarboxylate transporters (MCTs) in the central nervous system, represented by the astrocyte-neuron lactate shuttle (ANLS), is crucial for the maintenance of brain functions, including memory formation. Previously, we have reported that MCT1 contributes to L-lactate transport in normal human astrocytes. Therefore, in this study, we aimed to identify transporters that contribute to L-lactate transport in human neurons. SH-SY5Y cells, which are used as a model for human neurons, were differentiated using all-trans-retinoic acid. L-Lactate uptake was measured using radiolabeled L-lactate, and the expression of MCT proteins was confirmed Western blotting. L-Lactate transport was pH-dependent and saturated at high concentrations. Kinetic analysis suggested that L-lactate uptake was biphasic. Furthermore, MCT1, 2 selective inhibitors inhibited L-lactate transport. In addition, the expression of MCT1 and 2 proteins, but not MCT4, was confirmed. In this study, we demonstrated that MCT1 and 2 are major contributors to L-lactate transport in differentiated human neuroblastoma SH-SY5Y cells from the viewpoint of kinetic analysis. These results lead to a better understanding of ANLS in humans, and further exploration of the factors that can promote MCT1 and 2 functions is required.

Atorvastatin Exerts More Selective Inhibitory Effects on hMCT2 than on hMCT1 and hMCT4.

BACKGROUND/AIM: Human monocarboxylate transporter 1 (hMCT1), hMCT2, and hMCT4 transport monocarboxylates, such as L-lactate and pyruvate, with pH dependency. They are often over-expressed in various cancer cells and mediate the energy balance and pH homeostasis. Therefore, hMCT inhibitors can potentially be used as anticancer drugs. However, isoform-selective inhibitors have not yet been well-characterized. In addition, several statins and 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors have been reported to inhibit hMCTs, but their selectivity has not yet been evaluated. In this study, we aimed to determine whether statins could inhibit hMCT1, hMCT2, and hMCT4. MATERIALS AND METHODS: We expressed hMCT1, hMCT2, and hMCT4 in a heterologous expression system of Xenopus oocytes and performed inhibitory experiments with various statins (fluvastatin, atorvastatin, simvastatin, rosuvastatin, pravastatin, and pitavastatin). As the three-dimensional structure of hMCT2 has been recently reported, docking simulations of statins and their structures were also performed to estimate the inhibition site. RESULTS: All statins inhibited the transport activities of hMCT1, hMCT2, and hMCT4. In addition, atorvastatin was found to be a potent isoform-selective inhibitor of hMCT2. Docking simulation indicated that atorvastatin could interact with a site surrounded by transmembrane (TM)-2, TM11, and intracellular helix in the TM6/7loop. Therefore, targeting this site may lead to the discovery of more potent hMCT2-selective inhibitors. CONCLUSION: Atorvastatin exerts selective inhibitory effects on hMCT2. These findings provide insights into the inhibitory mechanism of statins against hMCT1, hMCT2, and hMCT4 and may aid in the development of novel anticancer agents.

5-Oxoproline Enhances 4-Hydroxytamoxifen-induced Cytotoxicity by Increasing Oxidative Stress in MCF-7 Breast Cancer Cells.

BACKGROUND/AIM: Monocarboxylate transporters (MCTs) transport short-chain monocarboxylates, such as lactate, and have been reported to be related to poor prognosis in breast cancer. Our previous studies showed that a high glucose state altered MCT expression and changed the sensitivity of the tamoxifen active metabolite 4-hydroxytamoxifen (4-OHT) via hypoxia-inducible factor-1α (HIF-1α) protein expression. We hypothesized that MCT inhibitors affect 4-OHT-induced cytotoxicity under normal glucose conditions by decreasing HIF-1α protein expression. To test this hypothesis, we evaluated the combined effect of MCT inhibitor and 4-OHT using the estrogen receptor (ER)-positive breast cancer cell line MCF-7, under normal glucose conditions. MATERIALS AND METHODS: Expression of MCTs and oxidative stress markers was evaluated by real-time PCR. Cell viability was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Reactive oxygen species (ROS) were measured using the cell permeability probe 2',7'-dichlorodihydrofluorescein diacetate. RESULTS: MCT1 expression increased under normal glucose conditions. The MCT1 substrate/inhibitor, 5-oxoproline (5-OP), enhanced 4-OHT-induced cytotoxicity. Bindarit, a selective MCT4 inhibitor, decreased 4-OHT sensitivity, similar to results of our previous study under high glucose conditions. In contrast, the combination of 5-OP and 4-OHT decreased ATP levels compared with that by 4-OHT alone in MCF-7 cells. Furthermore, 5-OP significantly increased the ROS production induced by 4-OHT. CONCLUSION: 5-OP enhances 4-OHT-induced cytotoxicity in ER-positive breast cancer cells under normal glucose conditions.

Monocarboxylate transporter 4 involves in energy metabolism and drug sensitivity in hypoxia.

Metabolic reprogramming of cancer cells is a potential target for cancer therapy. It is also known that a hypoxic environment, one of the tumor microenvironments, can alter the energy metabolism from oxidative phosphorylation to glycolysis. However, the relationship between hypoxia and drug sensitivity, which targets energy metabolism, is not well known. In this study, A549 cells, a cell line derived from lung adenocarcinoma, were evaluated under normoxia and hypoxia for the sensitivity of reagents targeting oxidative phosphorylation (metformin) and glycolysis (α-cyano-4-hydroxycinnamic acid [CHC]). The results showed that a hypoxic environment increased the expression levels of monocarboxylate transporter (MCT) 4 and hypoxia-induced factor-1α (HIF-1α), whereas MCT1 and MCT2 expression did not vary between normoxia and hypoxia. Furthermore, the evaluation of the ATP production ratio indicated that glycolysis was enhanced under hypoxic conditions. It was then found that the sensitivity to metformin decreased while that to CHC increased under hypoxia. To elucidate this mechanism, MCT4 and HIF-1α were knocked down and the expression level of MCT4 was significantly decreased under both conditions. In contrast, the expression of HIF-1α was decreased by HIF-1α knockdown and increased by MCT4 knockdown. In addition, changes in metformin and CHC sensitivity under hypoxia were eliminated by the knockdown of MCT4 and HIF-1α, suggesting that MCT4 is involved in the phenomenon described above. In conclusion, it was shown that the sensitivity of reagents targeting energy metabolism is dependent on their microenvironment. As MCT4 is involved in some of these mechanisms, we hypothesized that MCT4 could be an important target molecule for cancer therapy.

Involvement of SLC16A1/MCT1 and SLC16A3/MCT4 in l-lactate transport in the hepatocellular carcinoma cell line.

Fourteen isoforms of the monocarboxylate transporter (MCT) have been reported. Among the MCT isoforms, MCT1, MCT2, and MCT4 play a role in l-lactate/proton cotransport and are involved in the balance of intracellular energy and pH. Therefore, MCT1, MCT2, and MCT4 are associated with energy metabolism processes in normal and pathological cells. In the present study, we evaluated the expression of MCT1, MCT2, and MCT4 and the contribution of these three MCT isoforms to l-lactate uptake in hepatocellular carcinoma (HCC) cells. In HepG2 and Huh-7 cells, l-lactate transport was pH-dependent, which is characteristic of MCT1, MCT2, and MCT4. Furthermore, l-lactate uptake was selectively inhibited by MCT1 and MCT4 inhibitors in HepG2 and Huh-7 cells. Kinetic analysis of HepG2 cells demonstrated that l-lactate uptake was biphasic. Although the knockdown of MCT1 and MCT4 in the HepG2 cells decreased the uptake of l-lactate, the knockdown of MCT2 had no effect on the uptake of l-lactate. Consequently, we concluded that both MCT1 and MCT4 were involved in the transport of l-lactate in HepG2 and Huh-7 cells at pH 6.0. In contrast, PXB-cells, freshly isolated hepatocytes from humanized mouse livers, showed lower MCT4 expression and l-lactate uptake at pH 6.0 compared to that in HCC cell lines. In conclusion, MCT4, which contributes to l-lactate transport in HCC cells, is significantly different in HCC compared to normal hepatocytes, and has potential as a target for HCC treatment.