Inhibition of L-lactate transport and band 3-mediated anion transport in erythrocytes by the novel stilbenedisulphonate N,N,N',N'-tetrabenzyl-4,4'-diaminostilbene-2,2'-disulpho nat e (TBenzDS)

Structural aspects of the glucose and monocarboxylate transporters involved in the Warburg effect

Cancer cells shift their glucose catabolism from aerobic respiration to lactic fermentation even in the presence of oxygen, and this is known as the "Warburg effect". To accommodate the high glucose demands and to avoid lactate accumulation, the expression levels of human glucose transporters (GLUTs) and human monocarboxylate transporters (MCTs) are elevated to maintain metabolic homeostasis. Therefore, inhibition of GLUTs and/or MCTs provides potential therapeutic strategies for cancer treatment. Here, we summarize recent advances in the structural characterization of GLUTs and MCTs, providing a comprehensive understanding of their transport and inhibition mechanisms to facilitate further development of anticancer therapies.

Studies on the DIDS-binding site of monocarboxylate transporter 1 suggest a homology model of the open conformation and a plausible translocation cycle

Site-directed mutagenesis of MCT1 was performed on exofacial lysines Lys³⁸, Lys⁴⁵, Lys²⁸², and Lys⁴¹³. K38Q-MCT1 and K38R-MCT1 were inactive when expressed at the plasma membrane of Xenopus laevis oocytes, whereas K45R/K282R/K413R-MCT1 and K45Q/K282Q/K413Q-MCT1 were active. The former exhibited normal reversible and irreversible inhibition by DIDS, whereas the latter showed less reversible and no irreversible inhibition. K45Q/K413Q-MCT1 retained some irreversible inhibition, whereas K45Q/K282Q-MCT1 and K282Q/K413Q-MCT1 did not. These data suggest that the two DIDS SO₃⁻ groups interact with positively charged Lys²⁸² together with Lys⁴⁵ and/or Lys⁴¹³. This positions one DIDS isothiocyanate group close to Lys³⁸, leading to its covalent modification and irreversible inhibition. Additional mutagenesis revealed that DIDS cross-links MCT1 to its ancillary protein embigin using either Lys³⁸ or Lys²⁹⁰ of MCT1 and Lys¹⁶⁰ or Lys¹⁶⁴ of embigin. We have modeled a possible structure for the outward facing (open) conformation of MCT1 by employing modest rotations of the C-terminal domain of the inner facing conformation modeled previously. The resulting model structure has a DIDS-binding site consistent with experimental data and locates Lys³⁸ in a hydrophobic environment at the bottom of a substrate-binding channel. Our model suggests a translocation cycle in which Lys³⁸ accepts a proton before binding lactate. Both the lactate and proton are then passed through the channel via Asp³⁰²⁻ and Asp³⁰⁶⁺, an ion pair already identified as important for transport and located adjacent to Phe³⁶⁰, which controls channel selectivity. The cross-linking data have also been used to model a structure of MCT1 bound to embigin that is consistent with published data.

Newly identified structurally disparate modulators of osmosensitive taurine efflux inhibit cell cycle progression

FACS analysis and [14C]-taurine efflux were used to determine whether activation of the volume-sensitive organic osmolyte/anion channel plays a role in cell cycle progression. This was achieved by examining the effects of a collection of (i) H(1) antagonists and tricyclic antidepressants with a known inhibitory effect on cell cycle progression, and (ii) antidepressants and oestrogen receptor modulators with molecular structures likely to confer inhibition of the volume-sensitive organic osmolyte/anion channel. Of the 13 compounds examined in this study, the following showed no cytotoxicity following a 48-h exposure, and specifically inhibited osmosensitive taurine efflux (over lactate transport and anion exchange) with IC(50) values of (in microM): fluoxetine, approximately 14; fluvoxamine, approximately 24; amitriptyline, approximately 32; imipramine, approximately 32; mianserin, approximately 40. A 48-h application of these compounds at these concentrations significantly increased arrest in the G0/1 stage of the cell cycle by approximately 10%. The uniformity and specificity of the response elicited by these compounds strongly reinforces a correlation between cell cycle progression and osmosensitive taurine efflux activation.

Proton gradient-dependent transport of valproic acid in human placental brush-border membrane vesicles

PURPOSE

To investigate the transport mechanism of valproic acid across the human placenta, we used human placental brush-border membrane vesicles and compared them with that of lactic acid.

METHODS

Transport of [3H]valproic acid and [14C]lactic acid was measured by using human placental brush-border membrane vesicles.

RESULTS

The uptakes of [3H]valproic acid and [14C]lactic acid into brush-border membrane vesicles were greatly stimulated at acidic extravesicular pH. The uptakes of [3H]valproic acid and [14C]lactic acid were inhibited by various fatty acids, p-chloromercuribenzene sulfonate, alpha-cyano-4-hydroxycinnamate, and FCCP. A kinetic analysis showed that it was saturable, with Michaelis constants (Kt) of 1.04 +/- 0.41 mM and 1.71 +/- 0.33 mM for [3H]valproic acid and [14C]lactic acid, respectively. Furthermore, lactic acid competitively inhibited [3H]valproic acid uptake and vice versa.

CONCLUSION

These results suggest that the transport of valproic acid across the microvillous membrane of human placenta is mediated by a proton-linked transport system that also transports lactic acid. However, some inhibitors differentially inhibited the uptakes of [3H]valproic acid and [14C]lactic acid, suggesting that other transport systems may also contribute to the elevated fetal blood concentration of valproic acid in gravida.

Lactate transporters (MCT proteins) in heart and skeletal muscles

Lactate traverses the cell membranes of many tissues, including the heart and skeletal muscle via a facilitated monocarboxylate transport system that functions as a proton symport and is stereoselective for L-lactate. In the past few years, seven monocarboxylate transporters have been cloned. Monocarboxylate transporters are ubiquitously distributed among many tissues, and the transcripts of several monocarboxylate transporters are present within many of the same tissues. This complicates the identification of their metabolic function. There is also evidence that that there is some species specificity, with differences in MCT tissue distributions in hamsters, rats, and humans. MCT1 and MCT3-M/MCT4 are present in rat and human muscles, and MCT1 expression is highly correlated with the oxidative capacity of skeletal muscles and with their capacity to take up lactate from the circulation. MCT1 is also present in heart and is located on the plasma membrane (in subdomains), T-tubules, and in caveolae. With training, MCT1 is increased in rat and human muscle, and in rat hearts, resulting in an increased uptake of lactate from the buffers perfused through these tissues and an increase in lactate efflux out of purified vesicles. In humans, the training-induced increases in MCT1 are associated with an increased lactate efflux out of muscle. MCT3-M/MCT4 is not correlated with the muscles' oxidative capacities but is equally abundant in Type IIa and IIb muscles, whereas it is markedly lower in slow-twitch (Type I) muscles. Clearly, we are at the threshold of a new era in understanding the regulation of lactate movement into and out of skeletal muscle and cardiac cells.

The kinetics, substrate and inhibitor specificity of the lactate transporter of Ehrlich-Lettre tumour cells studied with the intracellular pH indicator BCECF

1. Suspensions of cultured Ehrlich-Lettre tumour cells were loaded with the pH-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and changes in intracellular pH upon addition of L-lactate and other monocarboxylates were continuously monitored by fluorimetry using dual-wavelength excitation (450/500 nm) and single-wavelength emission (> 520 nm). 2. The rapid fluorescence changes were analysed by first-order regression analysis, and with suitable calibration procedures this enabled calculation of initial rates of proton uptake associated with monocarboxylate transport. 3. The stoichiometry was shown to be one proton per lactate molecule transported. 4. The kinetics of carrier-mediated transport of a wide range of monocarboxylates were determined at 25 degrees C. The Km values for L-lactate, pyruvate and D-lactate were found to be 4.54, 0.72 and 27.5 mM respectively, similar to values found previously for rat erythrocytes. This similarity was shared with a wide range of variously substituted C2, C3 and C4 monocarboxylates, all of which were transported with similar Vmax. No stereoselectivity was found in the Km values for D- and L-2-chloropropionate (0.75 mM) or D- and L-3-hydroxybutyrate (11 mM), but in the latter case the Vmax. of the D-isomer was twice that of the L-isomer. 5. The temperature-dependence of L-lactate transport demonstrated a transition point, with activation energies of 60 and 109 kJ.mol-1 above and below 19 degrees C respectively The Km for L-lactate below the transition temperature was about half that above it. 6. Inhibition of lactate transport into tumour cells by a wide range of compounds known to inhibit the erythrocyte monocarboxylate carrier was analysed. Patterns of inhibition were similar to those seen in the erythrocyte, but the Ki values were 2-4-fold higher in the tumour cells. 7. It is concluded that tumour cells contain an isoform of the monocarboxylate carrier with functional properties almost identical with that found in erythrocytes. This is probably identical with MCT1, which was recently cloned and sequenced from Chinese Hamster Ovary cells [Kim Garcia, Goldstein, Pathak, Anderson and Brown (1994) Cell 76, 865-873].

Characterization of the inhibition by stilbene disulphonates and phloretin of lactate and pyruvate transport into rat and guinea-pig cardiac myocytes suggests the presence of two kinetically distinct carriers in heart cells

1. The kinetics of transport of pyruvate (Km 0.20 mM), L-lactate (Km 2.2 mM) and D-lactate (Ki 10.2 mM) into rat cardiac myocytes were studied and compared with those for guinea-pig heart cells [Poole, Halestrap, Price and Levi (1989) Biochem. J. 264, 409-418] whose equivalent values were 0.07, 2.3 and 6.6 mM respectively. Maximal rates of transport were about 5-fold higher in the rat heart cells. 2. 4,4'-Dibenzamidostilbene-2,2'-disulphonate (DBDS), a powerful inhibitor of monocarboxylate transport into erythrocytes [Poole & Halestrap (1991) Biochem. J. 275, 307-312], was found to be a potent but apparently partial inhibitor of lactate and pyruvate transport, with an apparent Ki value at 0.5 mM L-lactate of about 16 microM in both species. Maximal inhibition was 50% and 80% in rat and guinea-pig cells respectively. 3. The maximal extent of inhibition and apparent Ki values were dependent on both the substrate transported and its concentration. Maximum inhibition was less and the Ki was greater at higher substrate concentrations. 4. A variety of other stilbene disulphonates were studied which showed different Ki values and maximal extents of inhibition. 5. Phloretin was a significantly less potent inhibitor of transport into both rat (Ki 25 microM) and guinea-pig (Ki 16 microM) heart cells than into rat erythrocytes (Ki 1.4 microM). In the rat but not the guinea-pig heart cells, inhibition appeared partial (maximal inhibition 84%). 6. We demonstrate that our results can be explained by the presence of two monocarboxylate carriers in heart cells, both with Km values for L-lactate of about 2 mM and inhibited by alpha-cyano-4-hydroxycinnamate, but with different affinities for other substrates and inhibitors. One carrier is sensitive to inhibition by stilbene disulphonates and has lower Km values for pyruvate (0.05-0.10 mM) and D-lactate (5 mM), whereas the other has higher Km values for pyruvate (0.30 mM) and D-lactate (25 mM), and is relatively insensitive to stilbene disulphonates. Rat heart cells possess more of the latter carrier and guinea-pig heart cells more of the former. 7. The significance of these results for the study of lactate transport in the perfused heart is discussed.

Identification and partial purification of the erythrocyte L-lactate transporter

1. Intact erythrocytes were incubated with 100 microM-4,4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS), a concentration sufficient to inhibit lactate transport irreversibly by 65%. DIDS-labelled proteins were detected by immunoblotting of erythrocyte membrane proteins with an anti-DIDS antibody. Labelled polypeptides of 35-45 kDa in rat erythrocytes, and of 40-50 kDa in rabbit and guinea pig erythrocytes, were detected by this technique. In human erythrocytes, which have 10-fold less transport activity, no labelled polypeptide in this molecular mass range was detected. 2. Labelling of these 35-50 kDa polypeptides was decreased markedly in the presence of the specific inhibitors of lactate transport alpha-cyano-4-hydroxycinnamate and 4,4'-dibenzamidostilbene-2,2'-disulphonate (DBDS), which compete with DIDS for binding to the transporter. However, the weakly bound inhibitor 4,4'-dinitrostilbene-2,2'-disulphonate (DNDS) afforded little protection against labelling by DIDS. 3. The lactate transporter from rat erythrocytes was solubilized with decanoyl-N-methyl glucamide (MEGA-10) and partially purified by Mono-Q anion-exchange chromatography, with transport activity eluting at 0.1-0.15 M-NaCl. The 35-45 kDa DIDS-labelled polypeptide from rat erythrocytes was eluted in the same peak of protein as lactate transporter activity during Mono-Q chromatography. 4. These observations provide strong evidence that the lactate transporter is a polypeptide of 35-45 kDa in rat erythrocytes and of 40-50 kDa in rabbit and guinea pig erythrocytes.