Tumors contain oxygenated and hypoxic regions, so the tumor cell population is heterogeneous. Hypoxic tumor cells primarily use glucose for glycolytic energy production and release lactic acid, creating a lactate gradient that mirrors the oxygen gradient in the tumor. By contrast, oxygenated tumor cells have been thought to primarily use glucose for oxidative energy production. Although lactate is generally considered a waste product, we now show that it is a prominent substrate that fuels the oxidative metabolism of oxygenated tumor cells. There is therefore a symbiosis in which glycolytic and oxidative tumor cells mutually regulate their access to energy metabolites. We identified monocarboxylate transporter 1 (MCT1) as the prominent path for lactate uptake by a human cervix squamous carcinoma cell line that preferentially utilized lactate for oxidative metabolism. Inhibiting MCT1 with α-cyano-4-hydroxycinnamate (CHC) or siRNA in these cells induced a switch from lactate-fueled respiration to glycolysis. A similar switch from lactate-fueled respiration to glycolysis by oxygenated tumor cells in both a mouse model of lung carcinoma and xenotransplanted human colorectal adenocarcinoma cells was observed after administration of CHC. This retarded tumor growth, as the hypoxic/glycolytic tumor cells died from glucose starvation, and rendered the remaining cells sensitive to irradiation. As MCT1 was found to be expressed by an array of primary human tumors, we suggest that MCT1 inhibition has clinical antitumor potential.
Pierre Sonveaux, Frédérique Végran, Thies Schroeder, Melanie C. Wergin, Julien Verrax, Zahid N. Rabbani, Christophe J. De Saedeleer, Kelly M. Kennedy, Caroline Diepart, Bénédicte F. Jordan, Michael J. Kelley, Bernard Gallez, Miriam L. Wahl, Olivier Feron, Mark W. Dewhirst
Submitter: Simon N. Thornton | Simon.Thornton@scbiol.uhp-nancy.fr
INSERM, U684, Vandoeuvre les Nancy, France. | Université Henri Poincaré, Nancy Université, Nancy, France.
Published January 14, 2009
I was concerned to read the recent research article by Pierre Sonveaux and colleagues entitled “Targeting lactate-fueled respiration selectively kills hypoxic tumor cells in mice” (J Clin Invest. 2008; 118 (12): 3930-3942)(1). In the work published here the authors use alpha-cyano-4-hydirxycinnamate (CHC) to inhibit monocarboxylate transporter 1 (MCT-1) and in so doing deprive human cervix squamous carcinoma cells of the substrate lactate for oxidative metabolism thus killing them. Although the work shows this as an effective treatment in this model I am a little concerned to the overall outcome if ever this is applied to humans. Lactate has a vital function in brain metabolism where it is the preferred substrate for neurons being supplied by the surrounding glial cells (2). For this function there is an abundance of the MCT family of transporters, notably MCT-1 and MCT-4. There is also evidence that these MCT transporters are in the blood brain barrier and could thus take up lactate from the blood (2). There are many published articles about the cellular metabolic functions of lactate (for review see 3).
Furthermore, physical activity has been shown to increase lactate turnover and to increase MCT content in muscle (4) and even within the mitochondria membranes there are members of the MCT family, notably MCT-1 (5, 6). This would suggest that an inhibitor of MCT-1 would affect not only the cancer cells referred to in the article but also practically all other cells in the body, especially the muscles and the neurons.
In conclusion, the idea of using a global MCT-1 transporter inhibitor for cancer cell treatment does not appear to me to be such a good idea knowing that lactate and the MCT family of transporters are vitally important not only in muscle cell metabolism but also neuronal metabolism and overall healthy cell mitochondrial function. I would suggest more detailed behavioural testing in conscious behaving animals would be needed before general application of this strategy was approved.
Submitter: Andrew P. Halestrap | A.Halestrap@bristol.ac.uk
University of Bristol, Department of Biochemistry
Published December 18, 2008
Sonveaux et al report that some oxygenated tumor cells can oxidise lactate as a respiratory fuel. They propose that this allows lactate produced by glycolysis in the hypoxic cells at the centre of a tumor to be removed whilst providing energy for the oxygenated peripheral cells. To test their hypothesis they used alpha-cyano-4-hydroxycinnamate (CHC) to inhibit monocarboxylate transporter 1 (MCT1) that mediates lactic acid transport into cells. However, CHC is a very potent inhibitor of the mitochondrial pyruvate carrier (MPC) that transports pyruvate into the mitochondria (1) as noted by the authors in their supplementary material. Indeed, the KI for MPC inhibition is about 5 microM (1), nearly two orders of magnitude lower than that for inhibition of MCT1 (2). Thus in the presence of 5 mM CHC the MPC will be completely blocked and this provides an alternative, and perhaps more likely, explanation of the potent inhibition of lactate oxidation by CHC. The authors argue that they have eliminated mitochondrial effects of CHC through the use of methylpyruvate that enters the cell independently of MCT1, but methylpyruvate will also enter mitochondria independently of the MPC. They might also argue that glucose oxidation usually requires pyruvate to enter the mitochondria via the MPC, and thus that CHC should inhibit respiration in the presence of glucose as well as lactate if it targets the MPC rather than MCT1. This was not observed. However, the metabolism of tumor cells is unusual (3) and their very active pentose phosphate pathway may allow them to oxidise glucose to CO2 with the production of NADPH that can then be oxidised by the mitochondria using redox shuttles.
Despite these reservations, the data of Sonveaux et al remain important because they provide further evidence that the unique metabolism of tumor cells may be their Achillee’s heel (3). The availability of much more potent and specific inhibitors than CHC of both the MPC (4) and MCT1 (5) should make it possible to establish which of these transport mechanisms has the greater promise as a future chemotherapeutic target.
Submitter: Pierre Sonveaux | email@example.com
Authors: Olivier Feron, PhD, and Mark W. Dewhirst, DVM, PhD
Unit of Pharmacology & Therapeutics, Université catholique de Louvain, Brussels, Belgium.
Published December 18, 2008
In reply to Andrew P. Halestrap: “Inhibiting lactate oxidation in tumor cells”.
We recently reported that lactate recycling by oxidative tumor cells (TCs) accounts for a metabolic symbiosis in tumors and that the symbiont can be targeted therapeutically through inhibition of monocarboxylate transporter 1 (MCT1).(1) In some experiments, it was achieved using alpha-cyano-4-hydroxycinnamate (CHC), the most selective, commercially available and in vivo compatible MCT1 inhibitor.(2)
In his comment, Halestrap questions the selectivity of MCT1 inhibition by CHC. We agree that CHC may also potently inhibit the mitochondrial pyruvate carrier (MPC) that transports pyruvate into mitochondria,(3,4) although this activity in intact cells cannot straightforwardly be predicted from in vitro assays on isolated rat mitochondria.(3) In our experiments, MPC inhibition by CHC could indeed be limited by membranes acting as barriers preventing CHC binding to its inhibitory site on MPC inside mitochondria(5) and by potential structural MPC differences between species.(6)
In our manuscript, we have provided a series of independent evidence that inhibition of MCT1, not MPC, supports the glycolytic switch induced by CHC in oxidative TCs.(1) We indeed documented that silencing MCT1 using siRNA and shRNA recapitulated the phenotypic shift observed with CHC. CHC potently blocked lactate-fueled but not glucose-fueled respiration in oxidative TCs. In this condition, Halestrap suggests that respiration could be rescued by the pentose phosphate pathway (PPP) that promotes MPC-independent NADPH oxidation. This is not consistent with our data: in the presence of glucose and lactate, oxidative TCs exposed to CHC started to stoichiometrically produce 2 lactate molecules for 1 glucose,(1) indicating that glucose mainly served to fuel aerobic glycolysis, not the PPP. In vivo, CHC administration induced extensive tumor reoxygenation, further ruling out a major contribution of the oxidative arm of the PPP. Finally, we(1) and others(2) did not identify signs of muscular toxicity or major off-target effects in mice chronically treated with CHC. The antitumor effects of CHC were absent in MCT1-negative tumors.
We therefore believe that MCT1, not MPC, is the primary target responsible for the antitumor effects of CHC, but we acknowledge the need for more potent and selective MCT1 inhibitors than CHC for further clinical trials.