Original article Inhibition of tumor lactate oxidation: Consequences for the tumor microenvironment Morten Busk a,⇑ , Stefan Walenta d , Wolfgang Mueller-Klieser d , Torben Steiniche c , Steen Jakobsen b , Michael Robert Horsman a , Jens Overgaard a a Department of Experimental Clinical Oncology; b PET Centre; c Institute of Pathology, Aarhus University Hospital, Denmark; d Institute of Physiology and Pathophysiology, University Medical Center of the Johannes Gutenberg University of Mainz, Mainz, Germany article info Article history: Received 16 May 2011 Received in revised form 24 May 2011 Accepted 25 May 2011 Available online xxxx Keywords: Metabolic symbiosis MCT1 Lactate Glucose Hypoxia a-Cyano-hydroxycinnamic acid Pasteur effect Radiosensitization Warburg effect abstract Background and purpose: Tumor cells are recognized as being highly glycolytic. However, recently it was suggested that lactate produced in hypoxic tumor areas may be taken up by the monocarboxylate trans- porter MCT1 and oxidized in well-oxygenated tumor parts. Furthermore, it was shown that inhibition of lactate oxidation using the MCT1 inhibitor a-cyano-hydroxycinnamate (CHC) can radio-sensitize tumors possibly by forcing a switch from lactate oxidization to glycolysis in oxygenated cells, which in turn improves tumor oxygenation and indirectly kills radio-resistant hypoxic tumor cells from glucose starva- tion. Material and methods: To provide direct evidence for the existence of a targetable energetic symbiosis, mice bearing SiHa or FaDu dd tumors were treated with CHC for different time periods. One hour prior to sacrifice, mice were administered with the glucose analog fluorodeoxyglucose (FDG) and the hypoxia-marker pimonidazole. Tumor cryosections were analyzed for regional glucose retention (FDG autoradiograms), hypoxia (pimonidazole retention) and glucose and lactate levels (bioluminescence imaging). Results: Treatment did not influence metabolite concentrations, necrosis or extent of hypoxia, but pixel- by-pixel analysis comparing FDG retention and hypoxia (a measure of the apparent in vivo Pasteur effect) showed that CHC treatment caused a transient reduction in the Pasteur effect in FaDu dd 1.5 h following CHC administration whereas a reduction was only observed in SiHa following repeated treatments. Conclusions: In summary, our data show that CHC is able to influence the intratumoral distribution of glucose use between hypoxic and non-hypoxic tumor areas. That is in accordance with a functional tumor lactate-shuttle, but the absence of any detectable changes in hypoxic extent and tissue metabolites was unexpected and warrants further investigation. Ó 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology xxx (2011) xxx–xxx Nearly a century ago the German biochemist Otto Warburg showed that the basal energy metabolism in tumor cells is aberrant since most tumors produce ATP by converting excessive amounts of glucose into lactate, even in the presence of oxygen [1]. More recent research has verified these findings, and the special tumor energy metabolism, referred to as the Warburg effect or aerobic glycolysis, is now considered as an established hallmark of cancers, and this fundamental property of cancer now forms the basis of fluorodeoxy- glucose (FDG) positron emission tomography [2,3]. Several genetic changes have been linked to the Warburg phenotype, including p53 mutations which interfere with p53’s role as a positive regulator of respiration and increased activity of the oncogenes Akt, Myc, Src and RAS that are able to activate glucose transporters and glycolytic enzymes (for a review see Ref. [4]). Although a number of mitochon- drial mutations have been documented in tumor cells, oxygen uptake is not generally reduced in malignant compared to normal cells. Respiration can be stimulated pharmacologically in most tumor cells suggesting that up-regulated glycolysis is not to com- pensate for an insufficient respiratory capacity. Increased reliance on glycolysis is paradoxical since only a minor part of the energy stored in glucose is exploited, but high glycolytic flux may result in a more balanced supply of ATP and building blocks required in proliferating cells [5]. Other advantages of increased glycolysis in- clude (i) reduced ROS formation, (ii) the ability of glycolysis to pro- duce ATP at a high speed when glucose is unlimited, (iii) the acidotic efflux which assists extracellular matrix breakdown and kills non- adapted normal cells, and (iv) adaptation to cycling hypoxia. A recent and controversial study by Sonveaux et al. [6,7] challenged the traditional view of tumor cells simply as greedy consumers of glucose. In a series of elegant in vitro experiments 0167-8140/$ - see front matter Ó 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2011.05.053 ⇑ Corresponding author. Address: Department of Experimental Clinical Oncology, Aarhus University Hospital (AUH), Aarhus, Denmark. E-mail address: morten@oncology.dk (M. Busk). Radiotherapy and Oncology xxx (2011) xxx–xxx Contents lists available at ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com Please cite this article in press as: Busk M et al. Inhibition of tumor lactate oxidation: Consequences for the tumor microenvironment. Radiother Oncol (2011), doi:10.1016/j.radonc.2011.05.053