Inhibition of tumor lactate oxidation: Consequences for the tumor microenvironment

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 transporter MCT1 and oxidized in well-oxygenated tumor parts. Furthermore, it was shown that inhibition of lactate oxidation using the MCT1 inhibitor α-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 starvation.

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.