Abstract 1158: Real-time assessment of uptake and utilization of lactate in intact human breast cancer cells using a 1H-NMR-based assay
Authors: Wholey Emily G. , Parkes Harold G. , Workman Paul , Leach Martin O. , Beloueche-Babari Mounia 
Published: August 2015

Objective: The purpose of this study was to evaluate the feasibility of monitoring lactate uptake and utilization in intact human cancer cells using a dynamic 1H-NMR assay and [3-13C]-lactate. This would provide a simple, real-time method for monitoring enzyme kinetics in cells as an alternative to spectrophotometry - which cannot be applied to intact cells - or dynamic nuclear polarisation-NMR, which requires complex mathematical modelling. The ability to better understand and measure lactate metabolism in cancer cells could be applied to monitor MCT1 inhibition.

Methods: 3×10^7 MDA-MB-231 human breast cancer cells were suspended in 400μl serum-free DMEM plus 100μl D2O in a 5mm NMR tube. A mixture of 10mM [3-13C]-lactate combined with varying concentrations of unlabelled 12C-pyruvate (0mM, 5mM or 10mM) was added and proton spectra were acquired every minute for 30 minutes on a 500MHz Bruker spectrometer. For comparison, cells were also incubated with 10mM [3-13C]-pyruvate and either 0mM or 10mM 12C-lactate.

Results: The presence of 13C-pyruvate was detected in real-time following addition of [3-13C]-lactate indicating that 13C-label had been exchanged from lactate to pyruvate via the enzyme lactate dehydrogenase. The optimisation of reagent concentrations, however, revealed that this observation was dependent on the addition of unlabelled 12C-pyruvate and the apparent rate of 13C-label exchange was proportional to the concentration of 12C-pyruvate added.

The data show that no 13C-pyruvate signal is detected when 10mM 13C-lactate is added alone. However, 6% of the 10mM 13C-lactate is exchanged with 13C-pyruvate when 5mM 12C-pyruvate is added (rate = 1.1×10-3mM s-1). This increases to 12% when the 12C-pyruvate concentration was increased to 10mM (rate = 2.2×10-3 mM s-1). The total pyruvate concentration (12C + 13C-pyruvate) remained constant, which suggests that there is no net production of pyruvate. In comparison, the 13C-pyruvate experiment was less dependent on the addition of unlabelled 12C-lactate for 13C-label exchange to be observed.

Conclusion: The fact that unlabelled pyruvate was required for a reaction to be observed following addition of 13C-lactate suggests that the position of the isotopic equilibrium favours the label to remain in the lactate pool. Therefore a large amount of unlabelled pyruvate is required for the exchange process to be observed. This would also explain why the addition of unlabelled lactate was not required in the 13C-pyruvate experiment, since to reach isotopic equilibrium, a large proportion of the 13C-pyruvate needs to immediately exchange with the naturally larger lactate pool. Nevertheless, these findings demonstrate the feasibility of using this 1H-NMR assay to measure [3-13C]-lactate uptake in real-time. This could inform on metabolic pathway activity in response to MCT1 inhibition in intact cells.

Citation Format: Emily G. Wholey, Harold G. Parkes, Paul Workman, Martin O. Leach, Mounia Beloueche-Babari. Real-time assessment of uptake and utilization of lactate in intact human breast cancer cells using a 1H-NMR-based assay. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1158. doi:10.1158/1538-7445.AM2015-1158