The quantitative relationship between isotopic and net contributions of lactate and glucose to the TCA cycle

  1. Xun Hu1*
  1. 1Cancer Institute, Zhejiang University, China
  2. 2Cancer Institute, the Second Affiliated Hospital, Zhejiang University School of Medicine
  1. * Corresponding author; email: huxun{at}zju.edu.cn
  1. Author contributions: M.Y. and X.H. conceptualization; M.Y. formal analysis; M.Y. validation; M.Y. and C.G. methodology; M.Y. and X.H. writing-original draft; M.Y. and X.H. writing-review and editing; C.G. software; X.H. supervision; X.H. funding acquisition; X.H. project administration.

Abstract

Whether growing cancer cells prefer lactate as a fuel over glucose or vice versa is an important yet controversial issue. Labeling of tricarboxylic acid (TCA) cycle intermediates with glucose or lactate isotope tracers is often used to report the relative contributions of these two metabolites to the TCA cycle. However, this approach may not yield accurate results, as isotopic labeling may not accurately reflect net contributions of each metabolite. This may be due to isotopic exchange occurring during the conversion between pyruvate and lactate. To evaluate this quantitatively, we used an equation (CG - CG’ = CL’ - CL) assessing the relationship between isotopic labeling and net consumption measurements in vitro. CG and CL refer to the contributions of glucose and lactate to the TCA cycle as measured by their net consumption, whereas CG’ and CL’ refer to glucose and lactate’s contributions determined with isotopic labeling. We found that the isotopic labeling data overestimate the net contribution of lactate to the TCA cycle and underestimate that of glucose. The overestimated amount is equal to the isotopic exchange amount between pyruvate and lactate. After excluding the interference of isotopic exchange, the major carbon contribution (i.e. acetyl-CoA) to the TCA cycle comes from glucose rather than lactate in vitro. We propose that these relative contributions of glucose and lactate may also be present in cancer cells in vivo.

  • Received February 1, 2019.
  • Accepted April 30, 2019.

This Article

  1. jbc.RA119.007841.
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  3. All Versions of this Article:
    1. RA119.007841v1
    2. 294/24/9615 (most recent)

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