Is higher lactate an indicator of tumor metastatic risk? A pilot MRS study using hyperpolarized (13)C-pyruvate

Abstract

Rationale and objectives: Cancer cells generate more lactate than normal cells under both aerobic and hypoxic conditions-exhibiting the so-called Warburg effect. However, the relationship between the Warburg effect and tumor metastatic potential remains controversial. We intend to investigate whether the higher lactate reflects higher tumor metastatic potential.

Materials and methods: We used hyperpolarized (13)C-pyruvate magnetic resonance spectroscopy (MRS) to compare lactate (13)C-labeling in vivo in mouse xenografts of the highly metastatic (MDA-MB-231) and the relatively indolent (MCF-7) human breast cancer cell lines. We obtained the kinetic parameters of the lactate dehydrogenase (LDH)-catalyzed reaction by three methods of data analysis including the differential equation fit, q-ratio fit, and ratio fit methods.

Results: Consistent results from the three methods showed that the highly metastatic tumors exhibited a smaller apparent forward rate constant (k(+) = 0.060 ± 0.004 s(-1)) than the relatively indolent tumors (k(+) = 0.097 ± 0.013 s(-1)). The ratio fit generated the greatest statistical significance for the difference (P = .02). No significant difference in the reverse rate constant was found between the two tumor lines.

Conclusions: The result indicates that the less metastatic breast tumors may produce more lactate than the highly metastatic ones from the injected (13)C-pyruvate and supports the notion that breast tumor metastatic risk is not necessarily associated with the high levels of glycolysis and lactate production. More studies are needed to confirm whether and how much the measured apparent rate constants are affected by the membrane transporter activity and whether they are primarily determined by the LDH activity.

Keywords: NMR; Pyruvate; Warburg effect; ratio fit; two-site exchange model.

Fig. 1

A representative time series of ¹³C-MR spectra from a breast tumor. The peak of 1-¹³C-pyruvate is nearby 171ppm and 1-¹³C-lactate is nearby 182 ppm. The time interval between adjacent spectra is 1s.

Figure 2

The typical time courses of hyperpolarized pyruvate (*) and lactate (◊) and the DE fits (left), the qR fit (middle) and the ratio fit (right) results of a MDA-MB-231 tumor. All rates are in s⁻¹.

Figure 3

The typical time courses of hyperpolarized pyruvate (*) and lactate (◊), the DE fit (left) and the qR fit (middle) and the ratio fit (right) results of a MCF-7 tumor. All rates are in s⁻¹.

Figure 4

The comparison of rate constants (k⁺, k⁻) and ratios (k⁺/k⁻) between the MCF-7 and MDA-MB-231 tumor xenografts. These results were obtained with the RF (ratiofit) method shown in Table 1.