Autophagy is a highly regulated, energy dependent cellular process where
proteins, organelles and cytoplasm are sequestered in autophagosomes and digested to sustain cellular homeostasis. We hypothesized that during autophagy induced in
cancer cells by i)
starvation through serum and
amino acid deprivation or ii) treatment with
PI-103, a class I PI3K/mTOR inhibitor, glycolytic metabolism would be affected, reducing flux to
lactate, and that this effect may be reversible. We probed metabolism during autophagy in colorectal HT29 and HCT116 Bax knock-out cells using hyperpolarized (13)C-magnetic resonance spectroscopy (MRS) and steady-state (1)H-MRS. 24 hr
PI103-treatment or
starvation caused significant reduction in the apparent forward rate constant (k(PL)) for
pyruvate to
lactate exchange compared with controls in HT29 (100 μM
PI-103: 82%, p = 0.05) and HCT116 Bax-ko cells (10 μM
PI-103: 53%, p = 0.05; 20 μM
PI-103: 42%, p<0.0001;
starvation: 52%, p<0.001), associated with reduced
lactate excretion and intracellular
lactate in all cases, and unchanged
lactate dehydrogenase (LDH) activity and increased
NAD+/
NADH ratio following
PI103 treatment or decreased LDH activity and unchanged
NAD+/
NADH ratio following
starvation. After 48 hr recovery from
PI103 treatment, k(PL) remained below control levels in HT29 cells (74%, p = 0.02), and increased above treated values, but remained below 24 hr vehicle-treated control levels in HCT116 Bax-ko cells (65%, p = 0.004) both were accompanied by sustained reduction in
lactate excretion, recovery of
NAD+/
NADH ratio and intracellular
lactate. Following recovery from
starvation, k(PL) was significantly higher than 24 hr vehicle-treated controls (140%, p = 0.05), associated with increased LDH activity and total cellular
NAD(H). Changes in k(PL) and cellular and excreted
lactate provided measureable indicators of the major metabolic processes accompanying
starvation- and
drug-induced autophagy. The changes are reversible, returning towards and exceeding control values on cellular recovery, which potentially identifies resistance. k(PL) (hyperpolarized (13)C-MRS) and
lactate ((1)H-MRS) provide useful
biomarkers for the autophagic process, enabling non-invasive monitoring of the Warburg effect.