Low intracellular folate levels diminish the growth rate of HT-29 human colon cancer cells. This is accompanied by a metabolic shift from cytosolic glycolysis towards mitochondrial oxidative phosphorylation, as demonstrated by a lower lactate production and an increased mitochondrial oxygen consumption rate. To obtain insight in the molecular effects underlying these changes, the steady state gene expression profiles of HT-29 cells with different intracellular folate concentrations were compared. The gene expression profile of HT-29 cells with low intracellular folate levels (grown for 3 weeks in 10 ng/ml folic acid (PGA)) was clearly distinct from that of the other exposure conditions, which provide sufficient intracellular folate levels (100 ng/ml PGA, 10 ng/ml methyltetrahydrofolate (MTHF) or 100 ng/ml MTHF). Intracellular folate deficiency, contrary to expectation, did not lead to major changes in expression of genes involved in energy metabolism. This suggests that the shift towards mitochondrial oxidative phosphorylation is not mediated at the transcription level. Furthermore, only minor changes in the expression of folate metabolism related genes were observed. The changes that were observed were consistent with nucleotide salvage and in agreement with nucleotide need of the slow-growing folate-deficient HT-29 cells. The major observed effects were on cell cycle related gene expression, which was increased and interferon-responsive gene expression, which was reduced. The increase in cell cycle related gene expression seems compensatory to the reduced cell growth. Down-regulation of the interferon-response may be explained by decreased expression of signal transducer and activator of transcription 1 upon folate deficiency.
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