Abstract
Metabolic reprogramming has been coined as a hallmark of cancer, accompanied by which the alterations in metabolite levels have profound effects on gene expression, cellular differentiation and the tumor environment. Yet a systematic evaluation of quenching and extraction procedures for quantitative metabolome profiling of tumor cells is currently lacking. To achieve this, this study is aimed at establishing an unbiased and leakage-free metabolome preparation protocol for Hela carcinoma cell. We evaluated 12 combinations of quenching and extraction methods from three quenchers (liquid nitrogen, -40°C 50% methanol, 0.5°C normal saline) and four extractants (80% methanol, methanol: chloroform: water (1:1:1, v/v/v), 50% acetonitrile, 75°C 70% ethanol) for global metabolite profiling of adherent Hela carcinoma cells. Based on the isotope dilution mass spectrometry (IDMS) method, gas/liquid chromatography in tandem with mass spectrometry was used to quantitatively determine 43 metabolites including sugar phosphates, organic acids, amino acids, adenosine nucleotides and coenzymes involved in central carbon metabolism. According to the number, amount, and reproducibility of extracted metabolites, we found that 50% acetonitrile was an excellent extractant for Hela cells while 1:1:1 methanol: chloroform: water behaved the worst extraction efficiency. The results showed that the total amount of the intracellular metabolites in cell extracts obtained using different sample preparation procedures with the IDMS method ranged from 21.51 to 295.33 nmol/million cells. Consistent with previous study, metabolite leakage was found to be maximal with cold methanol as the quencher. Among 12 combinations, cells that washed twice with phosphate buffered saline (PBS), quenched with liquid nitrogen, and then extracted with 50% acetonitrile was found to be the most optimal method to acquire intracellular metabolites with minimal loss during sample preparation. In addition, the same conclusion was drawn as these 12 combinations were applied to obtain quantitative metabolome data from three-dimensional (3D) tumor spheroids. Furthermore, a case study was carried out to evaluate the effect of doxorubicin (DOX) on both adherent cells and 3D tumor spheroids using quantitative metabolite profiling. Extracellular metabolite concentrations showed that 3D cells performed an enhanced glucose consumption and lactate excretion rate than two-dimensional (2D) cells before and after the addition of DOX. While, the addition of DOX did not interfere with extracellular glutamine concentration in both 2D and 3D cells. Pathway enrichment analysis using targeted metabolomics data showed that DOX exposure would significantly affect amino acid metabolism-related pathways, which might be related to the mitigation of redox stress. Strikingly, our data suggested that compared to 2D cells the increased intracellular glutamine level in 3D cells benefited replenishing the tricarboxylic acid (TCA) cycle when the glycolysis was limited after dosing with DOX. Taken together, this study provides a well-established quenching and extraction protocol for quantitative metabolome profiling of Hela carcinoma cell under 2D and 3D cell culture conditions. Based on this, quantitative time-resolved metabolite data can serve to the generation of hypotheses on metabolic reprogramming to reveal its important role in tumor development and treatment.
Keywords : Hela; metabolomics; sample preparation; 3D tumor spheroids; quenching; extraction; isotope dilution mass spectrometry