A systematic evaluation of quenching and extraction procedures for quantitative metabolome profiling of HeLa carcinoma cell under 2D and 3D cell culture conditions

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°C 80% methanol, 0.5°C methanol/chloroform/water [1:1:1 v/v/v], 0.5°C 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 (AAs), adenosine nucleotides, and coenzymes involved in central carbon metabolism. 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 per million cells. Among 12 combinations, cells that washed twice with phosphate buffered saline (PBS), quenched with liquid nitrogen, and then extracted with 50% acetonitrile were found to be the most optimal method to acquire intracellular metabolites with high efficiency of metabolic arrest and 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. Pathway enrichment analysis using targeted metabolomics data showed that DOX exposure would significantly affect AA metabolism-related pathways, which might be related to the mitigation of redox stress. Strikingly, our data suggested that compared to two-dimensional (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: 3D tumor spheroids; HeLa; extraction; isotope dilution mass spectrometry; metabolomics; quenching; sample preparation.