Characterisation of bioenergetic pathways and related regulators by multiple assays in human tumour cells

BACKGROUND: Alterations in cellular metabolism are considered as hallmarks of cancers, however, to recognize these alterations and understand their mechanisms appropriate techniques are required. Our hypothesis was to determine whether dominant bioenergetic mechanism may be estimated by comparing the substrate utilisation with different methods to detect the labelled carbon incorporation and their application in tumour cells. METHODS: To define the bioenergetic pathways different metabolic tests were applied: (a) measuring CO(2) production from [1-(14)C]-glucose and [1-(14)C]-acetate; (b) studying the effect of glucose and acetate on adenylate energy charge; (c) analysing glycolytic and TCA cycle metabolites and the number of incorporated (13)C atoms after [U-(13)C]-glucose/[2-(13)C]-acetate labelling. Based on [1-(14)C]-substrate oxidation two selected cell lines out of seven were analysed in details, in which the highest difference was detected at their substrate utilization. To elucidate the relevance of metabolic characterisation the expression of certain regulatory factors, bioenergetic enzymes, mammalian target of rapamycin (mTOR) complexes (C1/C2) and related targets as important elements at the crossroad of cellular signalling network were also investigated. RESULTS: Both [U-(13)C]-glucose and [1-(14)C]-substrate labelling indicated high glycolytic capacity of tumour cells. However, the ratio of certain (13)C-labelled metabolites showed detailed metabolic differences in the two selected cell lines in further characterisation. The detected differences of GAPDH, β-F1-ATP-ase expression and adenylate energy charge in HT-1080 and ZR-75.1 tumour cells also confirmed the altered metabolism. Moreover, the highly limited labelling of citrate by [2-(13)C]-acetate—representing a novel functional test in malignant cells—confirmed the defect of TCA cycle of HT-1080 in contrast to ZR-75.1 cells. Noteworthy, the impaired TCA cycle in HT-1080 cells were associated with high mTORC1 activity, negligible protein level and activity of mTORC2, high expression of interleukin-1β, interleukin-6 and heme oxygenase-1 which may contribute to the compensatory mechanism of TCA deficiency. CONCLUSIONS: The applied methods of energy substrate utilisation and other measurements represent simple assay system using (13)C-acetate and glucose to recognize dominant bioenergetic pathways in tumour cells. These may offer a possibility to characterise metabolic subtypes of human tumours and provide guidelines to find biomarkers for prediction and development of new metabolism related targets in personalized therapy.