Metformin Sensitivity in Glucose- versus Galactose-Grown HepG2 Cells
An energy-metabolism laboratory grows HepG2 hepatoma cells for 24 h in DMEM containing either 25 mM glucose or 10 mM galactose. After 6 h, half of each group receives 2 mM metformin. At 24 h, medium lactate is 18.4 mM in glucose controls and 4.1 mM in galactose controls. Basal oxygen consumption rate (OCR) is 9 pmol min−1 per 10^4 cells in glucose and 31 pmol min−1 per 10^4 cells in galactose. ATP content, normalized to glucose controls, is 58% in galactose. Metformin lowers ATP to 82% in glucose but to 21% in galactose; OCR falls to 48% in glucose and to 6% in galactose. The laboratory must decide whether the metformin effect is mainly complex I inhibition or a secondary consequence of the Crabtree effect, and whether galactose should replace glucose in the screening assay.
1. Calculate the within-group changes in OCR and ATP caused by metformin in glucose and galactose, and state which medium better exposes mitochondrial dependence.
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In glucose, OCR falls from 9 to 9 × 0.48 = 4.3 pmol min−1 per 10^4 cells, a 52% fall. ATP falls from 100% to 82%, an 18% fall. In galactose, OCR falls from 31 to 31 × 0.06 = 1.9 pmol min−1 per 10^4 cells, a 94% fall. ATP in galactose controls is 58% of the glucose control; after metformin it is 21%, so within galactose the ATP remaining is 21/58 = 36%, a 64% fall. Thus metformin produces a much larger bioenergetic collapse in galactose. The key comparison is not absolute ATP alone but ATP relative to the same medium's control: galactose-grown cells have higher OCR and depend more on oxidative phosphorylation, so complex I inhibition cannot be buffered by glycolytic ATP. Galactose is therefore the better screening condition for mitochondrial inhibitors, with the caveat that it is a more stressful medium and may exaggerate toxicity.
2. Explain the Crabtree effect and why 25 mM glucose can make metformin appear less toxic.
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The Crabtree effect is the suppression of respiration by high extracellular glucose. High glucose drives high glycolytic flux, so pyruvate is largely converted to lactate, regenerating NAD+ and producing glycolytic ATP. The cells do not need to oxidize pyruvate in the mitochondria to meet ATP demand, so oxygen consumption is lower. If complex I is partially inhibited by metformin, the cell can still generate ATP from glycolysis and is less dependent on the electron transport chain. This masks the mitochondrial lesion: ATP falls only 18% in glucose, while OCR is already low, so a further fall has less functional impact. In galactose, glycolysis yields far less ATP, lactate is low, and cells must oxidize pyruvate to support ATP production; complex I inhibition then causes a large ATP and OCR drop. The apparent protection by glucose is therefore a metabolic shift, not evidence that metformin spares mitochondria.
3. The core proposes adding 1 µM oligomycin to confirm that the residual OCR in galactose plus metformin is mitochondrial. Predict the result and explain what it proves and does not prove.
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Oligomycin inhibits mitochondrial ATP synthase. In coupled mitochondria, ATP-linked respiration falls when ATP synthase is blocked, so a substantial oligomycin-sensitive OCR component indicates that respiration is driving ATP synthesis. In galactose plus metformin, residual OCR is already only 6% of the galactose control, so the absolute oligomycin-sensitive fraction will be small; a further fall is expected if the remaining respiration is ATP-linked. If OCR is unchanged by oligomycin, the residual oxygen consumption is more likely proton leak or non-mitochondrial oxidase activity. Oligomycin proves coupling and mitochondrial ATP turnover, but it does not identify complex I. To test complex I specifically, rotenone should abolish complex I-dependent respiration, while succinate can bypass complex I and support respiration through complex II. The correct interpretation therefore needs both oligomycin and substrate/inhibitor experiments.