Metabolic Compensation in Rotenone-Treated CHO Cells
Researchers study mitochondrial dysfunction in Chinese hamster ovary (CHO) cells cultured in DMEM with 25 mM glucose and 4 mM glutamine. Cells are exposed to rotenone, a complex I inhibitor, for 24 h. At 10 nM rotenone, the oxygen consumption rate (OCR) falls by 40%, yet the ATP/ADP ratio is 95% of control and extracellular lactate doubles. At 100 nM, OCR falls by 85%, ATP/ADP drops to 38% of control, and caspase-3 activity rises fourfold. The team considers two follow-up manipulations: adding AZD3965, an MCT1 inhibitor that blocks lactate export, or replacing glucose with galactose. Galactose is metabolized slowly through glycolysis, so cells must rely on oxidative phosphorylation for ATP. The medium pH is 7.4 at the start. No other inhibitors are present.
1. Explain why the ATP/ADP ratio remains near control at 10 nM rotenone even though OCR is reduced by 40%. Identify the pathway that compensates and the substrate source.
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At 10 nM, rotenone only partially inhibits complex I. The cells shift ATP production toward glycolysis: lactate secretion doubles, showing increased conversion of pyruvate to lactate. This regenerates NAD+ so glycolysis can continue, and glycolysis produces ATP. The ATP/ADP ratio is a balance between consumption and production; a near-control ratio means glycolytic ATP plus residual oxidative phosphorylation and glutamine-supported oxidation (for example, FADH2 via complex II) compensate for the 40% OCR loss. At 100 nM, compensation fails, ATP/ADP falls, and apoptosis markers rise.
2. Predict the effect of adding AZD3965 at 10 nM rotenone on intracellular lactate, intracellular pH, and ATP/ADP. Justify your prediction.
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AZD3965 blocks MCT1-mediated lactate export. At 10 nM rotenone, cells are already relying on lactate production; inhibiting export will cause intracellular lactate and H+ to accumulate, lowering intracellular pH. The pH drop and product accumulation inhibit glycolysis (for example, phosphofructokinase), reducing NAD+ regeneration and ATP yield. Therefore ATP/ADP should fall, possibly below the 95% seen with rotenone alone, and the cells may become more sensitive to rotenone.
3. Predict whether replacing glucose with galactose will increase, decrease, or leave unchanged the cells' sensitivity to 10 nM rotenone. Explain.
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Galactose slows entry into glycolysis, so cells cannot upregulate glycolytic ATP production as effectively. They rely more on oxidative phosphorylation. Because rotenone inhibits complex I, the same 10 nM dose should cause a larger drop in ATP/ADP and greater sensitivity. Thus switching to galactose will increase sensitivity to rotenone.