title: Biguanide-induced mitochondrial dysfunction yields increased lactate production and cytotoxicity of aerobically-poised HepG2 cells and human hepatocytes in vitro.
abstract: As a class, the biguanides induce lactic acidosis, a hallmark of mitochondrial impairment. To assess potential mitochondrial impairment, we evaluated the effects of metformin, buformin and phenformin on: 1) viability of HepG2 cells grown in galactose, 2) respiration by isolated mitochondria, 3) metabolic poise of HepG2 and primary human hepatocytes, 4) activities of immunocaptured respiratory complexes, and 5) mitochondrial membrane potential and redox status in primary human hepatocytes. Phenformin was the most cytotoxic of the three with buformin showing moderate toxicity, and metformin toxicity only at mM concentrations. Importantly, HepG2 cells grown in galactose are markedly more susceptible to biguanide toxicity compared to cells grown in glucose, indicating mitochondrial toxicity as a primary mode of action. The same rank order of potency was observed for isolated mitochondrial respiration where preincubation (40 min) exacerbated respiratory impairment, and was required to reveal inhibition by metformin, suggesting intramitochondrial bio-accumulation. Metabolic profiling of intact cells corroborated respiratory inhibition, but also revealed compensatory increases in lactate production from accelerated glycolysis. High (mM) concentrations of the drugs were needed to inhibit immunocaptured respiratory complexes, supporting the contention that bioaccumulation is involved. The same rank order was found when monitoring mitochondrial membrane potential, ROS production, and glutathione levels in primary human hepatocytes. In toto, these data indicate that biguanide-induced lactic acidosis can be attributed to acceleration of glycolysis in response to mitochondrial impairment. Indeed, the desired clinical outcome, viz., decreased blood glucose, could be due to increased glucose uptake and glycolytic flux in response to drug-induced mitochondrial dysfunction.
authors.affiliation: Drug Safety Research and Development, Pfizer, Inc., Ramsgate Rd. Sandwich, CT13 9NJ, UK. jamesdykens@pfizer.com
authors.forename: James A
authors.initials: JA
authors.lastname: Dykens
authors.forename: Joseph
authors.initials: J
authors.lastname: Jamieson
authors.forename: Lisa
authors.initials: L
authors.lastname: Marroquin
authors.forename: Sashi
authors.initials: S
authors.lastname: Nadanaciva
authors.forename: Puja A
authors.initials: PA
authors.lastname: Billis
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Will
mesh_terms: D000140:Acidosis, Lactic / Q000139:chemically induced; D000818:Animals; D001786:Blood Glucose / Q000187:drug effects; D002026:Buformin / Q000008:administration & dosage / Q000633:toxicity*; D002460:Cell Line; D019069:Cell Respiration / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D003470:Culture Media; D005978:Glutathione / Q000187:drug effects / Q000378:metabolism; D006019:Glycolysis / Q000187:drug effects; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007004:Hypoglycemic Agents / Q000008:administration & dosage / Q000633:toxicity*; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D008687:Metformin / Q000008:administration & dosage / Q000633:toxicity*; D008930:Mitochondria, Liver / Q000187:drug effects*; D010084:Oxidation-Reduction / Q000187:drug effects; D010629:Phenformin / Q000008:administration & dosage / Q000633:toxicity*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017382:Reactive Oxygen Species / Q000378:metabolism
chemical_list: D001786:Blood Glucose; D003470:Culture Media; D007004:Hypoglycemic Agents; D017382:Reactive Oxygen Species; D019344:Lactic Acid; D008687:Metformin; D010629:Phenformin; D005978:Glutathione; D002026:Buformin