Abstract
ABSTRACTMitochondrial DNA mutations accumulate with age and may play a role in stem cell aging as suggested by the premature aging phenotype of mitochondrial DNA polymerase gamma (POLG) exonuclease-deficient mice. Therefore, E1A immortalized murine embryonic fibroblasts (MEFs) from POLG exonuclease-deficient and wild-type (WT) mice were constructed. Surprisingly, when some E1A immortalized MEF lines were cultured in pyruvate-containing media they slowly became addicted to the pyruvate. The POLG exonuclease-deficient MEFs were more sensitive to several mitochondrial inhibitors and showed increased reactive oxygen species (ROS) production under standard conditions. When cultured in pyruvate-containing media, POLG exonuclease-deficient MEFs showed decreased oxygen consumption compared to controls. Increased AMP-activated protein kinase (AMPK) signaling and decreased mammalian target of rapamycin (mTOR) signaling delayed aging and influenced mitochondrial function. Therefore, the effects of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), an AMPK activator, or rapamycin, an mTOR inhibitor, on measures of mitochondrial function were determined. Rapamycin treatment transiently increased respiration only in WT MEFs and, under most conditions, increased ATP levels. Short term AICAR treatment transiently increased ROS production and, under most conditions, decreased ATP levels. Chronic AICAR treatment decreased respiration and ROS production in WT MEFs. These results demonstrate the context-dependent effects of AICAR and rapamycin on mitochondrial function.
Highlights
Several lines of evidence suggest the involvement of mitochondrial dysfunction in the aging process (Dai et al, 2012; Park and Larsson, 2011; Prolla and Denu, 2014)
The mitochondrial dysfunction that occurs in stem cells of polymerase gamma (POLG) exonucleasedeficient mice is likely responsible for many of the premature aging phenotypes that arise in this model (Ahlqvist et al, 2012), the stem cell alterations were somewhat distinct from those that occur during normal mouse aging (Norddahl et al, 2011)
Proper p53 function may be important for the identification of therapies that maintain or enhance mitochondrial function with aging as p53 is required for mitochondrial biogenesis (Matoba et al, 2006), the efficient repair of mitochondrial DNA (mtDNA) (Achanta et al, 2005; de Souza-Pinto et al, 2004) and for full mitochondrial pyruvate dehydrogenase activity (Contractor and Harris, 2012)
Summary
Several lines of evidence suggest the involvement of mitochondrial dysfunction in the aging process (Dai et al, 2012; Park and Larsson, 2011; Prolla and Denu, 2014). Evidence indicates that the mtDNA base substitution mutations in aged cells are primarily caused by DNA polymerase gamma (POLG) errors early in development that clonally expand over the lifespan (Larsson, 2010). The mitochondrial dysfunction that occurs in stem cells of POLG exonucleasedeficient ( called mtDNA mutator) mice is likely responsible for many of the premature aging phenotypes that arise in this model (Ahlqvist et al, 2012), the stem cell alterations were somewhat distinct from those that occur during normal mouse aging (Norddahl et al, 2011). Since endurance exercise rescued the premature aging phenotype and oxidative damage to mtDNA without substantially changing the mtDNA mutation load in these mice, mitochondrial RNA polymerase errors caused by oxidative damage to mtDNA is a potential mechanism driving the premature aging phenotype (Safdar et al, 2016a). Endurance exercise increases mitochondrial biogenesis and mtDNA copy number, which may be partly responsible for protection (Jiang et al, 2017; Safdar et al, 2011)
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