Age-dependent increases in oxidative damage to DNA, lipids, and proteins in human skeletal muscle

Abstract

A role for oxidative damage in normal aging is supported by studies in experimental animals, but there is limited evidence in man. We examined markers of oxidative damage to DNA, lipids, and proteins in 66 muscle biopsy specimens from humans aged 25 to 93 years. There were age-dependent increases in 8-hydroxy-2-deoxyguanosine (OH⁸dG), a marker of oxidative damage to DNA, in malondialdehyde (MDA), a marker of lipid peroxidation, and to a lesser extent in protein carbonyl groups, a marker of protein oxidation. The increases in OH⁸dG were significantly correlated with increases in MDA. These results provide evidence for a role of oxidative damage in human aging which may contribute to age-dependent losses of muscle strength and stamina.

Introduction

An age-dependent attrition of muscle strength and stamina for sustained physical effort is a well-established feature of aging in humans and other species [1]. One hypothesis of aging proposes that the physiological changes of aging are a consequence of the accumulation of random oxidative damage to DNA, lipids, and proteins. The free radical theory of aging was first proposed by Harman [2], and has, subsequently, been focused on mitochondria as the major site of free radical generation and damage [3], [4], [5]. Evidence in support of this contention has accumulated in concert with advances in methodologies for measuring oxidative damage. To date, however, most of the evidence comes from studies in species other than humans [6].

Two tissues that may be particularly prone to oxidative damage are muscle and the central nervous system. Both tissues contain postmitotic cells, which are liable to accumulate oxidative damage over time, and both account for a large share of the body’s total oxygen consumption at rest. We previously showed an age-dependent accumulation of oxidative damage to both nuclear and mitochondrial DNA in human cerebral cortex [7]. The increase was 10-fold greater in mitochondrial DNA as compared with nuclear DNA, consistent with prior work in aging rat liver. [8] Similarly, an age-dependent increase in protein carbonyls was found in brain in human and animal models [9], [10].

Other studies in humans showed age-dependent decreases in state 3 and state 4 respiration in liver mitochondria [11]. There are age-dependent increases in numbers of cytochrome oxidase deficient myocytes in skeletal and cardiac muscle, and reduced activities of several electron transport chain complexes in skeletal muscle mitochondria [12], [13], [14], [15] Age-dependent increases in mitochondrial deletions and rearrangements are also found in human skeletal muscle [14], [16]. A correlation between content of mitochondrial DNA deletions and declines in respiratory chain enzyme activities in human skeletal muscle was reported [17]. The accumulation of mitochondrial deletions overall is low but high levels are found in association with cytochrome oxidase negative muscle fibers, which accumulate with human aging [18]. In human cardiac and diaphragmatic muscle an age-dependent increase in 8-hydroxy-2-deoxyguanosine in mitochondrial DNA was reported [19], [20]. Mitochondrial proteins in human skeletal muscle are particularly susceptible to free radical induced oxidative damage [21]. The rate of mitochondrial protein synthesis in human skeletal muscle shows age-dependent decreases [22]. In the present study, we provide additional evidence for age-dependent oxidative damage in human skeletal muscle. We examined three well-established markers of oxidative damage to DNA, lipids and protein in muscle biopsies obtained from 66 patients aged 25 to 93 years.