Antiretroviral nucleoside and nucleotide analogues and mitochondria

Introduction This review is intended to provide understanding of the function of mitochondria, the advantages and disadvantages of available techniques for assessing mitochondrial function and quantity and to discuss the main clinical toxicities thought to be associated with mitochondrial dysfunction(definition) and how they may be managed or their prevalence reduced. Mitochondria are the key organelles in energy production in all human cells except erythrocytes. Energy, in the form of ATP, is produced through the highly efficient oxidative phosphorylation pathway. Additionally, mitochondria perform a range of other biological functions and carry a number of factors involved in cell apoptosis. Both HIV infection and antiretroviral nucleoside analogues (nucleoside reverse transcriptase inhibitors; NRTI) are known to affect mitochondrial DNA content and other aspects of mitochondrial function. A number of important clinical events occurring in individuals with HIV infection and on antiretroviral therapy have been linked to mitochondrial injury and dysfunction. In vitro studies have demonstrated that NRTI may differ in their effects on mitochondria and may affect mitochondria in different cell lines in different ways. This is likely to influence the clinical syndromes associated with toxicity to these agents. Dideoxy-NRTI have the greatest affinity for mitochondrial DNA polymerase-γ, the enzyme responsible for mitochondrial DNA replication, whereas other nucleoside analogues may influence mitochondrial function also through other mechanisms. These differences may be important in choosing techniques to evaluate the impact of antiretroviral agents on mitochondria. Mitochondria: roles and regulation The mitochondrion, from the Greek mito, thread and khóndrion, granule, was identified at the end of the nineteenth century. It was subsequently established that the mitochondrion was responsible for the majority of cellular energy production (in form of ATP) through the process of oxidative phosphorylation, driven by mitochondrial membrane potential (chemiosmosis) [1,2]. Subsequently, other biochemical and biological contributions of mitochondria to eukaryotic cellular function have been described (summarized in Table 1). More recently, we have expanded our understanding of the genetic basis of human diseases associated with mitochondrial DNA (mtDNA) mutations and the mechanism(s) that regulate the numbers and mass of these organelles, as well as the duplication of the DNA that these organelles contain [3].Table 1: Main roles and characteristics of mitochondria.The number, mass and morphology of mitochondria is partially controlled by fusion and fission events regulated by a variety of protein messengers [4]. Intracellular mechanisms exist to regulate the distribution of mitochondria during the cell cycle and cell division, their morphology, the replication and the inheritance of mtDNA into daughter cells. The morphology of mitochondria shows variability across different cell types [5]. Cells of hemopoietic origin (such as lymphocytes, monocytes, or platelets) typically display cigar-like organelles, while in muscle cells mitochondria are seen in rows between sarcolemma, in sperm cells they form spirals in the tail region, in fibroblasts and preadipocytes mitochondria form a reticulum. They may fuse or increase in size to form giant mitochondria or megamitochondria, and tend to locate near the structures where energy is required. Additionally, mitochondrial appearance and number in a tissue may change in response to changing tissue needs, most evident in cold adaptation [6] or under the situation of high metabolic activity. Conversely, the number of mitochondria can be reduced by pyknosis, ballooning, or autophagolysosome formation. Recent data underline the importance of mitochondria morphology and its correlation with the organelle's functionality, as the pleomorphicity of such organelle is probably linked to cell cycle stage or to the metabolic state of the organelle itself [7]. The mitochondrial matrix contains circular DNA of 16 569 base pairs, in punctate structures called ‘nucleoids'. Each nucleoid may contain several copies of mtDNA. Replication of mtDNA is performed by DNA polymerase-γ, a nuclear DNA (nDNA) encoded enzyme. In humans, several factors are required for transcription initiation, including the human mitochondrial promoter, h-mtRNA polymerase, and the DNA binding mitochondrial transcription factors, h-mtTF-A and -B [8,9]. Additional regulatory factors are hypothesized. Little is known of the factors that regulate the production and neogenesis of mitochondria. Studies are complicated by the fact that their stability varies considerably from tissue to tissue (their half-life in liver is suggested to be 3–5 days, in brain 30 days [10,11]), and that their components have different turnover rates. More is known about the mitochondrial changes provoked by agents which cause functional impairment or dysruption. When altered by a stimulus that does not provoke the necrotic death of the cell, mitochondria generate or amplify signals that lead to programmed cell death/apoptosis. Mitochondrial apoptosis, mitoptosis [12], occurs following stimuli provided by reactive oxygen species or tumour necrosis factor (TNF)-α and can result in cellular apoptosis because of the loss of mitochondrial membrane potential (ΔΨm), and the release of apoptogenic proteins such as cytochrome c or the apoptosis-inducing factor [13–16]. Mitochondria also contain antiapoptotic proteins whose role is yet to be clarified [17–21]. Non-fatal insults that damage mtDNA can be repaired by a proficient base excision repair of oxidative DNA damage [22]. However, mtDNA damage can impair the capacity of the organelle to synthesize crucial enzymes of the oxidative phosphorylation, so provoking a greater dependence on lactate generating cytosolic metabolism of glucose and a reduced capacity to clear H+ formed from the hydrolysis of ATP. M