Mitochondrial AZT metabolism

Environmental Chemical Exposures and Mitochondrial Dysfunction: a Review of Recent Literature

PURPOSE OF REVIEW

Mitochondria play various roles that are important for cell function and survival; therefore, significant mitochondrial dysfunction may have chronic consequences that extend beyond the cell. Mitochondria are already susceptible to damage, which may be exacerbated by environmental exposures. Therefore, the aim of this review is to summarize the recent literature (2012-2022) looking at the effects of six ubiquitous classes of compounds on mitochondrial dysfunction in human populations.

RECENT FINDINGS

The literature suggests that there are a number of biomarkers that are commonly used to identify mitochondrial dysfunction, each with certain advantages and limitations. Classes of environmental toxicants such as polycyclic aromatic hydrocarbons, air pollutants, heavy metals, endocrine-disrupting compounds, pesticides, and nanomaterials can damage the mitochondria in varied ways, with changes in mtDNA copy number and measures of oxidative damage the most commonly measured in human populations. Other significant biomarkers include changes in mitochondrial membrane potential, calcium levels, and ATP levels. This review identifies the biomarkers that are commonly used to characterize mitochondrial dysfunction but suggests that emerging mitochondrial biomarkers, such as cell-free mitochondria and blood cardiolipin levels, may provide greater insight into the impacts of exposures on mitochondrial function. This review identifies that the mtDNA copy number and measures of oxidative damage are commonly used to characterize mitochondrial dysfunction, but suggests using novel approaches in addition to well-characterized ones to create standardized protocols. We identified a dearth of studies on mitochondrial dysfunction in human populations exposed to metals, endocrine-disrupting chemicals, pesticides, and nanoparticles as a gap in knowledge that needs attention.

SAMHD1 has differential impact on the efficacies of HIV nucleoside reverse transcriptase inhibitors

Sterile alpha motif- and histidine/aspartic acid domain-containing protein 1 (SAMHD1) limits HIV-1 replication by hydrolyzing deoxynucleoside triphosphates (dNTPs) necessary for reverse transcription. Nucleoside reverse transcriptase inhibitors (NRTIs) are components of anti-HIV therapies. We report here that SAMHD1 cleaves NRTI triphosphates (TPs) at significantly lower rates than dNTPs and that SAMHD1 depletion from monocytic cells affects the susceptibility of HIV-1 infections to NRTIs in complex ways that depend not only on the relative changes in dNTP and NRTI-TP concentrations but also on the NRTI activation pathways.

Zidovudine exposure in HIV-1 infected Tanzanian women increases mitochondrial DNA levels in placenta and umbilical cords

BACKGROUND

Zidovudine (AZT) constitutes part of the recommended regimens for prevention and treatment of HIV-1 infection. At the same time, AZT as well as HIV-1 infection itself may induce mitochondrial damage. In this study, we analyzed the impact of prenatal AZT-exposure on mitochondrial alterations in HIV-infected women and their infants.

METHODS

Mitochondrial DNA (mtDNA) levels in placentas of HIV-1 infected Tanzanian women with and without prenatal AZT exposure, and in the umbilical cords of their AZT-exposed/unexposed infants were quantified using real-time PCR. Furthermore, we checked for the most common mitochondrial deletion in humans, the 4977 base pair deletion (dmtDNA4977) as a marker for mitochondrial stress.

RESULTS

83 women fulfilled the inclusion criteria. 30 women had been treated with AZT (median duration 56 days; IQR 43-70 days) while 53 women had not taken AZT during pregnancy. Baseline maternal characteristics in the two groups were similar. The median mtDNA levels in placentas and umbilical cords of women (311 copies/cell) and infants (190 copies/cell) exposed to AZT were significantly higher than in AZT-unexposed women (187 copies/cell; p = 0.021) and infants (127 copies/cell; p = 0.037). The dmtDNA4977 was found in placentas of one woman of each group and in 3 umbilical cords of AZT-unexposed infants but not in umbilical cords of AZT-exposed infants.

CONCLUSIONS

Antenatal AZT intake did not increase the risk for the common mitochondrial deletion dmtDNA4977. Our data suggests that AZT exposure elevates mtDNA levels in placentas and umbilical cords possibly by positively influencing the course of maternal HIV-1 infection.

A review comparing deoxyribonucleoside triphosphate (dNTP) concentrations in the mitochondrial and cytoplasmic compartments of normal and transformed cells

The deoxyribonucleoside triphosphate (dNTP) pools that support the replication of mitochondrial DNA are physically separated from the rest of the cell by the double membrane of the mitochondria. Perturbed homeostasis of mitochondrial dNTP pools is associated with a set of severe diseases collectively termed mitochondrial DNA depletion syndromes. The degree of interaction of the mitochondrial dNTP pools with the corresponding dNTP pools in the cytoplasm is currently not clear. We reviewed the literature on previously reported simultaneous measurements of mitochondrial and cytoplasmic deoxyribonucleoside triphosphate pools to investigate and quantify the extent of the influence of the cytoplasmic nucleotide metabolism on mitochondrial dNTP pools. We converted the reported measurements to concentrations creating a catalog of paired mitochondrial and cytoplasmic dNTP concentration measurements. Over experiments from multiple laboratories, dNTP concentrations in the mitochondria are highly correlated with dNTP concentrations in the cytoplasm in normal cells in culture (Pearson R = 0.79, p = 3 × 10(-7)) but not in transformed cells. For dTTP and dATP there was a strong linear relationship between the cytoplasmic and mitochondrial concentrations in normal cells. From this linear model we hypothesize that the salvage pathway within the mitochondrion is only capable of forming a concentration of approximately 2 μM of dTTP and dATP, and that higher concentrations require transport of deoxyribonucleotides from the cytoplasm.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Mechanisms of antiretroviral therapy-induced mitochondrial dysfunction

PURPOSE OF REVIEW

To discuss novel developments related to the mechanisms of antiretroviral therapy-related mitochondrial toxicity, describe some apparent paradoxes in the current understanding of this field, and present questions that should be addressed by future research.

RECENT FINDINGS

The early polymerase gamma hypothesis states that nucleoside reverse transcriptase inhibitors can inhibit mitochondrial DNA replication and cause mitochondrial toxicity through mtDNA depletion. This mechanism is supported by a large body of evidence. Clinical manifestations of mitochondrial dysfunction are not always associated with mtDNA depletion. Increased mtDNA levels after nucleoside reverse transcriptase inhibitor exposure, as well as seemingly severe mtDNA depletion in individuals who show no clinical toxicity, have been reported. These and other observations suggest that additional mechanisms are involved in antiretroviral therapy toxicity, a notion supported by recent studies. Individuals given the same antiretroviral regimen can differ vastly with respect to the development of mitochondrial toxicity symptoms, reflecting interindividual variability. Some factors that may modulate this variability will be discussed.

SUMMARY

Mitochondrial toxicity induced by nucleoside reverse transcriptase inhibitors and their metabolic intermediates is probably mediated through many direct and indirect mechanisms. Depending on the mechanisms at play, the long-term health consequences of this toxicity may vary.

Disruption of mitochondrial DNA replication in Drosophila increases mitochondrial fast axonal transport in vivo

Mutations in mitochondrial DNA polymerase (pol γ) cause several progressive human diseases including Parkinson's disease, Alper's syndrome, and progressive external ophthalmoplegia. At the cellular level, disruption of pol γ leads to depletion of mtDNA, disrupts the mitochondrial respiratory chain, and increases susceptibility to oxidative stress. Although recent studies have intensified focus on the role of mtDNA in neuronal diseases, the changes that take place in mitochondrial biogenesis and mitochondrial axonal transport when mtDNA replication is disrupted are unknown. Using high-speed confocal microscopy, electron microscopy and biochemical approaches, we report that mutations in pol γ deplete mtDNA levels and lead to an increase in mitochondrial density in Drosophila proximal nerves and muscles, without a noticeable increase in mitochondrial fragmentation. Furthermore, there is a rise in flux of bidirectional mitochondrial axonal transport, albeit with slower kinesin-based anterograde transport. In contrast, flux of synaptic vesicle precursors was modestly decreased in pol γ−α mutants. Our data indicate that disruption of mtDNA replication does not hinder mitochondrial biogenesis, increases mitochondrial axonal transport, and raises the question of whether high levels of circulating mtDNA-deficient mitochondria are beneficial or deleterious in mtDNA diseases.

A new drug binding subsite on human serum albumin and drug-drug interaction studied by X-ray crystallography

3'-Azido-3'-deoxythymidine (AZT) is the first clinically effective drug for the treatment of human immunodeficiency virus infection. The drug interaction with human serum albumin (HSA) has been an important component in understanding its mechanism of action, especially in drug distribution and in drug-drug interaction on HSA in the case of multi-drug therapy. We present here crystal structures of a ternary HSA-Myr-AZT complex and a quaternary HSA-Myr-AZT-SAL complex (Myr, myristate; SAL, salicylic acid). From this study, a new drug binding subsite on HSA Sudlow site 1 was identified. The presence of fatty acid is needed for the creation of this subsite due to fatty acid induced conformational changes of HSA. Thus, the Sudlow site 1 of HSA can be divided into three non-overlapped subsites: a SAL subsite, an indomethacin subsite and an AZT subsite. Binding of a drug to HSA often influences simultaneous binding of other drugs. From the HSA-Myr-AZT-SAL complex structure, we observed the coexistence of two drugs (AZT and SAL) in Sudlow site 1 and the competition between these two drugs in subdomain IB. These results provide new structural information on HSA-drug interaction and drug-drug interaction on HSA.

Targeted transgenic overexpression of mitochondrial thymidine kinase (TK2) alters mitochondrial DNA (mtDNA) and mitochondrial polypeptide abundance: transgenic TK2, mtDNA, and antiretrovirals

Mitochondrial toxicity limits nucleoside reverse transcriptase inhibitors (NRTIs) for acquired immune deficiency syndrome. NRTI triphosphates, the active moieties, inhibit human immunodeficiency virus reverse transcriptase and eukaryotic mitochondrial DNA polymerase pol-gamma. NRTI phosphorylation seems to correlate with mitochondrial toxicity, but experimental evidence is lacking. Transgenic mice (TGs) with cardiac overexpression of thymidine kinase isoforms (mitochondrial TK2 and cytoplasmic TK1) were used to study NRTI mitochondrial toxicity. Echocardiography and nuclear magnetic resonance imaging defined cardiac performance and structure. TK gene copy and enzyme activity, mitochondrial (mt) DNA and polypeptide abundance, succinate dehydrogenase and cytochrome oxidase histochemistry, and electron microscopy correlated with transgenesis, mitochondrial structure, and biogenesis. Antiretroviral combinations simulated therapy. Untreated hTK1 or TK2 TGs exhibited normal left ventricle mass. In TK2 TGs, cardiac TK2 gene copy doubled, activity increased 300-fold, and mtDNA abundance doubled. Abundance of the 17-kd subunit of complex I, succinate dehydrogenase histochemical activity, and cristae density increased. NRTIs increased left ventricle mass 20% in TK2 TGs. TK activity increased 3 logs in hTK1 TGs, but no cardiac phenotype resulted. NRTIs abrogated functional effects of transgenically increased TK2 activity but had no effect on TK2 mtDNA abundance. Thus, NRTI mitochondrial phosphorylation by TK2 is integral to clinical NRTI mitochondrial toxicity.