Mitochondria Are Sensors for HIV Drugs
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Review TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 Mitochondria are sensors for HIV drugs Fre´ de´ ric Petit1, Bernard Fromenty2, Andrew Owen3 and Je´ roˆ me Estaquier1 1Unite´ de Physiopathologie des Infections Lentivirales, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris cedex 15, France 2INSERM U481, Faculte´deMe´ decine Xavier Bichat, 16 rue Henri Huchard BP416, 75870 Paris cedex 18, France 3Department of Pharmacology and Therapeutics, 70 Pembroke Place, University of Liverpool, Liverpool L69 3GF, UK Highly active anti-retroviral therapy (HAART) has dras- care of HIV-1 infected individuals. However, drug toxicity tically altered the course of HIV-1 infection, resulting in a is one of the main obstacles to long-term therapy for HIV-1 major decrease in morbidity and mortality. However, infection [3]. Adverse effects of drugs can occur early adverse drug reactions and long-term toxicities associ- (within the first 3–6 months of therapy) or late (occurring ated with HAART are now a concern. A major toxicity in individuals who are established on, and tolerating, the that has been highlighted by the increased use of HAART drug treatment for some time). Most early toxicities such is related to mitochondrial side-effects. At the same as nausea, diarrhoea, rash and sleep disturbances are time, analysis of the biochemical pathways involved in predictable, transient and of mild-to-moderate intensity. programmed cell death has revealed that mitochondria Specifically, the lipodystrophy syndrome (LDS) consisting are main sensors in this process. In this article, the of dyslipidemia, metabolic abnormalities of insulin resist- regulation of mitochondrial damage following the use of ance and redistribution of body fat has become a central nucleoside analogue reverse transcriptase inhibitors issue in the primary care of HIV-infected patients [4,5]. (NRTIs) and protease inhibitors is discussed, with a particular focus on the putative molecular mechanisms involved. Preventive effects of HIV drugs on apoptosis and loss of mitochondrial membrane potential Highly active anti-retroviral therapy [HAART (a mixture of NRTIs and protease inhibitors)] aims to slow the rate of HIV-1 and its current therapy viral replication sufficiently to reduce the viral load and The human immunodeficiency virus type 1 (HIV-1) is a thereby stem the emergence of resistant forms of the retrovirus and is the causative agent of AIDS. The C virus. Thus, HAART produces a significant immune depletion of CD4 T cells is a major determinant of HIV-1 system reconstitution (with sustained increases in circu- pathogenicity [1]. Once HIV-1 has entered a cell, HIV lating levels of CD4C T cells) after a rapid drop in plasma reverse transcriptase converts the single-stranded viral levels of viral RNA and decreased apoptosis (Box 1). RNA into double-stranded DNA. The double-stranded A correlation between the magnitude of apoptosis observed DNA is then integrated into the host cell genome and in HIV-infected individuals and the stage of HIV disease has transcribed into a full-length mRNA, one of the primary been described [1,6,7]. However, protease inhibitors in translation products of which is the gag–pol polyprotein addition to exerting anti-viral effects can also have a direct product. Proteolytic cleavage of the gag–pol polyproteins effect on immune cells. Indeed, the susceptibility of by HIV-1 encoded aspartyl protease is necessary for the peripheral blood T cells to apoptosis is rapidly decreased 4 production of mature infectious virions. Currently days following the initiation of treatment with protease licensed anti-retroviral therapies focus on the HIV-1 inhibitors in adults and children receiving HAART [7]. reverse transcriptase and the HIV-1 protease as potential Therefore, protease inhibitors have clinical and immuno- targets (Table 1) [2]. Nucleoside reverse transcriptase logical benefits (even in the absence of sustained viral inhibitors [NRTIs (nucleoside analogues)] cause termin- suppression) that might be attributable to anti-apoptotic ation of the formation of the DNA chain, whereas non- properties. Several mechanisms might explain how pro- nucleoside reverse transcriptase inhibitors (NNRTIs) bind tease inhibitors decrease apoptosis. Protease inhibitors directly to, and inhibit, the action of the reverse such as ritonavir modulate proteasome activity and major transcriptase. However, anti-retroviral therapy directed histocompatibility complex (MHC) class I-restricted pres- at only the reverse transcriptase of HIV-1 has had limited entation [8], which might decrease immune activation and success because of drug toxicity and the emergence of viral resistance. Protease inhibitors inactivate the HIV-1 Table 1. Currently licensed anti-retroviral drugs protease and prevent the cleavage of gag–pol proteins. NRTIs NNRTIs Protease inhibitors Protease inhibitors, unlike reverse transcriptase inhibi- Tenofovir Efavirenz Indinavir tors, exhibit their effect late in the HIV replication cycle Emtricitabine Nevirapine Saquinavir and are active against chronically infected cells. Thus, Didanosine (ddI) Delavirdine Ritonavir combination anti-retroviral therapy of at least two NRTIs Stavudine (d4T) Atazanavir Lamivudine (3TC) Amprenavir and a protease inhibitor is considered to be the standard of Zidovudine (AZT) Lopinavir Corresponding author: Estaquier, J. ([email protected]). Abacadir (ABC) Available online 1 April 2005 Zalcitabine (ddc) www.sciencedirect.com 0165-6147/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2005.03.006 Review TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 259 and isolated mitochondria treated with the permeability Box 1. Apoptosis pathways transition pore complex (PTPC) inducers atractyloside Programmed cell death (PCD) and its main phenotype apoptosis is a and Vpr [18]. Therefore, protease inhibitors might prevent cell suicide programme that is essential for development and for in vivo mitochondrial injury, leading to the restoration of adult tissue homeostasis in all metazoan animals. Defects (inhibition C or exacerbation) in PCD are involved in several pathologies such as CD4 T cells. neurodegenerative diseases, cancers or AIDS. The stereotypical death throes of a cell undergoing apoptosis include DNA fragmenta- Side-effects of anti-retroviral drugs tion, nuclear condensation, cell shrinkage, blebbing and phospha- The efficacy and toxicity of any given NRTI analogue is the tidylserine externalization, all of which promote the physiologically result of many factors, including protein binding, trans- silent removal of the cell by its phagocytic neighbours. Mitochondria are implicated in the two major apoptotic pathways currently port (influx or efflux), and metabolic activation, incorpor- accepted as the models for cell death. The death receptor-mediated ation and degradation. Patients treated with one or more pathway (extrinsic pathway) leads to activation of caspase-8, which NRTI analogue experience a variety of side-effects, cleaves the pro-apoptotic Bcl-2 family member Bid, generating a including peripheral neuropathy, cardiac and skeletal truncated Bid (tBid). tBid translocates to the mitochondria where it muscle myopathy, pancreatitis, hepatic steatosis, lactic acts with the pro-apoptotic Bcl-2 family members Bax and Bak. Cellular deprival and stress-mediated apoptosis is regulated pre- acidosis and bone marrow suppression [4,5,19]. Indeed, dominantly at the mitochondrial level (intrinsic pathway). In both dose-dependent peripheral neuropathy is the major treat- pathways, mitochondrial injury is manifested by the release of ment-limiting adverse effect of NRTI drugs. Peripheral apoptogenic factors, leading to the apoptotic phenotype [1] neuropathy occurs in 34% of patients receiving NRTIs (Figure I). such as zalcitabine (ddC) and is manifested w6-8 weeks after starting therapy, often as the first indicator of Extrinsic pathway Intrinsic pathway toxicity. However, it is usually reversed following the Death receptors UV, drugs, withdrawal of the offending NRTIs [4,5]. Steatosis and (Fas, TNFR) deprival non-alcoholic steatohepatitis (NASH) can also be induced Caspase-8 by some NRTIs and provoke lactic acidosis, particularly Bcl-2 when zidovudine (AZT) is given as a high-dose mono- Bcl-xL therapy [4] or in patients receiving stavudine (d4T) and/or tBid C Bid Bax, Bak didanosine (ddI) [20]. Discordantly low CD4 T-cell BH-3 only restoration has been reported in HIV-1-infected patients proteins with low viral loads in response to HAART. It was Mitochondria hypothesized that AZT might limit CD4 restoration by a haematotoxic mechanism and a pilot study illustrated Release of apoptogenic factors that two patients switching from AZT to stavudine had reduced apoptosis and increased CD4C T-cell counts [21]. Many of the aforementioned drug complications are thought to be attributable to toxic effects on mitochondria, Chromatin condensation and the effects of NRTI-induced mitochondrial toxicity are and DNA fragmentation known to be tissue specific [4]. Direct examination of TRENDS in Pharmacological Sciences subcutaneous adipose tissue is likely to reveal the most conclusive information about the role of mitochondrial Figure I. Extrinsic and intrinsic pathways involved in apoptosis. Abbreviation: TNFR, tumour necrosis factor receptor. toxicity in the development of lipodystrophy syndrome. Cardiomyocyte apoptosis, which can be induced by myocardial cell stretch, oxygen radicals and cytokines thereafter decrease