A biochemical assessment of stress response following acute and prolonged exposure to antiretroviral drugs (nucleoside reverse transcriptase inhibitors) in vitro.
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Nucleoside reverse transcriptase inhibitors (NRTIs) are the most extensively used antiretroviral (ARV) drugs in highly active antiretroviral therapy (HAART). The long term use of HAART is associated with changes to metabolic parameters leading to lipodystrophy and metabolic syndrome, as well as toxicity to high energy demand organs e.g. liver, kidney, heart, and nervous system. Underlying the myriad of NRTI-associated adverse health outcomes is mitochondrial (mt) toxicity. Although inhibition of mtDNA synthesis was one of the first identified mechanisms of toxicity, it did not provide a holistic explanation for all NRTIs. Furthermore, variations in adaptive stress responses were observed following acute and chronic exposure to NRTIs. Insight gained from the molecular changes induced by NRTIs will enable effective management and limit adverse health outcomes. The human hepatoma (HepG2) cell line was used as an in vitro model to investigate changes to mt function, cellular redox status, and antioxidant response following acute [24 hour (h)] and prolonged (120 h) exposure to NRTIs – Zidovudine (AZT, 7.1μM); Stavudine (d4T, 4μM); Tenofovir (TFV, 1.2μM). Long term exposure to AZT and d4T reduced mtDNA levels (120h, AZT: 76.1%; d4T:36.1%, p<0.05) and mt function was compromised as evidenced by reduced ATP levels (AZT: 38%; d4T: 56.4%) and increased mt membrane depolarisation (p<0.02). Tenofovir compromised mt function at 120 h independently of depleting mtDNA levels. Oxidative stress parameters were significantly elevated by AZT and TFV at 24h; and all NRTIs at 120 h (p<0.05). Endogenous antioxidant response was highest in TFV in both time periods (120h; p<0.05). Once NRTI induced oxidative stress in HepG2 cells was established, protein homeostatic response to oxidative stress was investigated. Lon protease expression and related endoplasmic reticulum (ER) stress was evaluated. The data showed that ATP-dependent protein homeostasis responses Lon, heat shock protein 60 (HSP60) and ER stress were significantly increased at 24 h (>2 fold); but significantly decreased at 120 h for all NRTIs (p<0.005). The compromised ATP-dependent stress response then led to the assessment of an ATP- dependent drug transporter responsible for efflux of xenobiotics in hepatocytes. The transporter, ATP-binding cassette C4 (ABCC4), is regulated by microRNA (miR-) 124a. Regulation of ABCC4 by miR-124a has implications for bio-accumulation and resultant toxicity. An inverse relationship between miR-124a and ABCC4 mRNA levels in all treatments at both time periods was observed. All NRTIs elevated miR-124a levels at 24 h (p=0.0009) and this observation was consistent in d4T and TFV treated HepG2 cells at 120 h (p<0.0001). This was accompanied with a concomitant decline in ABCC4 mRNA levels (p<0.0001) relative to the control. Prolonged exposure to AZT caused a decrease in miR-124a and elevated ABCC4 mRNA levels. Protein expression of multi-drug resistance protein 4 (MRP4), coded for by ABCC4, did not correlate to mRNA expression. At 120 h, all NRTIs caused significant depletion of MRP4 (possibly due to oxidative cell membrane damage or ATP depletion). In conclusion, all three NRTIs compromised mt function and induced oxidative damage in HepG2 cells, with greater toxicity over 120 h. Reduced ATP levels compromised the ATP-dependent stress response proteins and xenobiotic detoxification. Tenofovir could be considered a safer alternative as it elicited the highest antioxidant response in spite of reduced mt function.