Beyond the polymerase-gamma theory: Respiratory chain inhibition and production of ROS as modes of NRTI induced mitochondrial toxicity

HIV-1 nucleoside reverse transcriptase inhibitor (NRTI) use is associated with severe adverse events. However, the exact mechanisms behind their toxicity has not been fully understood. Mitochondrial dysfunction after chronic exposure to NRTIs has predominantly been assigned to mitochondrial polymerase- inhibition by NRTIs. However, an increasing amount of data suggests that this is not the sole mechanism. Many NRTI induced adverse events have been linked to the incurrence of oxidative stress, although the causality of events leading to reactive oxygen species (ROS) production and their role in toxicity is unclear. In this study we show that short-term effects of these drugs, which are rarely discussed in the literature, include direct inhibition of the mitochondrial respiratory chain (MRC), decreased ATP levels and increased ROS production. Collectively these events affect fitness and longevity of C. elegans through mitohormetic signalling events. Furthermore, we demonstrate that these effects can be normalized by addition of the anti-oxidant N-acetylcysteine (NAC), which suggests that ROS likely influence the onset and severity of adverse events upon drug exposure. RNA-seq on Caenorhabditis elegans exposed to DMSO, 3'-azido-3'-deoxythymidine (zidovudine or AZT), 2',3'-didehydro-2',3'-deoxythymidine (stavudine or d4T), 3'-deoxy-3'-fluorothymidine (alovudine or FLT) or untreated control after 24 or 72 hours of exposure.