The role of mRNA translational regulation in models of hepatotoxicity

Over the past few years translational regulation has been shown to be critical, particularly in cases of cellular stress where it can elicit a rapid, efficient response at the protein level. It was, therefore, hypothesised that changes in mRNA translation might play an important role in compound-induced hepatotoxicity and that identifying such changes would provide a more complete understanding of the molecular mechanisms of such toxicity. The translational profiling technique frequently used for in vitro samples was modified slightly to enable its use with in vivo samples and, it revealed that, in cases of acute, compound-induced hepatotoxicity more mRNAs showed a change at the translational level than at the transcriptional level. This suggested that the liver was acting rapidly at the translational level to alter protein activity following toxic insult. One mRNA that exhibited translational repression was selected for further investigation. Following compound-induced hepatotoxicity, Dio3 demonstrated no change at the mRNA level, but a significant reduction in protein, thus, it was hypothesised that the translational repression of Dio3 was the predominant factor regulating the reduction in protein. Furthermore, it was proposed that due to the involvement of Dio3 in maintaining thyroid hormone levels, this was an important feedback mechanism by which the liver, upon early signs of damage, was acting rapidly to maintain its own energy equilibrium, avoiding global disruption of the hypothalamic-pituitary-thyroid axis. miRNAs have been identified as translational regulators; however, there was no evidence that the translational repression of Dio3 was miRNA-mediated. Global hepatic miRNA expression analysis did however, reveal that miRNAs appeared to mediate large networks important in the cellular response to compound-induced liver injury. Furthermore, by correlating miRNA and translational profiling data sets from the same models, it was possible to identify specific functionally-relevant miRNA-mRNA interactions. miRNA expression profiling was extended to the plasma of rats treated with one of the hepatotoxic compounds. A subset of miRNAs were differentially expressed following treatment and these appeared to mediate pathways involved in hepatic fibrosis and stellate cell activation, suggesting that they might function as predictive biomarkers following compound-induced hepatotoxicity. The work presented in this thesis has expanded the existing data available for a number of models of compound-induced hepatotoxicity to encompass global changes in both mRNA translation and miRNA expression. It has shown that mRNA translation, regardless of how it is regulated, plays an important role in the dynamic response of the liver to cellular stress induced by hepatotoxic compounds.