Vitamin D and the Molecular Pathogenesis of Non-alcoholic Fatty Liver Disease

Evidence from preclinical and clinical studies for the role of vitamin D in non-alcoholic fatty liver disease (NAFLD) pathogenesis is conflicting. Although microRNAs (miRNAs) are critical to the molecular pathogenesis of NAFLD and the cellular response to vitamin D, the role of vitamin D-regulated miRNAs in NAFLD pathogenesis has been relatively unexplored. This project aimed to investigate miRNAs modulated by vitamin D that might contribute to NAFLD progression. In addition to in vitro experiments, a two-sample bidirectional Mendelian randomisation (MR) analysis was conducted to determine whether circulating 25-hydroxyvitamin D [25(OH)D] status could be associated with NAFLD. First, a comprehensive literature review identified six miRNAs (miR-21, miR-30, miR-34, miR-122, miR-146 and miR-200) dysregulated in multiple independent human NAFLD studies. Focusing on miRNAs found dysregulated in more than one vitamin D and NAFLD-related human study, five miRNAs were identified (miR-27, miR-125, miR-146, miR-155 and miR-188). Secondly, the response of immortalised human hepatocytes (HepG2) and hepatic stellate cells (HSCs; LX-2) to fatty acid (FA) and vitamin D [1α,25(OH)2D] treatments was characterised by examining cell viability, intracellular lipid accumulation, vitamin D receptor (VDR) and cytochrome P450 24A1 (CYP24A1) mRNA and protein expression. These in vitro cellular models were then used to examine miRNA expression by TaqMan low-density array (TLDA) and bioinformatic analyses. After integrating the bioinformatic data with literature evidence, a subset of candidate miRNAs were followed up for independent verification. However, results were inconclusive, likely because of insufficient miRNA quality. Finally, a two-sample bidirectional MR study indicated no causal relationship between serum 25(OH)D status and NAFLD risk and vice versa. In conclusion, the modulation of miRNAs by vitamin D in the molecular progression of NAFLD has been understudied. As the TLDA approach has limitations, future work should investigate miRNAs involved in vitamin D metabolism and NAFLD progression using next-generation sequencing (NGS).