Profiling microRNA expression: A snapshot of nonalcoholic steatohepatitis and a recording of its pathogenesis

Abstract

We read with great interest the article by Cheung et al.1 In this report, the authors analyzed the expression profile of microRNAs in liver from patients affected by nonalcoholic steatohepatitis (NASH), and sought to find differentially expressed microRNAs and their potential role in NASH pathogenesis. Using microarray technology, microRNA profiles of 15 subjects with biopsy-proven NASH were normalized against 15 control subjects with a normal liver histology. Out of a total of 474 microRNAs analyzed, 46 were found differentially expressed in NASH patients. In particular, 23 were up-regulated (i.e., miR-34a and miR-146b), while 23 were down-regulated (i.e., miR-122) and a selection of differentially expressed microRNAs were further validated using quantitative real-time polymerase chain reaction (qRT-PCR). As expected, the predicted targets of these microRNAs include proteins belonging to complex intracellular pathways, which have been previously reported altered in NASH.2 In fact, the most recognized model of NASH pathogenesis, which is an advanced form of nonalcoholic fatty liver disease (NAFLD), considers the elevated plasma levels of free fatty acids as a first “hit” able to induce several secondary hits, including insulin resistance, increased oxidative stress on hepatocytes, and induction of proinflammatory cytokines.3 Interestingly, the authors demonstrated that miR-122 was significantly decreased (63%) in patients with NASH. The in vitro silencing of miR-122 affected the messenger RNA expression levels of predicted targets like FAS, sterol regulatory element binding protein-1c (SREBP-1c), SREBP-2, and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR). These proteins, which play roles in the lipid metabolism, have been already associated to NASH both in in vitro and in vivo models.4-6 The authors conclude that human NASH is associated with unbalanced hepatic microRNA expression and that the alteration of lipid metabolism, linked to miR-122 down-regulation, could contribute to development of NASH. Cheung's work is a clear example of the basic application of microRNA profiling to study a pathology such as NASH. However, although the reported results connected to miR-122 expression are sound, their significance is difficult to interpret because the modulation of this microRNA is not a specific sign of NASH. In fact, down-regulation of miR-122 has been previously described in hepatocarcinoma, suggesting that its function is associated with liver cancer development.7, 8 As previously underlined, we strongly believe in the power of bioinformatics in determining the interconnections among genes, proteins, and pathways potentially involved in NAFLD/NASH pathogenesis.9 However, microRNA profiling and validation is only the first step toward the comprehension of NAFLD/NASH pathogenesis. The experimental validation of target gene (putative targets) predictions is not an easy issue. Several software and algorithms based on different rationales have been proposed in the last several years (i.e., MiRanda, TargetScan, Diana-microT, and PicTar) to help find candidate targets regulated by microRNAs.10 Also the performance of individual programs and their combined use to achieve a common list of targets has been evaluated and reported.11 The development of such algorithms indeed represents a hot topic in biomedical informatics. Moreover, the scarcity of biological data linking specific microRNA action to the fate of their respective messenger RNA targets so far represents the most serious limitation. In fact, as correctly indicated by Cheung and colleagues, their findings “do not provide direct proof of the involvement of microRNAs, but they serve as a broad blueprint and resource for future hypothesis generation and hypothesis-driven studies of the role of microRNAs in the development of NASH.” Finally, we believe that microRNA technology could provide not only a snapshot of the current condition of NASH, but also a record of early pathogenetic events. Thus, a complete profiling of microRNAs in patients with simple hepatic steatosis normalized against NASH, or animal models that provide the possibility to study the evolution to NASH, might help solve these intricate molecular mechanisms. Anna Alisi Ph.D.*, Andrea Masotti Ph.D. , Valerio Nobili M.D.*, * Liver Unit, Bambino Gesù Children's Hospital and Research Institute, Rome, Italy, Gene Expression–Microarrays Laboratory, Bambino Gesù Children's Hospital and Research Institute, Rome, Italy.