Is miR‐21 a potent target for liver fibrosis?

MicroRNAs (miRNAs), small noncoding RNAs, regulate gene expression posttranscriptionally by binding to targeted sites within the 30 untranslated regions of messenger RNAs (mRNAs) and play a role in a wide range of physiological and pathophysiological conditions, including liver fibrosis. The miRNA miR-21 has been reported to be up-regulated in patients and mouse models of liver fibrosis, and the results of previous studies suggest that its deletion or suppression can reduce fibrosis severity. In contrast to these studies (Table 1), in the current issue of HEPATOLOGY, Caviglia et al. report that miR-21 is not crucial for the activation of hepatic stellate cells (HSCs) or the development of liver fibrosis. To reach this conclusion, the investigators performed rigorous examinations with genetic and pharmacologic manipulation of miR-21 in multiple models of liver fibrosis. For example, three models of fibrogenesis (carbon tetrachloride [CCl4] injection, multidrugresistant 2 knockout [KO] mice, and 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet) were applied to miR21 null (miR-21 KO) and wild-type mice. None of these models demonstrated significant differences in liver fibrosis and/or HSC activation among these mice as assessed by sirius red staining and hepatic hydroxyproline content as well as markers of HSC activation, including a-smooth muscle actin protein, Acta2, ColIa1, Lox, and Timp1 mRNA. Furthermore, to address a concern regarding the possible compensatory changes resulting from the absence of miR-21 throughout development and adulthood, they acutely suppressed miR-21 immediately before and after the onset of hepatic fibrosis with antisense DNA oligonucleotides (miR-21 knockdown [KD] antimirs) in four fibrotic models (the previous three models plus bile duct ligation [BDL]). Furthermore, they examined CCl4-induced fibrosis in a different line of miR-21 KO mice. All of these in vivo experiments and other studies including in vitro experiments and experiments using human HSC cells and liver slices provided consistent results that led to their conclusion. Given their finding on miR-21, the authors proceeded to examine the importance of miRNAs in HSC activation and liver fibrosis. For this purpose, they generated mice with HSC-specific deletion of Dicer1. Dicer1 is a key RNAse that generates mature miRNAs. Although deletion of Dicer1 decreased miRNAs including miR-21, miR-199a-3p, and let-7i in HSCs by at least 50%, HSCs exhibited a normal phenotype, and the effect of the knockout on HSC activation and liver fibrosis was minimal. Furthermore, given that miR-21 is known as an “oncomir,” the authors investigated the contribution of miR-21 to liver cancer with miR-21 KO and KD approaches, multiple liver cancer models, and human cancer cell lines. Although the experiments performed were not as rigorous as those studying liver fibrosis, miR-21 was shown to be dispensable for hepatic carcinogenesis. What would account for these discrepancies among this study and others? A simple explanation points to differences in experimental design. For example, two previous studies that described a profibrotic effect of miR-21 on the liver also used CCl4 injection to induce Abbreviations: BDL, bile duct ligation; CCl4, carbon tetrachloride; HSC, hepatic stellate cell; KD, knockdown; KO, knockout; miRNA, microRNA; mRNA, messenger RNA.