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Authors: Heike Bantel, Thomas Thum, Klaus Schulze‐Osthoff
Journal: Hepatology (2014)

Potential conflict of interest: Nothing to report. We thank Dr. Antoine and colleagues for their comment on our publication by John et al.1 Our study aimed at investigating the role of certain microRNAs (miRNAs), which have been implicated in liver cell death or regeneration, in sera from patients revealing either spontaneous (SR) or nonspontaneous recovery (NSR) from acute liver failure (ALF; n = 63). The time interval between acquisition of serum samples and the fulfillment of ALF criteria was 1‐2 days (n = 46; 66.7% of SR and 76.2% of NSR patients) or 3‐12 days (n = 17). We demonstrated that ALF patients with SR revealed significantly higher levels of the proregenerative miR‐21 and −221 compared to NSR. Compared to NSR patients, we also found increased miR‐122 serum levels in SR patients, although the role of this miRNA in liver regeneration remains less clear compared to miR‐21 and ‐221.1 Antoine et al. mention that deletion of Dicer—a regulator of miRNA processing—is associated with enhanced hepatocyte proliferation. As reported by others, however, elimination of Dicer results in overwhelming apoptosis and therefore primarily impairs cell survival, which was not only observed in hepatocytes but also other cell types.2 Likewise, deletion of miR‐122 first results in liver damage, hepatitis, and steatosis, even though under certain circumstances, i.e., the presence of oncogenic stimuli, Dicer or miR‐122 deletion might trigger compensatory processes promoting hepatocarcinogenesis.2 Similarly, hepatocyte‐specific deletion of another component of miRNA processing (DGCR8) resulted in delayed liver regeneration.5 This is in line with our observation that increased miR‐122 levels in sera of spontaneous survivors are associated with down‐regulated target genes that impair liver regeneration.1 It has to be noticed, however, that conclusions from genetically manipulated, i.e., miRNA‐deficient, mice remain limited and that target genes of various miRNAs might differ between mice and humans. Antoine et al. found elevated miR‐122 serum levels in patients with acetaminophen‐mediated acute liver injury.6 Acute liver injury is a strong trigger for liver regeneration and the duration or intensity of the cytotoxic stimulus might determine whether cell death overwhelms the regenerative capacity of the liver. Thus, the enhanced miR‐122 serum levels in patients with acetaminophen‐induced liver injury might either reflect liver cell death or regeneration. In a mouse model of acetaminophen intoxication, enhanced miR‐122 serum levels were associated with a reciprocal decrease in liver tissues following 24 hours of acetaminophen application.7 The changes of miR‐122 levels in that study were dependent on the dose and duration of drug exposure, albeit the intoxication outcome was not reported. In our study, we found significantly decreased levels of miR‐221 and −21 in liver tissues of SR compared to NSR patients, whereas miR‐122 levels were not reduced in the SR patients.1 Differences in the kinetic and release of miRNAs might explain this observation. Nevertheless, the time between sampling of serum and liver tissues did not exceed more than 1 day in all of our SR patients and 50% of the NSR patients. Only from two NSR patients biopsies were taken 5 or 6 days later than the sera, and in just one NSR patient the sampling of the serum and biopsy differed by 4 weeks. We agree that analysis of a larger number of liver tissues would be desirable, although this remains difficult considering the higher risk of liver biopsy in critically ill patients. It is well documented that ALF outcome is influenced by etiology.8 Thus, it is not surprising that etiologies differ between survivors and nonsurvivors in our study.1 We have therefore provided detailed information about parameters routinely considered for assessment of patient outcome. Understanding the molecular mechanisms that contribute to liver regeneration and ALF outcome might open up novel diagnostic and therapeutic strategies. Consideration of novel prognostic biomarkers, i.e. miRNAs, together with established scores and parameters might allow for an earlier and more precise selection of ALF patients requiring liver transplantation.