Two independent research groups this week reported in The Journal of Clinical Investigation on the role of microRNA-122 in the liver, showing that the non-coding RNA is critical for fat and cholesterol metabolism in the organ and that it may play an important role in tumor suppression.
The most abundant miRNA in the liver, miR-122 has been shown by antisense-based inhibition to regulate cholesterol and fatty acid metabolism, while data linking it to the replication of hepatitis C virus has made it a target for therapeutic intervention for companies including Regulus Therapeutics and Santaris Pharma.
However, “while much is known about the broad effects of miR-122 in the adult liver, the intrinsic roles of miR-122 in liver physiology have not been formally addressed,” a team lead by National Yang-Ming University researcher Ann-Pin Tsou wrote in one of the JCI papers.
To explore this issue, Ohio State University's Kalpana Ghoshal and collaborators generated and characterized mice with germline knockout or liver-specific knockout of the miR-122 locus.
“Both KO and LKO mice develop normally and are viable, and, consistent with studies performed using antisense-mediated miR-122 inhibition, exhibit reduced serum cholesterol,” they wrote in the second paper. “In contrast to transient inhibition studies, however, [the knockout] animals develop microsteatosis and liver inflammation in early adult life that progresses to steatohepatitis, fibrosis, and spontaneous tumors” resembling hepatocellular carcinoma, or HCC.
According to the OSU-led investigators, prolonged loss of miR-122 function leads to the accumulation of hepatic triglycerides in young mice — an abnormality associated with up-regulation of several gene products that catalyze triglyceride biosynthesis and storage, including two newly identified direct targets of the miRNA.
As the knockout mice aged, “hepatic inflammation ensued, preceding the progressive onset of fibrosis and, eventually, tumors resembling HCC,” they wrote. “Although a tumor suppressor role for miR-122 has previously been proposed based on in vitro studies and expression analyses of human HCC samples, these findings provide the first in vivo evidence to our knowledge that loss of this miRNA is sufficient to initiate highly penetrant HCC development.”
Notably, miR-122 demonstrated tumor-suppressor activity when it was administered via an adeno-associated viral vector into the liver of a non-inflammatory MYC-driven HCC mouse model, showing that the miRNA “performs a tumor suppressor function that is independent of its role in reducing inflammation and maintaining hepatocyte integrity,” according to Ghoshal and colleagues. “This activity is likely mediated by the ability of miR-122 to directly and indirectly control a broad program of gene expression, which includes key factors that influence HCC pathogenesis.”
Overall, these findings help expand the overall understanding of the functions of miR-122, and establish its role as a tumor suppressor in vivo, the authors concluded. Importantly, they also reveal potential consequences of miR-122 inhibition that may be important for its application as an HCV treatment.
Even though Santaris' phase II miR-122-targeting drug miravirsen has proven safe in humans and chimpanzees over short periods of time, the work by Ghoshal et al. suggests that genetic deletion of the miRNA ultimately results in liver cancer, requiring further investigation.
In their paper, Tsou and colleagues took a similar tack, generating a mutant mouse with a germline deletion of miR-122, finding “wide-ranging effects” on liver homeostasis.
“Although there is a marked reduction in the expression of genes involved in lipid biogenesis, an accumulation of both cholesterol and [triglycerides] in the liver was present” in the mice, they wrote.
“The hypothesis that miR-122a plays a critical role in proper lipoprotein secretion via [microsomal triglyceride transfer protein] regulation is supported by the data demonstrating the successful reversal of hepatic steatosis after the in vivo restoration of [the MTTP gene] or the re-expression of miR-122a,” the team noted. “Our results further suggest that the hepatic steatosis observed in miR-122a-deficient mice is the consequence of the global impairment of lipid metabolism and lipoprotein assembly and secretion as demonstrated by the repression of multiple genes involved in lipid metabolism.”
The rescue experiment by the National Yang-Ming University investigators also suggests that miR-122 regulates liver fibrosis, with the expression of three fibrogenic factors and increased collagen deposition observed in the knockout mice, but inflammation and fibrosis reduced, with the resolution of MTTP.
Like Ghoshal's team, Tsou et al. also found that knocking out miR-122 caused “disruptions in a range of pathways, many of which closely resemble the disruptions found in human HCC,” while the miRNA's re-expression reduced disease manifestation and tumor incidence — all of which suggests that “the restoration of miR-122 constitutes a novel approach that could be used in differentiation therapy for chronic liver diseases and HCCs that express low levels of miR-122.”
Overall, the findings presented in the two JCI studies “definitively describe the phenotype of miR-122 loss in the quiescent liver,” according to a commentary co-authored by Children's Hospital of Philadelphia's Jessica Wen and the University of Pennsylvania's Joshua Friedman.
They added that miR-122's tumor-suppressive function illustrated by the two studies “suggests that the restoration of miR-122 expression may be of benefit in HCC — but as with many other tumor suppressor miRNAs, delivery of the miRNA remains a major challenge.”