The present work has provided evidence to suggest the involvement of a differentiated cell-specific miR-122 as a modulator of hESC self-renewal and HCC proliferation through a direct translational suppression of a gene, Pkm2, that is commonly enriched in these classes of cells. Both loss of Pkm2 and gain of miR-122 function in hESCs and HCCs lead to a severe deficiency in self-renewal and proliferation, and during the differentiation process of hESCs into hepatocytes, a reciprocal expression pattern is observed between miR-122 and Pkm2. These findings suggest a possible role for miR-122 as a modulator of self-renewal during hepatic lineage specification, and that a failure to properly attenuate the expression of this miRNA is observed in proliferating HCCs. An examination of the genomic region up-steam of miR-122 uncovered hyper-methylation in hESCs and HCCs, while the same region is de-methylated and occupied by a transcription initiating protein, RNA polymerase II (RNAPII) in hPHs. Our findings suggest that hyper-methylation of the genomic region up-stream of miR-122 may interfere with the binding of RNAPII, which in turn, inhibits a proper initiation of miR-122 transcription (). The coordinated interplay between miR-122 and Pkm2 suggests a novel and elegant mechanism for controlling the expression of a gene that may be beneficial for stem cells, but becomes undesirable as stem cells transition into a differentiated quiescent hepatic cell fate. By initiating the expression of miR-122, the differentiating cells of a hepatic lineage have evolved an effective method for reducing the cell of a transcript that they have outgrown, in order to pave a path to ushering in a new phenotype for which self-renewal is undesired. A failure to elevate the expression of miR-122 is observed in hepatocellular carcinoma cells, and its effect on cellular proliferation suggests that miR-122 embodies a function that is reminiscent of a tumor suppressor.
The activation of a specific metabolic pathway involving the enzyme pyruvate kinase (PK) in highly proliferating cancer cells has been described decades ago by Otto Warburg 
– a metabolic process that is now known as aerobic glycolysis or the Warburg effect 
. Since the initial discovery, various studies have shown that while Pkm2
expression is primarily elevated in rapidly proliferating embryonic and progenitor stem cells, in terminally differentiated hepatocytes, PK is altered into a different isozyme, PK-L. Recent studies have demonstrated that in several cancer cells, the primary Pkm2
transcript is re-expressed, however, in an altered isoform that is capable of interacting with a phosphor-tyrosine (pTyr) motif, which enables these cells to redirect oxidative phosphorylation towards aerobic glycolysis for growth and proliferation 
. The ramifications of these findings are significant in reference to this study, as they suggest that in differentiated normal hepatocytes, miR-122 may function as a critical tumor suppressor by negatively regulating the level of Pkm2
transcripts in the cell, thereby minimizing the likelihood that PKM2 may interact with pTyr. Hence, in addition to the many homeostatic functions that have been ascribed to miR-122 in the liver, this miRNA may also function as an early defense mechanism against a possible neoplastic transformation.
It remains to be seen whether silencing miR-122 in terminally differentiated hepatocytes may give rise to tumorigenesis. Such analysis is hampered at present by the lack of experimental strategy for keeping hPHs stable and/or alive for more than a few days after the isolation procedure from the cadaveric liver or hepatic resections, and the lack of techniques to identify, isolate, and culture mature hepatic derivatives from hESCs in vitro
. Although several studies have knocked-down miR-122 expression in the mouse liver using ‘antagomirs,’ Pkm2
up-regulation was not reported 
. For this reason, examining the relevance of miR-122 in stem and cancer cells of human origin helps elucidate the important tumor suppressor potential of this miRNA during the differentiation process of hESCs into quiescent hepatocytes. Such information may also provide a useful tool as a biomarker to evaluate the safety and efficacy of hepatic derivatives of hESCs in an in vitro
differentiation culture system. Furthermore, studies like this would be helpful for exploring additional avenues through which to study liver cancer development, and facilitate the designing of early prognostic tools and treatment strategies for therapeutic intervention. Given the multifaceted role of miR-122 in the liver 
, it is likely that this miRNA suppresses genes other than Pkm2
to modulate hESC self-renewal and HCC proliferation. For example, studies have shown that Igf1r
is a direct target of miR-122, and may function as a mediator of HCC proliferation 
). Future challenge will be to further elucidate the involvement of miR-122 as a regulator of hESC self-renewal and HCC proliferation through direct translational suppression of genes in addition to Pkm2
, as well as the role of various other differentiated cell-specific miRNAs that have yet to be explored.