Despite clear evidence that exosomal microRNAs (miRNAs) are able to modulate the cellular microenvironment and that exosomal RNA cargo selection is deregulated in pathological conditions, the mechanisms controlling specific RNA sorting into extracellular vesicles are still poorly understood. Here, we identified the RNA binding protein SYNCRIP (synaptotagmin-binding cytoplasmic RNA-interacting protein; also known as hnRNP-Q or NSAP1) as a component of the hepatocyte exosomal miRNA sorting machinery. SYNCRIP knockdown impairs sorting of miRNAs in exosomes. Furthermore, SYNCRIP directly binds to specific miRNAs enriched in exosomes sharing a common extra-seed sequence (hEXO motif). The hEXO motif has a role in the regulation of miRNA localization, since embedment of this motif into a poorly exported miRNA enhances its loading into exosomes. This evidence provides insights into the mechanisms of miRNA exosomal sorting process. Moreover, these findings open the way for the possible selective modification of the miRNAs exosomal cargo.

The epithelial-to-mesenchymal transition (EMT) is a crucial process, occurring both during development and tumor progression, by which an epithelial cell undergoes a conversion to a mesenchymal phenotype, dissociates from initial contacts and migrates to secondary sites. We recently reported that in hepatocytes the multifunctional cytokine TGFbeta induces a full EMT characterized by (i) Snail induction, (ii) E-cadherin delocalization and down-regulation, (iii) down-regulation of the hepatocyte transcriptional factor HNF4alpha and (iv) up-regulation of mesenchymal and invasiveness markers. In particular, we showed that Snail directly causes the transcriptional down-regulation of E-cadherin and HNF4, while it is not sufficient for the up-regulation of mesenchymal and invasiveness EMT markers. In this paper, we show that in hepatocytes TGFbeta induces a Src-dependent activation of the focal adhesion protein FAK. More relevantly, we gathered results indicating that FAK signaling is required for (i) transcriptional up-regulation of mesenchymal and invasiveness markers and (ii) delocalization of membrane-bound E-cadherin. Our results provide the first evidence of FAK functional role in TGFbeta-mediated EMT in hepatocytes.

The concept that cellular terminal differentiation is stably maintained once development is complete has been questioned by numerous observations showing that differentiated epithelium may undergo an epithelial-to-mesenchymal transition (EMT) program. EMT and the reverse process, mesenchymal-to-epithelial transition (MET), are typical events of development, tissue repair, and tumor progression. In this study, we aimed to clarify the molecular mechanisms underlying these phenotypic conversions in hepatocytes. Hepatocyte nuclear factor 4a (HNF4a) was overexpressed in different hepatocyte cell lines and the resulting gene expression profile was determined by real-time quantitative polymerase chain reaction. HNF4a recruitment on promoters of both mesenchymal and EMT regulator genes was determined by way of electrophoretic mobility shift assay and chromatin immunoprecipitation. The effect of HNF4a depletion was assessed in silenced cells and in the context of the whole liver of HNF4 knockout animals. Our results identified key EMT regulators and mesenchymal genes as new targets of HNF4a. HNF4a, in cooperation with its target HNF1a, directly inhibits transcription of the EMT master regulatory genes Snail, Slug, and HMGA2 and of several mesenchymal markers. HNF4a-mediated repression of EMT genes induces MET in hepatomas, and its silencing triggers the mesenchymal program in differentiated hepatocytes both in cell culture and in the whole liver. Conclusion: The pivotal role of HNF4a in the induction and maintenance of hepatocyte differentiation should also be ascribed to its capacity to continuously repress the mesenchymal program; thus, both HNF4a activator and repressor functions are necessary for the identity of hepatocytes. (HEPATOLOGY 2011;53:2063-2074 E pithelial-to-mesenchymal transition (EMT) is the process by which polarized cells of the epithelium lose cell-cell connections and acquire the mesenchymal characteristics of motility and invasiveness. The reverse process, mesenchymal-to-epithelial transition (MET), often occurs at a site secondary to the original EMT population. The dynamic EMT/MET processes are essential for embryonic development and wound repair and initiate the pathological states of fibrosis and metastatic cancer.