MicroRNA-based regulation of epithelial–hybrid–mesenchymal fate determination

Abstract

Forward and backward transitions between epithelial and mesenchymal phenotypes play crucial roles in embryonic development and tissue repair. Aberrantly regulated transitions are also a hallmark of cancer metastasis. The genetic network that regulates these transitions appears to allow for the existence of a hybrid phenotype (epithelial/mesenchymal). Hybrid cells are endowed with mixed epithelial and mesenchymal characteristics, enabling specialized capabilities such as collective cell migration. Cell-fate determination between the three phenotypes is in fact regulated by a circuit composed of two highly interconnected chimeric modules-the miR-34/SNAIL and the miR-200/ZEB mutual-inhibition feedback circuits. Here, we used detailed modeling of microRNAbased regulation to study this core unit. More specifically, we investigated the functions of the two isolated modules and subsequently of the combined unit when the two modules are integrated into the full regulatory circuit. We found that miR-200/ZEB forms a tristable circuit that acts as a ternary switch, driven by miR-34/ SNAIL, that is a monostable module that acts as a noise-buffering integrator of internal and external signals. We propose to associate the three stable states-(1,0), (high miR-200)/(low ZEB); (0,1), (low miR-200)/(high ZEB); and (1/2,1/2), (medium miR-200)/(medium ZEB)-with the epithelial, mesenchymal, and hybrid phenotypes, respectively. Our (1/2,1/2) state hypothesis is consistent with recent experimental studies (e.g., ZEB expression measurements in collectively migrating cells) and explains the lack of observed mesenchymalto-hybrid transitions in metastatic cells and in induced pluripotent stem cells. Testable predictions of dynamic gene expression during complete and partial transitions are presented. multistable decision circuit | partial EMT | cancer systems biology | microRNA modeling | metastable intermediate phenotypes U nderstanding cell-fate decisions during embryonic development and tumorigenesis remains a major research challenge in modern developmental and cancer biology (1). Over recent years, we have witnessed rapid progress in mapping the gene regulatory networks associated with cellular phenotype with applications to transitions from epithelial to mesenchymal modalities, to the differentiation of pluripotent stem cells into progenitor cells, to the existence and role of cancer stem-like cells (CSC), and to the production of induced pluripotent stem cells (iPSCs). Cellfate determinations in all of these examples involve changes in expression of various transcription factors (TFs) and microRNAs (miRNAs) that regulate cascades of regulatory networks, ultimately generating genome-wide gene-expression patterns and concomitant protein levels corresponding to a particular cell lineage (fate).The E/M Hybrid Phenotype. An archetypal example of cell-fate decisions concerns the forward and backward transitions between the epithelial (E) and mesenchymal (M) phenotypes (EMT and MET), transitions that play a critical role in embryonic deve...

Towards elucidating the connection between epithelial–mesenchymal transitions and stemness

“…transition into a hybrid epithelial/mesenchymal (E/M) phenotype [4]. This hybrid E/M phenotype, often referred to as a partial EMT (pEMT) state, has also been observed experimentally in ovarian carcinoma cell lines [5].…”

“…Shifted Hill functions [4] are defined as weighted sum of excitatory Hill function H þ (X ) and inhibitory Hill function…”

“…Therefore, the LIN28/let-7 circuit can operate as a three-way switch. The EMT regulatory circuit miR-200/ZEB can also operate as a three-way switch between epithelial (E), mesenchymal (M) and hybrid E/M phenotypes [4]. These phenotypes correspond to (high miR-200, low ZEB), (low miR-200, high ZEB) and (medium miR-200, medium ZEB) states, respectively [4].…”

“…EMTs are regulated by the miR-200/ZEB mutually inhibitory circuit that operates as a three-way switch with the three possible states being the epithelial, mesenchymal and hybrid E/M phenotypes [4]. Similarly, stemness is regulated by a mutually inhibitory circuit between RNA binding factor LIN28 and microRNA let-7 [14].…”

“…To devise the theoretical framework presented here, we started from our previously devised framework for modelling miRNA-mediated translational repression [4,29]. We have generalized this framework to include the novel modes of regulation in the LIN28/let-7 circuit mentioned earlier.…”