The epigenetics of epithelial-mesenchymal plasticity in cancer

Abstract

During the course of malignant cancer progression, neoplastic cells undergo dynamic and reversible transitions between multiple phenotypic states, the extremes of which are defined by the expression of epithelial and mesenchymal phenotypes. This plasticity is enabled by underlying shifts in epigenetic regulation. A small cohort of pleiotropically acting transcription factors is widely recognized to effect these shifts by controlling the expression of a constituency of key target genes. These master regulators depend on complex epigenetic regulatory mechanisms, notably the induction of changes in the modifications of chromatin-associated histones, in order to achieve the widespread changes in gene expression observed during epithelial-mesenchymal transitions (EMTs). These associations indicate that an understanding of the functional interactions between such EMT-inducing transcription factors and the modulators of chromatin configuration will provide crucial insights into the fundamental mechanisms underlying cancer progression and may, in the longer term, generate new diagnostic and therapeutic modalities for treating high-grade malignancies.

Figure 1

Connecting extracellular signals to EMT transcription factors. Contextual signals, such as TGF-β, WNT proteins, platelet-derived growth factors (PDGFs) and interleukin-6 (IL-6), arising from autocrine or paracrine signaling networks can activate intracellular signaling factors that influence the activation or maintenance of the EMT transcription factor network during an EMT. TGF-βR1 and TGF-βR2 are two TGF-β receptors; PDGF-CC is a specific member of the PDGF family; PDGFR-α/β indicates two distinct receptor serine/threonine kinases; STAT3 is signal transducer and activator of transcription 3; IL-6R is the IL-6 receptor; gp130 is a membrane glycoprotein; SMAD2/3 indicates SMAD2 and SMAD3; c-JUN/FRA1 are heterodimeric subunits of the AP-1 complex (please note that AP-1 has been defined earlier in Box 1); NK cells are natural killer cells.

Figure 2

Epithelial-mesenchymal plasticity allows cancer cells to undergo functional adaptations during the invasion-metastasis cascade. In response to EMT-promoting signals, a subpopulation of epithelial cells at the invasive edge of the tumor may lose epithelial traits. As these cells detach further from the bulk of the tumor, they become less exposed to epithelial signals and acquire more mesenchymal properties in the presence of EMT signals supplied by stromal cells^–. The metastable mesenchymal cells are suited for invasion into surrounding tissues. A fully mesenchymal phenotype facilitates intravasation into blood capillaries or draining lymphatic vessels. In some instances, this process may be aided by macrophages. The disseminating cancer cell is also more resistant to environmental and genotoxic stresses, a characteristic that is crucial for survival in circulation. After arrival at a distant organ, the mesenchymal phenotype facilitates extravasation and invasion into the foreign tissue. Here disseminated cells are exposed to signals different from those of the primary tumor, and the mesenchymal state may confer survival advantages to single cancer cells or alternatively may support long-term dormancy. When the appropriate contextual signals become available, disseminated cells may undergo an MET and gradually reacquire epithelial properties such as rapid proliferative capabilities^,. Epithelial signals are reinforced through autocrine and paracrine signals, resulting in the stabilization of an epithelial phenotype. This facilitates the outgrowth of macrometastases that are composed predominantly of epithelial cells.

Figure 3

The epigenetic landscape governs the stability of epithelial-mesenchymal plasticity. The epithelial phenotype is a default state of epithelial cells. Contextual signals promote the epigenetic repression of key epithelial genes (for example, that encoding E-cadherin) by introducing histone modifications, which help define the plasticity of epithelial cells and the residency of cells in a given phenotypic state during the transition. The gain of an increasingly stable mesenchymal phenotype depends on the sustained presence of potent EMT-promoting signals. In their absence, metastable mesenchymal cells may simply revert to a more epithelial phenotype unless they are supported by the appropriate epigenetic modifications. H3Kac, histone H3 lysine acetylation.

Figure 4

Interactions between transcription factors and epigenetic regulators. (a) PRC1- and PRC2-mediated silencing of epithelial genes such as that encoding E-cadherin involves the initial recruitment of an EMT-TF (for example, SNAIL) to the gene promoter. SNAIL recruits PRC2, which catalyzes conversion to the repressive H3K27me3 mark that is recognized by the PRC1. (b) EMT-TFs repress gene activity through the deacetylation of gene promoters. EMT-TFs associate with the NuRD complex, which contains HDACs that catalyze the removal of acetyl groups from lysine residues of histones. H3K9/14, histone H3 lysine 9 and lysine 14. (c) SNAIL-mediated recruitment of LSD1 to target genes can result in opposing functional outcomes. LSD1 may catalyze the removal of methyl groups from H3K4, resulting in the loss of transcription activation, or cause the conversion of repressive H3K9me2 or H3K9me3 to H3K9me1, thereby permitting transcriptional activity in conjunction with additional epigenetic modifications. H3K4me1/2, methylation or dimethylation of H3K4. (d) SNAIL mediates stable silencing by recruiting G9a and SUV39H1, which cooperatively result in the trimethylation of H3K9. The H3K9me3 mark is a prerequisite for the consequent recruitment of DNMTs, which leads to CpG methylation of gene promoters. The conversion of euchromatin or facultative heterochromatin to constitutive heterochromatin stably blocks transcription activity. SUV39H1/2, SUV39H1 and SUV39H2.

Figure 5

Integral microRNA transcription regulator networks control epithelial-mesenchymal plasticity. MicroRNAs such as miR-34, miR-200 and let-7 promote the EMT or MET by interacting with certain transcription factors and epigenetic regulators^,,,,–. Reciprocal negative feedback loops appear to be a common feature that regulates the bi-stable residence of cells in two distinct states.