HDAC3 at the fulcrum of an epithelial-mesenchymal balance

Abstract

In this issue of Molecular Cell, Wu et al. (2011) reveal an essential role for a chromatin modifier, histone deacetylase 3 (HDAC3), in hypoxia-induced epithelial-mesenchymal transition (EMT); HIF-activated HDAC3 integrates with WDR5 to impose chromatin modifications that culminate in EMT.

The Epithelial-Mesenchymal Transition (EMT) has received significant attention in the last several years. Following the initial discovery that epithelial-like cells transiently acquire mesenchymal properties and undergo EMT during embryonic development, numerous studies uncovered similar events in multiple pathological conditions including tissue pathogenesis, wound healing and cancer (Kalluri, 2003; Thiery, 2009). These studies have led to an improved understanding of how these conditions arise and progress. Of these pathological conditions, EMT events are perhaps best described in cancer metastasis, where there has been significant progress towards the identification of molecular alterations that are sufficient and/or necessary for EMT. Although molecular markers of EMT may be observed in tumor specimens, causal evidence for their role in EMT in vivo is largely lacking. During embryonic development, EMT events are transient in nature, but whether this is also true in cancer is not clear. While some tumor cells may exhibit a permanent EMT phenotype, for example claudin-low breast cancer (Hennessy, 2009), other cancer cells likely undergo a transient EMT, as suggested by the epithelial nature of macroscopic metastases. Therefore, some EMT events are likely sustained by transient molecular changes induced by extracellular cues from the tumor microenvironment, hypoxia for example, and not by permanent genetic alterations. Consequently, it is likely that epigenetic adjustments and chromatin modifications play a key role in EMT and cancer metastasis. To address these questions, Wu et al. (2011) pursued the identification and characterization of epigenetic regulators, particularly chromatin modifiers, which play essential roles in hypoxia-induced EMT. Their findings are published in this issue of Molecular Cell.

EMT has been linked to regulation by oxygen demand or hypoxia in both developmental and pathological processes (Haase, 2008; Yang, 2008). Wu et al. (2011) initiated in silico and molecular studies to uncover hypoxia-regulated genes that encode histone modifiers and reveal essential mechanisms of epigenetic alterations during hypoxia-induced EMT. They found that HIF1α-mediated activation of Class I histone deacetylase HDAC3 expression, in response to hypoxia, occurs in both epithelial and mesenchymal cells and is essential for EMT and metastasis (Figure 1). In epithelial cells, hypoxia induces HDAC3 expression and its enzymatic activity toward H3K4ac, for removal of acetyl groups and repression of epithelial-associated gene chromatin structure. Transcription factors Snail and Twist are likewise induced by hypoxia, independently of HDAC3, and bind chromatin with HDAC3 to repress epithelial genes, such as E-cadherin, leading to loss of apical-basal polarity and cell-cell contacts. In parallel, HDAC3 is also hypoxia-induced in mesenchymal cells but, in this cell-specific background, HDAC3 collaborates with another hypoxia-responsive modifier of histone post-translational modifications (PTMs): WDR5, a histone methyl transferase (HMT) that methylates H3K4 to create gene activation-associated PTMs H3K4me2 and H3K4me3 (Wysocka, 2005). HDAC3 interacts with HIF1/2-induced WDR5 and mediates its recruitment to repressed mesenchymal genes, such as N-cadherin and Vimentin, in collaboration with hypoxia-activated Twist. These interactions led to WDR5-dependent H3K4me2/3 and activation of mesenchymal gene expression (Wu, 2011). In an HDAC3-dependent manner, hypoxia-regulated EMT in clonal cell lines occurred with increased migration and invasion, supporting development of a metastatic phenotype. Thus, hypoxia-activated HDAC3 lies at the center of chromatin alterations in both epithelial and mesenchymal cells, in one case to repress epithelial genes and the other to activate mesenchymal gene expression and induce EMT.

Figure 1.

Hypoxia tips the epithelial-mesenchymal balance via HDAC3. Both epithelial and mesenchymal cells respond to low levels of oxygen with HIF-dependent induction of HDAC3. In epithelia, increased HDAC3 deacetylates H3K4ac to repress chromatin structure and expression of epithelial genes. In parallel, WDR5 and HDAC3 levels coordinately increase in mesenchymal cells. In these cells, HDAC3 interacts with WDR5; the complex is targeted by Twist to silent mesenchymal genes in order to activate expression (not shown). This combination of events culminates in EMT, and is associated with migration, invasion and a metastatic phenotype.

Like many key players in cellular homeostasis, HDAC3 expression and functions may be highly regulated, as both decreased and increased levels have serious outcomes. Previous studies suggest important roles for HDAC3 in a broad array of normal development and chronic conditions, including negative regulation of long-term memory, inflammation, nuclear receptor signaling and tissue-specific regulation (Hennesey, 2009; McQuown, 2011). Loss of HDAC3 in conditionally targeted mouse models revealed HDAC3 functions in maintenance of genome stability, S-phase progression and DNA repair (Bhaskara, 2008). In the current study, Wu, et al. (2011) establish that a critical balance in HDAC3, and chromatin structure of both active epithelial and repressed mesenchymal genes, is shifted by hypoxic stress. Importantly, the authors present a convincing case for the prognostic significance of hypoxia/HIF1 and HDAC3 in human cancers. Sets of human head-and-neck squamous cell carcinomas (HNSCC) were analyzed for HIF1 and HDAC3 expression, along with WDR5 association. These studies show a strong correlation for these markers in patients who have a significantly shorter metastasis-free period, following treatment, and worse outcomes.

The studies of Wu, et al. (2011) answered several pertinent questions, and also raised a set of new questions. How other EMT-inducing signals, such as TGFß, Wnt, Notch and other signaling pathways, coordinately suppress the epithelial phenotype and induce a mesenchymal program, as shown here for hypoxic induction of HDAC3, and whether the loss/gain of both expression programs is necessary for cancer metastasis remain unclear. A link between EMT and stem cells was recently unmasked (Mani, 2008). Since a stem cell state correlates with bivalent, activating/repressing histone modifications of H3K4me/H3K27me, unraveling whether this characteristic is connected to HDAC3-mediated functions in repressing epithelial genes or activating mesenchymal genes remains for future studies. At a molecular level, how the enzymatic activity of HDAC3 is regulated and induced to target H3K4ac, in a specific manner, remains unknown. Finally, since Wu et al. mainly utilized cell lines and ectopic expression of hypoxia genes to decipher the biochemical events underlying EMT, the next step will be exploring this paradigm using animal models of tumor hypoxia, as well as establishing the clinical relevance in multiple tumors types, including breast cancer. The clinical applications of HDAC-inhibitors and development of additional HDAC3-specific inhibitors for the treatment of cancers are well supported by the studies of Wu, et al, published in this issue of Molecular Cell.