Connecting the dots from fatty acids to nonalcoholic steatohepatitis: Epigenetics in the spotlight

In 1964, Vincent Allfrey discovered histone acetylation and prophetically predicted its impact on gene transcription (1). It was not until 1990 that the phenotypic effects of histone deacetylase (HDAC) inhibitors were first demonstrated in cancer cells (2). Then, in late 2006, Vorinostat became the first HDAC inhibitor to be approved by the FDA for human use (cutaneous T-cell lymphoma) (3). Although most of the pre-clinical data and clinical trials to date with HDAC inhibitors are in cancer, emerging evidence suggests their potential therapeutic role in nonmalignant disease. For example, due to their effects on transcription, HDAC inhibitors were recently implicated in inflammatory conditions, such as rheumatoid arthritis (4, 5). In the field of non alcoholic steatohepatitis (NASH), however, in the 49 years since Allfrey’s observation, only nine manuscripts containing “NASH” and “histone” in their main text were published. The study by Tian et al. in this month’s edition of Hepatology is one of these 9 papers (6). Multiple recent studies revealed several potential therapeutic targets in NASH, such as fatty acids binding protein – FABP (7), peroxisome proliferator-activated receptors alpha and gamma – PPARα and PPARγ (8), glucagon-like peptide-1 receptor (9) and others. The current study adds to previous knowledge and is among the first to raise the importance of chromatin regulation and other epigenetic phenomena in NASH to front page news. Brahma (Brm) and Brahma related gene 1 (Brg1) were discovered relatively recently, and shown to activate transcription, when fused to a DNA-binding domain (10). Interestingly, they are intimately involved in modulating the embryonic stem cell epigenetic landscape, and therefore implicated in the balance of self-renewal and differentiation (11). Given the ability of Brm and Brg1 to modulate the chromatin environment, it is not surprising that they were found to play salient roles in neural, heart and muscle, and immune system development, as well as in hematopoiesis and cancer (12-15). Now, Tian et al. implicate Brm and Brg1 in the pathogenesis of NASH through demonstration of their roles in maintaining a chromatin microenvironment primed for transcription in hepatocytes. In response to palmitate, Brm and Brg1 are recruited to promoters of inflammatory genes, such as IL1, IL-6, TNF-α and MCP-1. Interestingly, elimination of the p65 subunit of NF-kB by RNAi abrogates the recruitment of Brm and Brg1 to these promoters. In addition, depletion of Brg1 or Brm by RNAi also decreases the ability of p65 to bind to its promoters, suggesting a dynamic complex between Brg 1 and NF-kB.. The role played by Brm and Brg1 appears center-stage, since sh/siRNA against either abolishes inflammatory responses as assessed by downregulation of inflammatory cytokines IL1, IL-6, TNF-α and MCP-1. Aside from the landmark discovery of a mechanistic link between diet and NASH, Brm and Brg1 also represent a tempting new therapeutic target. Furthermore, although less significantly explored in the present manuscript, Brg1 ablation resulted in diminished fibrogenesis in vivo, which represents a potentially major target in the “holy grail” of hepatology. This manuscript is a step towards understanding epigenetic mechanisms in NASH, however, multiple questions linger. For example, while Brg1 is known to mediate inflammatory responses in macrophages (16) and the work by Tian et al. now strongly argues for its similar functions within hepatocytes, the question regarding the role played by Brg1 in Kupffer cells in the context of NASH, for now, remains unanswered. Along similar lines, it is not entirely clear if the lentiviral construct used by Tian et al. transduced only hepatocytes, or if Kupffer cells or stellate cells were also transduced. In addition, while the interaction between Brm/Brg1 and NF-kB is elegantly demonstrated, it would be of great interest to evaluate what other pathways and/or cytokines downstream of Brm/Brg1 are modified. In this manuscript, the focus is on Brg1 and Brm. However, the fact that elimination of p65 through RNAi decreases the recruitment of Brm and Brg1 to inflammatory promoters raises the possibility that NF-kB itself could be a valid therapeutic target in NASH. Moreover, the impact of Brg1/Brm on fibrosis is an extremely exciting future direction. After all, it is not NASH in itself, but rather the ensuing fibrosis that eventually can progress to cirrhosis, end stage liver disease and other complications with high morbidity and mortality. Another potentially interesting venue to investigate would be the connection between Brm/Brg1 and hepatocellular cancer (HCC) development. While the causal connection between fibrosis and HCC is well-documented, the specific mechanisms linking the two are not. Of note, Brg1 has been recently demonstrated to be required for liver progenitor cell reprogramming efficiency (17). Therefore, an interesting speculation, that deserves experimental validation, places Brg1 at the intersection between diet, obesity, NASH, fibrosis and carcinogenesis. Last, utilization of tissue-specific Brg1 null mice (18) may shed additional light regarding involvement of Brg1 in specific liver cells. The study by Tian et al. may be the harbinger of a fresh perspective in the controversial, but highly relevant, field of NASH biology and therapeutics. Invoking a mechanistic substrate for the link between diet and NASH, through Brm and Brg1-mediated chromatin modifications, this study will hopefully mark the beginning of a new era of improved mechanistic understanding of NASH. In addition, the added value of understanding chromatin modifications in NASH flows from the rich knowledge in other areas, such as cancer, that could be easily “transplanted” to NASH, especially since a plethora of clinical trials employing chromatin modifiers is already currently underway (19). In conclusion, studying epigenetics in NASH appears to be of paramount importance. We wonder - how long will it be until a NASH clinical trial employing a chromatin modifying agent, such as Vorinostat, is started?