In 1964, Vincent Allfrey discovered histone acetylation and prophetically predicted its effect on gene transcription. It was not until 1990 that the phenotypic effects of histone deacetylase (HDAC) inhibitors were first demonstrated in cancer cells. Then, in late 2006, Vorinostat became the first HDAC inhibitor to be approved by the U.S. Food and Drug Administration for human use (cutaneous T-cell lymphoma). Although most of the preclinical data and clinical trials to date with HDAC inhibitors are in cancer, emerging evidence suggests their potential therapeutic role in nonmalignant disease. For example, because of their effects on transcription, HDAC inhibitors were recently implicated in inflammatory conditions, such as rheumatoid arthritis.[4, 5] However, in the field of nonalcoholic steatohepatitis (NASH), 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 nine papers.
Multiple recent studies revealed several potential therapeutic targets in NASH, such as fatty-acid–binding protein, peroxisome proliferator-activated receptors (PPARs) alpha and gamma (PPAR-α and PPAR-γ), glucagon-like peptide 1 receptor, 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 were shown to activate transcription, when fused to a DNA-binding domain. Interestingly, they are intimately involved in modulating the embryonic stem cell epigenetic landscape and are therefore implicated in the balance of self-renewal and differentiation. 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, muscle, and immune system development, as well as in hematopoiesis and cancer. 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 interleukin (IL)-1, IL-6, tumor necrosis factor alpha (TNF-α), and monocyte chemoattractant protein 1 (MCP-1). Interestingly, elimination of the p65 subunit of nuclear factor kappa B (NF-κB) by RNA interference (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 Brg1 and NF-κB. The role played by Brm and Brg1 appears center stage, because short hairpin or short interfering RNA against either abolishes inflammatory responses, as assessed by down-regulation 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 article, Brg1 ablation resulted in diminished fibrogenesis in vivo, which represents a potentially major target in the “holy grail” of hepatology.
This article is a step toward understanding epigenetic mechanisms in NASH; however, multiple questions linger. For example, whereas Brg1 is known to mediate inflammatory responses in macrophages, 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 (KCs) in the context of NASH, for now, remains unanswered. Along similar lines, it is not entirely clear whether the lentiviral construct used by Tian et al. transduced only hepatocytes or whether KCs or stellate cells were also transduced. In addition, whereas the interaction between Brm/Brg1 and NF-κB 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 article, 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-κB itself could be a valid therapeutic target in NASH. Moreover, the effect 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 carcinoma (HCC) development. Although 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. Therefore, an interesting speculation, which deserves experimental validation, places Brg1 at the intersection between diet, obesity, NASH, fibrosis, and carcinogenesis. Last, utilization of tissue-specific Brg1-null mice may shed additional light regarding the 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 an 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 because a plethora of clinical trials employing chromatin modifiers is already currently underway.
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?
Florin M. Selaru, M.D.1
Frank A. Anania, M.D., FACP2
1Division of Gastroenterology and Hepatology, Johns Hopkins University, Baltimore, MD
2Division of Digestive Diseases, Emory University School of Medicine, Atlanta, GA