Metabolic Homeostasis: HDACs Take Center Stage

Abstract

Hormonal regulation of glucose and lipid metabolism is pivotal for metabolic homeostasis and energy balance. Two studies in this issue of Cell (Mihaylova et al., 2011 and Wang et al., 2011) introduce a new conserved signaling mechanism controlling catabolic gene expression: class IIa histone deacetylases (HDACs) regulate Foxo activity in Drosophila and mice.

The hormones insulin and glucagon are central to regulating glucose homeostasis in vertebrates (Biddinger and Kahn, 2006). During states of fasting, glucagon initiates the production of glucose (i.e., gluconeogenesis) in the liver by increasing the transcription of gluconeogenic enzymes, such as glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK). When nutrients are increased, insulin then attenuates gluconeogenesis in the liver and initiates glucose uptake in peripheral tissues, stabilizing blood glucose concentrations while also promoting anabolic processes and energy storage. When this regulation of glucose homeostasis dysfunctions, metabolic disorders develop, such as type 2 diabetes and metabolic syndrome. Thus, deciphering the details of this signaling network promises to provide new strategies for treating these diseases. Now two studies in this issue of Cell take the first step toward bringing this promise to fruition; they uncover a pivotal role for histone deacetylases in glucose homeostasis, suggesting that established HDAC inhibitors may be effective at mitigating diabetes and metabolic syndrome.

In the liver, glucagon stimulates transcription of gluconeogenic genes through a signaling pathway (Figure 1, left) that includes the cAMP-dependent Protein Kinase A (PKA) and the AMP-dependent protein kinase family members AMPK (AMP-activated protein kinase), SIK1 (salt-inducible kinase 1), and SIK2 (Viollet et al., 2009). AMPK and its upstream kinase LKB1 (Liver Kinase 1) inhibit gluconeogenic gene transcription in hepatic cells by suppressing nuclear translocation of the transcriptional coactivator CRTC2 (CREB-regulated transcription coactivator 2)/TORC2. Once PKA blocks AMPK and SIK1/2, CRTC2 associates with the transcription factor CREB to induce the coactivator PGC-1α (peroxisome proliferator-activated receptor gamma coactivator-1 alpha), which in turn associates with the transcription factors Foxo1 (Forkhead box o1) and HNF4α (hepatocyte nuclear factor 4 alpha) to activate gluconeogenic gene expression (Altarejos and Montminy, 2011; Viollet et al., 2009).

Figure 1. Model for How Type IIa HDACs Regulate Foxo in Drosophila and Mice.

Left: In vertebrates, insulin and glucagon control glucose homeostasis by differentially regulating a well-characterized pathway that involves the cAMP-dependent Protein Kinase A (PKA), AMP-activated protein kinase (AMPK), SIK1/2 (salt-inducible kinase 1/2), and the Akt kinase. This signaling cascade ultimately leads to the regulation of the Forkhead box o (Foxo) transcription factors, which control transcription of gluconeogenic genes. Mihaylova et al. (2011) find that class IIa histone deacetylases (HDACs) also control gluconeogenic gene expression in mouse hepatocytes by regulating the acetylation state of Foxo.

Right: In invertebrates, glucose homeostasis is also regulated by insulin- and glucagon-like hormones, such as the Drosophila insulin-like peptides (dILPs) and the adipokinetic hormone (AKH). Wang et al. (2011) find that, in Drosophila, class IIa HDACs also regulate Foxo acetylation and ultimately gluconeogenic gene expression in the fly’s fat body. The fat body is the fly equivalent of the mammalian adipose tissue, but it also has functions similar to those of the liver.

Insulin attenuates glucose production by influencing the same pathway (Figure 1, left). Activation of the insulin-responsive Akt kinase results in the inhibition of CRTC2 via SIK1/2, as well as phosphorylation and inactivation of Foxo (Viollet et al., 2009).

In addition to this hormone-mediated control of glucose metabolism, the cell’s energy balance also influences glucose production through a family of class III histone deacetylases (HDACs), called Sirtuins, which couple deacetylation with NAD (nicotinamide adenine dinucleotide) hydrolysis and, thus, are sensitive to the cellular NAD:NADH ratio. These factors control the activity of a variety of transcription factors, including Foxo1. Previous studies have established that acetylation of Foxo modulates both its promoter specificity and transcriptional activity, which in turn influences metabolism, cell proliferation, and survival (Guarente, 2006; Haigis and Sinclair, 2010). Disrupting SIRT1 expression in the liver of mice reduces gluconeogenesis and fatty acid beta-oxidation, highlighting the importance of Sirtuins in metabolic regulation. However, the ultimate consequence of losing SIRT1 on glucose homeostasis remains controversial (Purushotham et al., 2009; Rodgers et al., 2008).

The findings reported by Mihaylova et al. and Wang et al. now implicate a second class of HDACs—the class IIa HDACs—in the regulation of Foxo during glucose and lipid metabolism in both mammals and the fruitfly Drosophila melanogaster. Strikingly, they find that the LKB/SIK1/2/AMPK module directly regulates these HDACs (Figure 1), activating Foxo to promote gluconeogenic gene expression in response to glucagon.

Using a combination of genetic and biochemical approaches, Mihaylova and colleagues find that deleting LKB1 in the mouse liver blocks basal phosphorylation of several class IIa HDACs: HDAC4, HDAC5, and HDAC7. Furthermore, injecting mice with metformin, an AMPK/LKB1 agonist, increases phosphorylation of HDACs 4/5/7 in the liver and promotes their retention in the cytoplasm. In response to fasting or glucagon treatment, these HDACs are dephosphorylated and accumulate in the nucleus of hepatocytes, where they act as scaffolds for the class I HDAC3. HDAC3 is catalytically active, and inside the nucleus it activates Foxo1 by deacetylation, ultimately leading to gluconeogenic gene expression (Figure 1, left).

Using Drosophila genetics, Wang and colleagues add one more step to this pathway; they identify an evolutionarily conserved role of SIK3, the fly homolog of mammalian SIK2. They find that SIK3 phosphorylates and inhibits HDAC4 during feeding states. As in mammals, the translocation of HDAC4 into the nucleus leads to Foxo deacetylation and activation of catabolic enzymes, including phosphoenolpyruvate carboxykinase (PEPCK) and the brummer (bmm) lipase, the fly homolog of adipose triglyceride lipase (ATGL).

Previous studies have reported that the SIK kinase regulates HDACs in mouse skeletal myocytes (Berdeaux et al., 2007), and now Wang and colleagues show that this regulation is conserved in flies. When SIK3 is mutated, HDAC4 is activated and translocates to the nucleus, where it subsequently switches on Foxo (Figure 1, right). This leads to lipolysis in the fat bodies of the mutant flies and significantly decreases lipid stores. The fat body is the fly equivalent of the mammalian adipose tissue and also has similar functions as the liver. Interestingly, in feeding flies, AKT stimulates SIK3 activity, indicating that SIK3 might integrate insulin- and glucagon-like stimuli in the fly to adjust HDAC4 activity in response to systemic energy requirements.

Thus, in both flies and mice, the regulation of Foxo function through class IIa HDACs emerges as a critical step in the control of energy metabolism under starvation conditions. Indeed, Mihaylova and colleagues directly demonstrate the significance of these HDACs in metabolic diseases, as reducing HDAC 4/5/7 expression in the liver by RNA interference strikingly improves glucose tolerance in several mouse models of metabolic diseases, including hyperglycemia induced by a high-fat diet and genetic models of obesity. This mechanism thus presents a unique opportunity to develop class II HDAC inhibitors as therapeutics for treating metabolic disorders. Given that pharmacological HDAC inhibitors are already in development as potential anticancer agents, this strategy seems particularly promising.

Importantly, the two new studies also raise a number of complex questions that are critical to understanding the regulation of metabolic homeostasis. First, the studies highlight the importance of acetylation in controlling Foxo function. Given Foxo’s involvement in numerous physiological processes, this regulatory mechanism is expected to influence not only metabolism but also cell growth, proliferation, apoptosis, and longevity. However, the relative contributions of Sirtuins and class IIa HDACs for controlling Foxo activity still remain unknown. It will also be important to understand why gluconeogenic gene expression is regulated by two distinct pathways: HDAC inhibition by AMPK/SIK1/2 and CRTC2 inhibition by SIK1/2. One speculation is that the perceived redundancy allows intricate, context-dependent regulation of gluconeogenesis or of Foxo function more generally. Finally, the physiologic consequences of this regulatory mechanism might reach beyond glucose and lipid homeostasis and may include Foxo-mediated control of life span, cell survival, and growth. The combined power of Drosophila and mouse genetics will clearly provide key insights into these intriguing questions.