Fasting-sensitive SUMO-switch on Prox1 controls hepatic cholesterol metabolism

Accumulation of excess nutrients hampers proper liver function and is linked to nonalcoholic fatty liver disease (NAFLD) in obesity. However, the signals responsible for an impaired adaptation of hepatocytes to obesogenic dietary cues remain still largely unknown. Post-translational modification by the small ubiquitin-like modifier (SUMO) allows for a dynamic regulation of numerous processes including transcriptional reprogramming. We demonstrate that specific SUMOylation of transcription factor Prox1 represents a nutrient-sensitive determinant of hepatic fasting metabolism. Prox1 is highly SUMOylated on lysine 556 in the liver of ad libitum and refed mice, while this modification is abolished upon fasting. In the context of diet-induced obesity, Prox1 SUMOylation becomes less sensitive to fasting cues. The hepatocyte-selective knock-in of a SUMOylation-deficient Prox1 mutant into mice fed a high-fat/high-fructose diet leads to a reduction of systemic cholesterol levels, associated with the induction of liver bile acid detoxifying pathways during fasting. The generation of tools to maintain the nutrient-sensitive SUMO-switch on Prox1 may thus contribute to the development of "fasting-based" approaches for the preservation of metabolic health.

At thermoneutrality, acute thyroxine-induced thermogenesis and pyrexia are independent of UCP1

OBJECTIVE

Hyperthyroidism is associated with increased metabolism ("thyroid thermogenesis") and elevated body temperature, often referred to as hyperthermia. Uncoupling protein-1 (UCP1) is the protein responsible for nonshivering thermogenesis in brown adipose tissue. We here examine whether UCP1 is essential for thyroid thermogenesis.

METHODS

We investigated the significance of UCP1 for thyroid thermogenesis by using UCP1-ablated (UCP1 KO) mice. To avoid confounding factors from cold-induced thermogenesis and to approach human conditions, the experiments were conducted at thermoneutrality, and to resemble conditions of endogenous release, thyroid hormone (thyroxine, T4) was injected peripherally.

RESULTS

Both short-term and chronic thyroxine treatment led to a marked increase in metabolism that was largely UCP1-independent. Chronic thyroxine treatment led to a 1-2 °C increase in body temperature. This increase was also UCP1-independent and was maintained even at lower ambient temperatures. Thus, it was pyrexia, i.e. a defended increase in body temperature, not hyperthermia. In wildtype mice, chronic thyroxine treatment induced a large relative increase in the total amounts of UCP1 in the brown adipose tissue (practically no UCP1 in brite/beige adipose tissue), corresponding to an enhanced thermogenic response to norepinephrine injection. The increased UCP1 amount had minimal effects on thyroxine-induced thermogenesis and pyrexia.

CONCLUSIONS

These results establish that thyroid thermogenesis is a UCP1-independent process. The fact that the increased metabolism coincides with elevated body temperature and thus with accelerated kinetics accentuates the unsolved issue of the molecular background for thyroid thermogenesis.

SUMOylation-dependent LRH-1/PROX1 interaction promotes atherosclerosis by decreasing hepatic reverse cholesterol transport

Reverse cholesterol transport (RCT) is an antiatherogenic process in which excessive cholesterol from peripheral tissues is transported to the liver and finally excreted from the body via the bile. The nuclear receptor liver receptor homolog 1 (LRH-1) drives expression of genes regulating RCT, and its activity can be modified by different posttranslational modifications. Here, we show that atherosclerosis-prone mice carrying a mutation that abolishes SUMOylation of LRH-1 on K289R develop less aortic plaques than control littermates when exposed to a high-cholesterol diet. The mechanism underlying this atheroprotection involves an increase in RCT and its associated hepatic genes and is secondary to a compromised interaction of LRH-1 K289R with the corepressor prospero homeobox protein 1 (PROX1). Our study reveals that the SUMOylation status of a single nuclear receptor lysine residue can impact the development of a complex metabolic disease such as atherosclerosis.

Abstract 2310: Epigenetic silencing of a potential tumor suppressor NR4A3 by aberrant JAK/STAT signaling predicts prognosis in gastric cancer

Gastric cancer is the second leading cause of cancer worldwide. Epigenetic silencing of tumor-suppressors has emerged as an important underlying mechanism in the gastric carcinogenesis. Previous studies showed that infection of H. pylori activates JAK/STAT3 signaling pathway in gastric cancer. However, the role of this aberrant signaling remains unclear. We hypothesized that activation of JAK/STAT signaling leads to epigenetic silencing of STAT3 target genes in gastric cancer. To test this hypothesis, a constitutively activated mouse STAT3 mutant (STAT3c) was transfected into MKN28 gastric cancer cells in which the JAK/STAT signaling pathway is inactive. STAT3c stable transfectant (S16) showing hyperphosphorylation of STAT3 demonstrated increased cell proliferation as compared to vector control (C9). Integrative expression microarray coupled with bioinformatic analysis identified putative STAT3 targets, NR4A3 that are down-regulated in S16 cells. In association with up-regulation of DNMT1, NR4A3 exhibited increased promoter methylation in S16 but not C9 cells as demonstrated by bisulphite sequencing and demethylation treatment. Interestingly, NR4A3 was also found to be epigenetically silenced in AGS cells where JAK/STAT signaling is constitutively activated. ChIP-PCR experiment revealed that STAT3 bound to the putative STAT3 binding site in NR4A3 promoter of AGS cells. Depletion of STAT3 by lenti-viral knockdown restored NR4A3 expression in this cell. Interestingly, luciferase reporter assay using the NR4A3 promoter containing putative STAT3 binding site exhibited a further 1.6 fold increment after deleting the STAT3 binding region (P < 0.005). Ectopic expression of NR4A3 in AGS cells reduced cancer cell growth in colony formation assay (P < 0.001). In clinical specimens, quantitative MSP demonstrated a significant correlation between the degree of NR4A3 methylation and STAT3 nuclear translocation in 72 gastric tumor samples (P < 0.05). Importantly, methylation of NR4A3 was significantly associated with patients with shorter survival (P < 0.05). In conclusion, our result demonstrated that aberrant JAK/STAT3 signaling confers epigenetic silencing of a potential tumor suppressor, NR4A3 in gastric cancer. Methylation of NR4A3 may be able to serve as a prognostic indicator in gastric cancer patients.

Citation Format: Michael W. Y. Chan, Li-Han Zeng, Liang-Yu Chang, Claudia Dittner, Jian-Liang Chou, Yao-Ting Huang, Alfred S.L. Cheng, Jiayuh Lin, Kun-Tu Yeh. Epigenetic silencing of a potential tumor suppressor NR4A3 by aberrant JAK/STAT signaling predicts prognosis in gastric cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2310. doi:10.1158/1538-7445.AM2014-2310

Abstract 1232: Aberrant JAK/STAT signaling confers epigenetic silencing of STAT3 targets in gastric cancer by integrated microarray and bioinformatic analysis

Gastric cancer is the 2nd leading cause of cancer worldwide. One of the major risk factors of gastric cancer is infection of H. pylori. Patients infected with CagA-positive H. pylori would have increased risk of gastric cancer. Previous studies including ours have demonstrated that infection of CagA-positive H. pylori induced activation of JAK/STAT3 signaling pathway. However, the role of aberrant activation of this signaling pathway in gastric cancer is not clear understood. We hypothesized that aberrant activation of JAK/STAT signaling may lead to epigenetic silencing of STAT3 target genes in gastric cancer. In this study, we transfected a constitutively activated mouse STAT3 mutant (STAT3c) into MKN28 gastric cancer cell line in which the JAK/STAT signaling pathway is not active. STAT3c stable transfectant MKN28/STATc-16 showing hyper-phosphorylation of STAT3 demonstrated increased cell proliferation as compared to vector control MKN28/C-9. In order to identify STAT3 target genes that are epigenetically down-regulated in MKN28/STATc-16 cells, expression microarray using Agilent 44K oligonucleotide microarray coupled bioinformatic analysis were performed. Our result showed that there are 2945 genes showing at least 1.5 fold down-regulation in MKN28/STATc-16 cells as compared with vector control. By whole genome bioinformatic analysis, 397 genes with STAT3 binding sites within 5kb of a CpG island were discovered. Integrated analysis revealed that there are 18 downregulated genes with a STAT3 binding site around the promoter CpG island. Further characterization of two STAT3 target, NR4A3 and LOXL4 demonstrated that promoter hypermethylation of these 2 genes were observed in MKN28/STATc-16 cells but not the vector control. Demethylation treatment using 5azaDC also restored expression of these 2 genes in MKN28/STATc-16 cells. These phenomena may be partially related to the up-regulation of DNMT1 in MKN28/STATc-16 cells. In conclusion, our result demonstrated that aberrant JAK/STAT3 signaling may lead to epigenetic silencing of its target gene in gastric cancer. The role of STAT3 in mediating epigenetic silencing of NR4A3 and LOXL4 and its clinical significance in gastric cancer patients is currently under investigation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1232.