Zbtb7c is a critical gluconeogenic transcription factor that induces glucose-6-phosphatase and phosphoenylpyruvate carboxykinase 1 genes expression during mice fasting.

Gluconeogenesis is essential for blood glucose homeostasis during fasting and is regulated by various enzymes, which are encoded by gluconeogenic genes. Those genes are controlled by various transcription factors. Zinc finger and BTB domain-containing 7c (Zbtb7c, also called Kr-pok) is a BTB-POZ family transcription factor with proto-oncogenic activity. Previous findings have indicated that Zbtb7c is involved in the regulation of fatty acid biosynthesis, suggesting an involvement also in primary metabolism. We found here that fasting induced Zbtb7c expression in the mouse liver and in primary liver hepatocytes. We also observed that Zbtb7c-knockout mice have decreased blood glucose levels, so we investigated whether Zbtb7c plays a role in gluconeogenesis. Indeed, differential gene expression analysis of Zbtb7c-knockout versus wild type mouse livers showed downregulated transcription of gluconeogenic genes encoding the glucose 6-phosphatase catalytic subunit (G6pc) and phosphoenolpyruvate carboxykinase 1 (Pck1), while Zbtb7c expression upregulated these two genes, under fasting conditions. Mechanistically, we found that when complexed with histone deacetylase 3 (Hdac3), Zbtb7c binds insulin response elements (IREs) within the G6pc and Pck1 promoters. Moreover, complexed Zbtb7c deacetylated forkhead box O1 (Foxo1), thereby increasing Foxo1 binding to the G6pc and Pck1 IREs, resulting in their transcriptional activation. These results demonstrate Zbtb7c to be a crucial metabolic regulator of blood glucose homeostasis, during mammalian fasting.

Prolactin regulatory element-binding (PREB) protein regulates hepatic glucose homeostasis.

Prolactin regulatory element-binding (PREB) protein is a transcription factor that regulates prolactin (PRL) gene expression. PRL, also known as luteotropic hormone or luteotropin, is well known for its role in producing milk. However, the role of PREB, in terms of hepatic glucose metabolism, is not well elucidated. Here, we observed expression of Preb in the mouse liver, in connection with glucose homeostasis. Morevoer, Preb was downregulated in db/db, ob/ob and high-fat diet-induced obese (DIO) mice, concurrent with upregulation of the liver genes glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase-1 (Pck). Administration of adenovirus-Preb (Ad-Preb) to db/db, ob/ob, and DIO mice diminished glucose, insulin, and pyruvate tolerance, which analogously, were impaired in normal (C57BL/6) mice knocked down for Preb, via infection with Ad-shPreb (anti-Preb RNA), indicating Preb to be a negative regulator of liver gluconeogenic genes. We further demonstrate that Preb negatively influences gluconeogenic gene expression, by directly binding to their promoters at a prolactin core-binding element (PCBE). A better understanding of Preb gene expression, during the pathogenesis of hepatic insulin resistance, could ultimately provide new avenues for therapies for metabolic syndrome, obesity, and type-2 diabetes mellitus, disorders whose worldwide incidences are increasing drastically.

The role of CREB3L4 in the proliferation of prostate cancer cells.

The incidence of prostate cancer (PC) is growing rapidly throughout the world, in probable association with the adoption of western style diets. Thus, understanding the molecular pathways triggering the development of PC is crucial for both its prevention and treatment. Here, we investigated the role of the metabolism-associated protein, CREB3L4, in the proliferation of PC cells. CREB3L4 was upregulated by the synthetic androgen, R1881, in LNCaP PC cells (an androgen-dependent cell line). Knockdown of CREB3L4 resulted in decreased androgen-dependent PC cell growth. LNCaP cells transfected with siCREB3L4 underwent G2/M arrest, with upregulation of the proteins cyclin B1, phospho-CDK1, p21Waf1/Cip1, and INCA1, and downregulation of cyclin D1. Moreover, depletion of CREB3L4 resulted in significantly decreased expression of a subset of androgen-receptor (AR) target genes, including PSA, FKBP5, HPGD, KLK2, and KLK4. We also demonstrated that CREB3L4 directly interacts with the AR, and increases the binding of AR to androgen response elements (AREs). We also identified a role for the unfolded protein response (and its surrogate, IRE1α), in activating CREB3L4. Cumulatively, we postulate that CREB3L4 expression is mediated by an AR-IRE1α axis, but is also directly regulated by AR-to-ARE binding. Thus, our study demonstrates that CREB3L4 plays a key role in PC cell proliferation, which is promoted by both AR and IRE1α.

Txnip contributes to impaired glucose tolerance by upregulating the expression of genes involved in hepatic gluconeogenesis in mice.

AIMS/HYPOTHESIS: Thioredoxin-interacting protein (TXNIP) is upregulated in the hyperglycaemic state and represses glucose uptake, resulting in imbalanced glucose homeostasis. In this study, we propose a mechanism of how TXNIP impairs hepatic glucose tolerance at the transcriptional level. METHODS: We administered adenoviral Txnip (Ad-Txnip) to normal mice and performed intraperitoneal glucose tolerance tests (IPGTT), insulin tolerance tests (ITT) and pyruvate tolerance tests (PTT). After Ad-Txnip administration, the expression of genes involved in glucose metabolism, including G6pc and Gck, was analysed using quantitative real-time PCR and western blot. To understand the increased G6pc expression in liver resulting from Txnip overexpression, we performed pull-down assays for TXNIP and small heterodimer partner (SHP). Luciferase reporter assays and chromatin immunoprecipitation using the Txnip promoter were performed to elucidate the interrelationship between carbohydrate response element-binding protein (ChREBP) and transcription factor E3 (TFE3) in the regulation of Txnip expression. RESULTS: Overabundance of TXNIP resulted in impaired glucose, insulin and pyruvate tolerance in normal mice. Ad-Txnip transduction upregulated G6pc expression and caused a decrease in Gck levels in the liver of normal mice and primary hepatocytes. TXNIP increased G6pc expression by forming a complex with SHP, which is known to be a negative modulator of gluconeogenesis. Txnip expression in mouse models of diabetes was decreased by Ad-Tfe3 administration, suggesting that TFE3 may play a negative role through competition with ChREBP at the E-box of the Txnip promoter. CONCLUSIONS/INTERPRETATION: We demonstrated that TXNIP impairs glucose and insulin tolerance in mice by upregulating G6pc through interaction with SHP.

Role of transcription factor modifications in the pathogenesis of insulin resistance.

Non-alcoholic fatty liver disease (NAFLD) is characterized by fat accumulation in the liver not due to alcohol abuse. NAFLD is accompanied by variety of symptoms related to metabolic syndrome. Although the metabolic link between NAFLD and insulin resistance is not fully understood, it is clear that NAFLD is one of the main cause of insulin resistance. NAFLD is shown to affect the functions of other organs, including pancreas, adipose tissue, muscle and inflammatory systems. Currently efforts are being made to understand molecular mechanism of interrelationship between NAFLD and insulin resistance at the transcriptional level with specific focus on post-translational modification (PTM) of transcription factors. PTM of transcription factors plays a key role in controlling numerous biological events, including cellular energy metabolism, cell-cycle progression, and organ development. Cell type- and tissue-specific reversible modifications include lysine acetylation, methylation, ubiquitination, and SUMOylation. Moreover, phosphorylation and O-GlcNAcylation on serine and threonine residues have been shown to affect protein stability, subcellular distribution, DNA-binding affinity, and transcriptional activity. PTMs of transcription factors involved in insulin-sensitive tissues confer specific adaptive mechanisms in response to internal or external stimuli. Our understanding of the interplay between these modifications and their effects on transcriptional regulation is growing. Here, we summarize the diverse roles of PTMs in insulin-sensitive tissues and their involvement in the pathogenesis of insulin resistance.

Role of resveratrol in FOXO1-mediated gluconeogenic gene expression in the liver.

During a state of fasting, the blood glucose level is maintained by hepatic gluconeogenesis. SIRT1 is an important metabolic regulator during nutrient deprivation and the liver-specific knockdown of SIRT1 resulted in decreased glucose production. We hypothesize that SIRT1 is responsible for the upregulation of insulin-suppressed gluconeogenic genes through the deacetylation of FOXO1. Treatment of primary cultured hepatocytes with resveratrol increased insulin-repressed PEPCK and G6Pase mRNA levels, which depend on SIRT1 activity. We found that the resveratrol treatment resulted in a decrease in the phosphorylation of Akt and FOXO1, which are independent of SIRT1 action. Fluorescence microscopy revealed that resveratrol caused the nuclear localization of FOXO1. In the nucleus, FOXO1 is deacetylated by SIRT1, which might make it more accessible to the IRE of the PEPCK and G6Pase promoter, causing an increase in their gene expression. Our results indicate that resveratrol upregulates the expression of gluconeogenic genes by attenuating insulin signaling and by deacetylating FOXO1, which are SIRT1-independent in the cytosol and SIRT1-dependent in the nucleus, respectively.