Regulation of hepatic gluconeogenesis by an ER-bound transcription factor, CREBH.
ABSTRACT Endoplasmic reticulum (ER)-bound transcription factor families are shown to be involved in the control of various metabolic pathways. Here, we report a critical function of ER-bound transcription factor, CREBH, in the regulation of hepatic gluconeogenesis. Expression of CREBH is markedly induced by fasting or in the insulin-resistant state in rodents in a dexamethasone- and PGC-1alpha-dependent manner, which results in the accumulation of active nuclear form of CREBH (CREBH-N). Overexpression of constitutively active CREBH activates transcription of PEPCK-C or G6Pase by binding to its enhancer site that is distinct from the well-characterized CREB/CRTC2 regulatory sequences in vivo. Of interest, knockdown of CREBH in liver significantly reduces blood glucose levels without altering expression of genes involved in the ER stress signaling cascades in mice. These data suggest a crucial role for CREBH in the regulation of hepatic glucose metabolism in mammals.
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ABSTRACT: Orphan nuclear receptor ERRγ is a member of nuclear receptor superfamily that regulates several important cellular processes including hepatic glucose and alcohol metabolism. However, mechanistic understanding of transcriptional regulation of the ERRγ gene remains to be elucidated. Here, we report that activating transcription factor 6α (ATF6α), an endoplasmic reticulum (ER)-membrane-bound basic leucine zipper (bZip) transcription factor, directly regulates ERRγ gene expression in response to ER stress. ATF6α binds to ATF6α responsive element in the ERRγ promoter. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) is required for this transactivation. Chromatin immunoprecipitation (ChIP) assay confirmed the binding of both ATF6α and PGC1α on the ERRγ promoter. ChIP assay demonstrated histone H3 and H4 acetylation occurs at the ATF6α and PGC1α binding site. Of interest, ERRγ along with PGC1α induce ATF6α gene transcription upon ER stress. ERRγ binds to an ERRγ responsive element in the ATF6α promoter. ChIP assay confirmed that both ERRγ and PGC1α bind to a site in the ATF6α promoter that exhibits histone H3 and H4 acetylation. Overall, for the first time our data show a novel pathway of cross talk between nuclear receptors and ER-membrane-bound transcription factors and suggest a positive feed-forward loop regulates ERRγ and ATF6α gene transcription.Nucleic Acids Research 05/2013; · 8.28 Impact Factor
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ABSTRACT: & Aims: Hepatic gluconeogenesis provides fuel during starvation, and is abnormally induced in obese individuals or those with diabetes. Common metabolic disorders associated with active gluconeogenesis and insulin resistance (obesity, metabolic syndrome, diabetes and non-alcoholic fatty liver disease) have been associated with alterations in iron homeostasis that disrupt insulin sensitivity and promote disease progression. We investigated whether gluconeogenic signals directly control Hepcidin, an important regulator of iron homeostasis, in starving mice (a model of persistently activated gluconeogenesis and insulin resistance). We investigated hepatic regulation of Hepcidin expression in C57BL/6Crl, 129S2/SvPas, BALB/c, and wild-type and Creb3l3-/- null mice. Mice were fed a standard, iron-balanced, chow diet or an iron-deficient diet for 9 days before sacrifice, or for 7 days before a 24-48 hr starvation period; liver and spleen tissues were then collected and analyzed by quantitative reverse transcription PCR and immunoblot analyses. Serum levels of iron, hemoglobin, Hepcidin and glucose were also measured. We analyzed human hepatoma (HepG2) cells and mouse primary hepatocytes to study transcriptional control of Hamp (the gene that encodes Hepcidin) in response to gluconeogenic stimuli using small interfering RNA, luciferase promoter, and chromatin immunoprecipitation analyses. Starvation led to increased transcription of Pck1 (encodes phosphoenolpyruvate carboxykinase 1, a protein involved in gluconeogenesis) in livers of mice, increased levels of Hepcidin, and degradation of Ferroportin, compared with non-starved mice. These changes resulted in hypoferremia and iron retention in liver tissue. Livers of starved mice also had increased levels of Ppargc1a mRNA and Creb3l3 mRNA, which encode a transcriptional co-activator incolved in energy metabolism and a liver-specific transcription factor, respectively. Glucagon and a cAMP analog increased promoter activity and transcription of Hamp in cultured liver cells; levels of Hamp were reduced following administration of small interfering RNAs against Ppargc1a and Creb3l3. PPARGC1A and CREB3L3 bound the Hamp promoter to activate its transcription in response to a cAMP analog. Creb3l3-/- mice did not upregulate Hamp or become hypoferremic during starvation. We identified a link between glucose and iron homeostasis, showing that Hepcidin is a gluconeogenic sensor in mice during starvation. This response is involved in hepatic metabolic adaptation to increased energy demands; it preserves tissue iron for vital activities during food withdrawal, but can cause excessive iron retention and hypoferremia in disorders with persistently activated gluconeogenesis and insulin resistance.Gastroenterology 12/2013; · 12.82 Impact Factor
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ABSTRACT: The orphan nuclear receptor estrogen-related receptor-γ (ERRγ) is a constitutively active transcription factor regulating genes involved in several important cellular processes, including hepatic glucose metabolism, alcohol metabolism, and the endoplasmic reticulum (ER) stress response. cAMP responsive element-binding protein H (CREBH) is an ER-bound bZIP family transcription factor that is activated upon ER stress and regulates genes encoding acute-phase proteins whose expression is increased in response to inflammation. Here, we report that ERRγ directly regulates CREBH gene expression in response to ER stress. ERRγ bound to the ERRγ response element (ERRE) in the CREBH promoter. Overexpression of ERRγ by adenovirus significantly increased expression of CREBH as well as C-reactive protein (CRP), whereas either knockdown of ERRγ or inhibition of ERRγ by ERRγ specific inverse agonist, GSK5182, substantially inhibited ER stress-mediated induction of CREBH and CRP. The transcriptional coactivator PGC1α was required for ERRγ mediated induction of the CREBH gene as demonstrated by the chromatin immunoprecipitation (ChIP) assay showing binding of both ERRγ and PGC1α on the CREBH promoter. The ChIP assay also revealed that histone H3 and H4 acetylation occurred at the ERRγ and PGC1α binding site. Moreover, chronic alcoholic hepatosteatosis, as well as the diabetic obese condition significantly increased CRP gene expression, and this increase was significantly attenuated by GSK5182 treatment. We suggest that orphan nuclear receptor ERRγ directly regulates the ER-bound transcription factor CREBH in response to ER stress and other metabolic conditions.PLoS ONE 01/2014; 9(1):e86342. · 3.73 Impact Factor