Endoplasmic Reticulum Stress-Induced Activation of Activating Transcription Factor 6 Decreases cAMP-Stimulated Hepatic Gluconeogenesis via Inhibition of CREB

Abstract

The expression of genes encoding key hepatic gluconeogenic enzymes, including phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), is regulated at the transcriptional level by a network of transcription factors and cofactors, including cAMP response element-binding protein (CREB). It has been suggested that increased endoplasmic reticulum (ER) stress in the liver impairs hepatic glucose metabolism. However, the direct effect of ER stress on hepatic gluconeogenesis is still not clear. Here, we investigated whether ER stress influences hepatic gluconeogenesis and whether this process is mediated by activating transcription factor 6 (ATF6) through the inhibition of cAMP-mediated activation of CREB. A cAMP stimulant, forskolin, and 8-bromoadenosine-cAMP increased PEPCK and G6Pase mRNA expression in H4IIE rat hepatoma cells, and ER stress induced by tunicamycin or thapsigargin decreased the expression of these genes in forskolin or 8-bromoadenosine-cAMP-treated cells. In a transient transfection study, ATF6 inhibited the PEPCK and G6Pase promoters. Also, adenovirus-mediated overexpression of ATF6 in H4IIE cells decreased forskolin-stimulated PEPCK and G6Pase gene expression. Moreover, the inhibition of endogenous ATF6 expression by small interfering RNAs restored the ER stress-induced suppression of PEPCK and G6Pase gene expression. Transient transfection of ATF6 inhibited transactivation by CREB on the PEPCK and G6Pase promoters, and a gel shift assay showed that Ad-ATF6 inhibits forskolin-stimulated CREB DNA-binding activity. Finally, we found that expression of ATF6 decreased fasting-induced PEPCK, G6Pase mRNA expression, and blood glucose levels in mice. Taken together, these data extend our understanding of ER stress and the regulation of liver gluconeogenesis by ATF6.

Mitochondrial Phosphoenolpyruvate Carboxykinase (PEPCK-M) Is a Pro-survival, Endoplasmic Reticulum (ER) Stress Response Gene Involved in Tumor Cell Adaptation to Nutrient Availability

“…XBP1 binds to the unfolded protein response element (UPRE) sequence, TGACTG(G/A), whereas ATF6 binds to the CCACG portion of the ER stress response element (ERSE) sequence, CCAAT-N9-CCACG. For example, PEPCK-C expression in the liver is down-regulated by ATF6 under ER stress (30,31). ATF6 downregulates PCK1 expression by competing for CREB binding at the C/EBP binding site with no similarity to PCK2 AARE.…”

The Effects of Palmitate on Hepatic Insulin Resistance Are Mediated by NADPH Oxidase 3-derived Reactive Oxygen Species through JNK and p38MAPK Pathways

“…It is known that increased hepatic gluconeogenesis is induced by overexpression of PEPCK, a key gluconeogenic enzyme, and the catalytic subunit glucose-6-phosphatase, which is regulated by transcriptional and nontranscriptional mechanisms (46,47). PEPCK catalyzes one of the rate-limiting steps of gluconeogenesis, the reaction of oxaloacetic acid to phosphoenolpyruvate, contributing to increased hepatic glucose output and elevated blood glucose levels in the initial stages of type 2 diabetes characterized by insulin resistance (48).…”

Curcumin Differentially Regulates Endoplasmic Reticulum Stress through Transcriptional Corepressor SMILE (Small Heterodimer Partner-interacting Leucine Zipper Protein)-mediated Inhibition of CREBH (cAMP Responsive Element-binding Protein H)

“…CREBH plays an important role in controlling iron metabolism in liver (23). Both CREBH and ATF6 have been reported to regulate glucose metabolism by regulating phosphoenolpyruvate carboxykinase and glucose 6-phosphatase expression (24,25). Tunicamycin (Tm) is a well known ER stress inducer that has been used to activate CREBH, ATF6, and other ER stress-regulated chaperon proteins (22,23,26).…”