Insulin inhibits liver expression of the CCAAT/enhancer-binding protein beta

High-fat diet-induced diabetes couples to Alzheimer's disease through inflammation-activated C/EBPβ/AEP pathway

Diabetes is a risk factor for Alzheimer's disease (AD), which is also called type 3 diabetes with insulin reduction and insulin resistance in AD patient brains. However, the molecular mechanism coupling diabetes to AD onset remains incompletely understood. Here we show that inflammation, associated with obesity and diabetes elicited by high-fat diet (HFD), activates neuronal C/EBPβ/AEP signaling that drives AD pathologies and cognitive disorders. HFD stimulates diabetes and insulin resistance in neuronal Thy1-C/EBPβ transgenic (Tg) mice, accompanied with prominent mouse Aβ accumulation and hyperphosphorylated Tau aggregation in the brain, triggering cognitive deficits. These effects are profoundly diminished when AEP is deleted from C/EBPβ Tg mice. Chronic treatment with inflammatory lipopolysaccharide (LPS) facilitates AD pathologies and cognitive disorders in C/EBPβ Tg but not in wild-type mice, and these deleterious effects were substantially alleviated in C/EBPβ Tg/AEP -/- mice. Remarkably, the anti-inflammatory drug aspirin strongly attenuates HFD-induced diabetes and AD pathologies in neuronal C/EBPβ Tg mice. Therefore, our findings demonstrate that inflammation-activated neuronal C/EBPβ/AEP signaling couples diabetes to AD.

Neuronal C/EBPβ/AEP pathway shortens life span via selective GABAnergic neuronal degeneration by FOXO repression

The age-related cognitive decline of normal aging is exacerbated in neurodegenerative diseases including Alzheimer's disease (AD). However, it remains unclear whether age-related cognitive regulators in AD pathologies contribute to life span. Here, we show that C/EBPβ, an Aβ and inflammatory cytokine-activated transcription factor that promotes AD pathologies via activating asparagine endopeptidase (AEP), mediates longevity in a gene dose-dependent manner in neuronal C/EBPβ transgenic mice. C/EBPβ selectively triggers inhibitory GABAnergic neuronal degeneration by repressing FOXOs and up-regulating AEP, leading to aberrant neural excitation and cognitive dysfunction. Overexpression of CEBP-2 or LGMN-1 (AEP) in Caenorhabditis elegans neurons but not muscle stimulates neural excitation and shortens life span. CEBP-2 or LGMN-1 reduces daf-2 mutant-elongated life span and diminishes daf-16-induced longevity. C/EBPβ and AEP are lower in humans with extended longevity and inversely correlated with REST/FOXO1. These findings demonstrate a conserved mechanism of aging that couples pathological cognitive decline to life span by the neuronal C/EBPβ/AEP pathway.

Insulin: The master regulator of glucose metabolism

Insulin is the master regulator of glucose, lipid, and protein metabolism. Following ingestion of an oral glucose load or mixed meal, the plasma glucose concentration rises, insulin secretion by the beta cells is stimulated and the hyperinsulinemia, working in concert with hyperglycemia, causes: (i) suppression of endogenous (primarily reflects hepatic) glucose production, (ii) stimulation of glucose uptake by muscle, liver, and adipocytes, (iii) inhibition of lipolysis leading to a decline in plasma FFA concentration which contributes to the suppression of hepatic glucose production and augmentation of muscle glucose uptake, and (iv) vasodilation in muscle, which contributes to enhanced muscle glucose disposal. Herein, the integrated physiologic impact of insulin to maintain normal glucose homeostasis is reviewed and the molecular basis of insulin's diverse actions in muscle, liver, adipocytes, and vasculature are discussed.

Metabolic Gene Expression in the Muscle and Blood Parameters of Broiler Chickens Stimulated In Ovo with Synbiotics

Simple Summary

Poultry production plays a major role in providing meat products to global markets. Hence, there is a continued interest of researchers in the possibilities of improving the production parameters of broiler chickens. A direct response from muscles and their metabolism to in ovo synbiotic stimulation on day 12 of egg incubation has already been widely documented. However, it is necessary to analyze the molecular mechanisms determining the phenotypic effects. The present research aimed to explain the molecular background of the quality of broiler chicken meat after injection of synbiotics based on Lactobacillus strains into the air chamber of the egg. Characterization of the meat quality is based on the signature of the metabolic gene expression closely related to muscles and basic physiological parameters.

Abstract

To better understand the effects of synbiotics administered at early stages of embryonic development in poultry, it is necessary to analyze direct effects (meat quality) and the molecular background. The molecular interpretation of poultry meat properties after in ovo administration of synbiotics remains to be reported. The purpose of the present study was to analyze the molecular background of meat quality based on gene expression and basic physiological parameters. Eggs were injected with (S1) Lactobacillus salivarius with galacto-oligosaccharides or (S2) Lactobacillus plantarum with raffinose family oligosaccharides. The pectoral muscle was collected at two time points (day 7 and day 42) and subjected to RNA isolation. Gene expression analysis was performed by RT-qPCR for a panel of eight genes associated with metabolism. The concentration of glucose and hormones (insulin, glucagon, and leptin (S1 p = 0.04)) was also increased. The obtained results showed that metabolic gene expression in the muscle was more differential due to synbiotic stimulation on day 7 (FST in S1 p = 0.03; PDK4 in S1 p = 0.02 and S2 p = 0.01; CEBPB in S1 p = 0.01 and S2 p = 0.008; PHKB in S1 p = 0.01; PRKAG3 in S1 p = 0.02) than on day 42 (PDK4 in S1 p = 0.04). On the basis of the results obtained, it can be concluded that in ovo stimulation with S1 triggered the most potent and favorable changes in the pectoral muscle gene expression in broiler chickens.

Metabolic impairment in response to early induction of C/EBPβ leads to compromised cardiac function during pathological hypertrophy

Chronic pressure overload-induced left ventricular hypertrophy in heart is preceded by a metabolic perturbation that prefers glucose over lipid as substrate for energy requirement. Here, we establish C/EBPβ (CCAAT/enhancer-binding protein β) as an early marker of the metabolic derangement that triggers the imbalance in fatty acid (FA) oxidation and glucose uptake with increased lipid accumulation in cardiomyocytes during pathological hypertrophy, leading to contractile dysfunction and endoplasmic reticulum (ER) stress. This is the first study that shows that myocardium-targeted C/EBPβ knockdown prevents the impaired cardiac function during cardiac hypertrophy led by maladaptive metabolic response with persistent hypertrophic stimuli, whereas its targeted overexpression in control increases lipid accumulation significantly compared to control hearts. A new observation from this study was the dual and opposite transcriptional regulation of the alpha and gamma isoforms of Peroxisomal proliferator activated receptors (PPARα and PPARγ) by C/EBPβ in hypertrophied cardiomyocytes. Before the functional and structural remodeling sets in the diseased myocardium, C/EBPβ aggravates lipid accumulation with the aid of the increased FA uptake involving induced PPARγ expression and decreased fatty acid oxidation (FAO) by suppressing PPARα expression. Glucose uptake into cardiomyocytes was greatly increased by C/EBPβ via PPARα suppression. The activation of mammalian target of rapamycin complex-1 (mTORC1) during increased workload in presence of glucose as the only substrate was prevented by C/EBPβ knockdown, thereby abating contractile dysfunction in cardiomyocytes. Our study thus suggests that C/EBPβ may be considered as a novel cellular marker for deranged metabolic milieu before the heart pathologically remodels itself during hypertrophy.

Vanadate inhibits transcription of the rat insulin receptor gene via a proximal sequence of the 5'flanking region

Vanadate, a protein tyrosine phosphatase inhibitor which elicits insulin-like effects, has previously been shown to inhibit expression of the insulin receptor gene at the transcriptional level in rat hepatoma cells. In an attempt to identify the DNA sequence and transcription factors potentially involved in this effect, a fragment of the proximal 5'flanking region of the IR gene (-1143/-252 upstream the ATG codon) has been cloned and functionally characterized. RNase protection allowed the identification of several transcription start sites in the conserved region of the gene, among which two major sites at -455 and -396. Upon fusion to the luciferase gene and transient transfection into hepatoma cells, the -1143/-252 fragment showed promoter activity. This was unaffected by deletion of the -1143/-761 sequence, but markedly decreased (90%) by additional deletion of the -760/-465 sequence. Treatment of hepatoma cells with vanadate led to a dose-dependent decrease in promoter activity of the 1143/-252, -760/-252 and -464/-252 constructs (change relative to untreated cells, 40, 55 and 23% at 125 μM, and 70, 85 and 62% at 250 μM, respectively). These data suggest that although the entire DNA sequence upstream the transcription start sites is probably involved in vanadate-induced inhibition, the short sequence downstream of position -464 and is sufficient for inhibition. Potential targets of vanadate are the transcription factors FoxO1 and HMGA1, two downstream targets of the insulin signaling pathway which have been shown to mediate the inhibitory effect of insulin on IR gene expression.

Expression of cardiac insulin signalling genes and proteins in rats fed a high-sucrose diet: effect of bilberry anthocyanin extract

Scope

Insulin resistance is associated with impaired cardiac function, but the underlying molecular abnormalities are largely unexplained. Bilberry anthocyanin (BAcn) may be protective, as it appears to potentiate insulin action.

Methods

Rats were randomly allocated to control, sucrose-fed (SF) or sucrose-fed + BAcn diets (SF-A) for 15 weeks. Cardiac insulin signalling genes and proteins were quantified using reverse transcription quantitative real-time polymerase chain reaction and western blots.

Results

Glucose tolerance was not different with treatment. SF showed lower (p < 0.05) ferric reducing antioxidant power, which increased with BAcn. SF resulted in significantly decreased (p < 0.05) expression of 10 genes: acetyl-coenzyme A carboxylase alpha; V-Akt murine thymoma viral oncogene homolog 1; Bcl2-like 1; cytosine-cytosine-adenosine-adenosine-thymidine/enhancer binding protein; FK506 binding protein 12-rapamycin associated; glycerol-3-phosphate dehydrogenase 1 (soluble); solute carrier family 2 (facilitated glucose transporter), member 1, 4; hexokinase 2; and thyroglobulin. SF-A prevented these changes. Compared to SF-A, SF up-regulated (p < 0.05) complement factor D and phosphoinositide-3-kinase, regulatory subunit1 (α); sterol regulatory element binding transcription factor 1 was down-regulated (p < 0.05). SF increased (p < 0.05) cardiac phospholamban and decreased phosphorylated troponin I, which were not attenuated by BAcn. Compared to control or SF, SF-A resulted in significantly lower (p < 0.05) 5′-AMP-activated protein kinase.

Conclusions

SF lowered antioxidant capacity and changed the expression of insulin signalling genes, which were modulated by BAcn.

Decreased Gluconeogenesis in the Absence of Cystathionine Gamma-Lyase and the Underlying Mechanisms

AIMS

To investigate the regulation of hepatic glucose production by cystathionine γ-lyase (CSE)-generated hydrogen sulfide (H2S) in hepatic glucose production under physiological conditions.

RESULTS

We found that CSE knockout (KO) mice had a reduced rate of gluconeogenesis, which was reversed by administration of NaHS (an H2S donor) (i.p.). Interestingly, isolated CSE KO hepatocytes exhibited a reduced glycemic response to chemical-induced activation of the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and glucocorticoid pathways compared with wild-type (WT) hepatocytes. Treatment with the inhibitors for PKA (KT5720) or glucocorticoid receptor (GR) (RU-486) significantly reduced H2S-stimulated glucose production from both WT and CSE KO mouse hepatocytes. NaHS treatment upregulated the protein levels of key gluconeogenic transcription factors, such as peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and CCAAT-enhancer-binding protein-β (C/EBP-β). Moreover, exogenous H2S augmented the S-sulfhydration of the rate-limiting gluconeogenic enzymes and PGC-1α and increased their activities, which were lower in untreated CSE KO hepatocytes. Finally, knockdown of PGC-1α, but not C/EBP-β, significantly decreased NaHS-induced glucose production from the primary hepatocytes.

INNOVATION

This study demonstrates the stimulatory effect of endogenous H2S on liver glucose production and reveals three underlying mechanisms; that is, H2S upregulates the expression levels of PGC-1α and phosphoenolpyruvate carboxykinase via the GR pathway; H2S upregulates the expression level of PGC-1α through the activation of the cAMP/PKA pathway as well as PGC-1α activity via S-sulfhydration; and H2S upregulates the expression and the activities (by S-sulfhydration) of glucose-6-phosphatase and fructose-1,6-bisphosphatase.

CONCLUSION

This study may offer clues for the homeostatic regulation of glucose metabolism under physiological conditions and its dysregulation in metabolic syndrome.

CCAAT/enhancer binding protein β in relation to ER stress, inflammation, and metabolic disturbances

The prevalence of the metabolic syndrome and underlying metabolic disturbances increase rapidly in developed countries. Various molecular targets are currently under investigation to unravel the molecular mechanisms that cause these disturbances. This is done in attempt to counter or prevent the negative health consequences of the metabolic disturbances. Here, we reviewed the current knowledge on the role of C/EBP-β in these metabolic disturbances. C/EBP-β deletion in mice resulted in downregulation of hepatic lipogenic genes and increased expression of β-oxidation genes in brown adipose tissue. Furthermore, C/EBP-β is important in the differentiation and maturation of adipocytes and is increased during ER stress and proinflammatory conditions. So far, studies were only conducted in animals and in cell systems. The results found that C/EBP-β is an important transcription factor within the metabolic disturbances of the metabolic system. Therefore, it is interesting to examine the potential role of C/EBP-β at molecular and physiological level in humans.

Decreased O-GlcNAcylation of the key proteins in kinase and redox signalling pathways is a novel mechanism of the beneficial effect of α-lipoic acid in diabetic liver

The present study aimed to investigate the effects of the treatment with a-lipoic acid (LA), a naturally occurring compound possessing antioxidant activity, on liver oxidant stress in a rat model of streptozotocin (STZ)-induced diabetes by examining potential mechanistic points that influence changes in the expression of antioxidant enzymes such as catalase (CAT) and CuZn/Mn superoxide dismutase(s) (SOD). LA was administered for 4 weeks by daily intraperitoneal injections (10 mg/kg) to STZ-induced diabetic rats, starting from the last STZ treatment. LA administration practically normalised the activities of the indicators of hepatocellular injury, alanine and aspartate aminotransferases, and lowered oxidative stress, as observed by the thiobarbituric acid-reactive substance assay, restored the reduced glutathione:glutathione disulphide ratio and increased the protein sulfhydryl group content. The lower level of DNA damage detected by the comet assay revealed that LA reduced cytotoxic signalling, exerting a hepatoprotective effect. The LA-treated diabetic rats displayed restored specific enzymatic activities of CAT, CuZnSOD and MnSOD. Quantitative real-time PCR analysis showed that LA restored CAT gene expression to its physiological level and increased CuZnSOD gene expression, but the gene expression of MnSOD remained at the diabetic level. Although the amounts of CAT and CuZnSOD protein expression returned to the control levels, the protein expression of MnSOD was elevated. These results suggested that LA administration affected CAT and CuZnSOD expression mainly at the transcriptional level, and MnSOD expression at the post-transcriptional level. The observed LA-promoted decrease in the O-GlcNAcylation of extracellular signal-regulated kinase, protein 38 kinase, NF-kB, CCAAT/enhancer-binding protein and the antioxidative enzymes themselves in diabetic rats suggests that the regulatory mechanisms that supported the changes in antioxidative enzyme expression were also influenced by post-translational mechanisms.

CCAAT/enhancer-binding protein beta: its role in breast cancer and associations with receptor tyrosine kinases

The CCAAT/enhancer-binding proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Encoded by an intronless gene, C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, LIP) whose expression is regulated by the differential use of several in-frame translation start sites. LAP1 and LAP2 are transcriptional activators and are associated with differentiation, whereas LIP is frequently elevated in proliferative tissue and acts as a dominant-negative inhibitor of transcription. However, emerging evidence suggests that LIP can serve as a transcriptional activator in some cellular contexts, and that LAP1 and LAP2 might also have unique actions. The LIP:LAP ratio is crucial for the maintenance of normal growth and development, and increases in this ratio lead to aggressive forms of breast cancer. This review discusses the regulation of C/EBPbeta activity by post-translational modification, the individual actions of LAP1, LAP2 and LIP, and the functions and downstream targets that are unique to each isoform. The role of the C/EBPbeta isoforms in breast cancer is discussed and emphasis is placed on their interactions with receptor tyrosine kinases.

Transcriptional regulation of the human hepatic CYP3A4: identification of a new distal enhancer region responsive to CCAAT/enhancer-binding protein beta isoforms (liver activating protein and liver inhibitory protein)

CCAAT/enhancer-binding proteins (C/EBPs) are key transcription factors involved in the constitutive expression of several cytochrome P450 genes in the liver. Their concentration and activity change in several pathophysiological conditions. For instance, during inflammation, released cytokines induce repressive C/EBPbeta-liver inhibitory protein (LIP), which antagonizes constitutive C/EBP transactivators [C/EBPalpha and C/EBPbeta-liver activating protein (LAP)], down-regulating genes such as CYP3A4. However, the mechanism by which hepatic C/EBP factors modulate transcription of the CYP3A4 gene is not known. To elucidate the mechanism of action, we cotransfected luciferase reporter vectors, containing 5'-flanking deletions of the CYP3A4 gene, along with expression vectors for C/EBPbeta-LAP, C/EBPbeta-LIP, and C/EBPalpha, in hepatic (HepG2) and nonhepatic (HeLa) cells. Analysis of the -3557 to -6954 base pair (bp) region demonstrated the existence of a 288-bp sequence at -5.95 kilobases (kb), which showed maximal response to C/EBPbeta-LAP ( approximately 30-fold increase in HepG2 cells). Coexpression of LAP with increasing amounts of LIP reduced the activating effect by approximately 70%. Site-directed mutagenesis of predicted C/EBPbeta binding sites demonstrated the presence of four functional C/EBPbeta-responsive motifs within this distal flanking region. Further experiments using chromatin immunoprecipitation proved the binding of endogenous C/EBPbeta to the -5.95-kilobase enhancer of the CYP3A4 gene in human hepatocytes. Expression of recombinant LAP and LIP by means of adenoviral vectors resulted in their binding to this region, which was followed by activation/repression of CYP3A4. Together, our results uncover a new distal enhancer site in the CYP3A4 gene where C/EBPbeta-LAP binds and activates transcription, whereas the truncated form, C/EBPbeta-LIP, antagonizes LAP activity and causes gene repression.

Insulin does not regulate the promoter of cholesteryl ester transfer protein (CETP) in HIRc/pCETP-CAT cells

Cholesteryl ester transfer protein (CETP) is a plasma enzyme involved in cholesterol metabolism. As a potential target in the treatment of atherosclerosis, a number of studies have focused how this enzyme is regulated. It has been postulated that insulin may regulate CETP gene expression, and these effects may be mediated through CCAAT/enhancer binding protein alpha (C/EBPalpha). The present study examines the effects of insulin on the activity of the CETP promoter in rat fibroblasts expressing the human insulin receptor (HIRc). HIRc cells were stably transfected with a chimeric construct containing 3.2 kb of the CETP promoter attached to the bacterial chloramphenicol acyltransferase gene (pCETP-CAT) without significantly affecting the expression of the insulin receptor. CAT activity was 8-fold higher in cultured HIRc/pCETP-CAT in the presence of 100 mg/dL LDL cholesterol, than those cultured without cholesterol (p < 0.05). However, culturing these cells in the presence of 100 nM insulin did not result in any change in CAT activity when compared to control cells. In HIRc/pCETP-CAT cells transiently transfected with a construct that constitutively expressed C/EBPalpha protein, a 3-fold increase in CAT activity was observed when compared to cells transiently transfected with non-specific DNA (p < 0.05). However, no observable effect on the CETP promoter was observed in the presence of insulin. Thus, in HIRc/pCETP-CAT cells, we were unable to substantiate the hypothesis that insulin regulates CETP gene transcription. These results suggest that the effects of insulin on CETP expression regulation may be downstream of transcription.

Increased insulin receptor substrate-1 and enhanced skeletal muscle insulin sensitivity in mice lacking CCAAT/enhancer-binding protein beta

CCAAT/enhancer-binding protein beta (C/EBPbeta) controls gene transcription and metabolic processes in a variety of insulin-sensitive tissues; however, its role in regulating insulin responsiveness in vivo has not been investigated. We performed hyperinsulinemic-euglycemic clamps in awake, non-stressed, chronically catheterized adult mice homozygous for a deletion in the gene for C/EBPbeta (C/EBPbeta(-/-)). Fasting plasma insulin, glucose, and free fatty acid (FFA) levels were significantly lower in C/EBPbeta(-/-) mice compared with wild-type (WT) controls. Acute hyperinsulinemia (4 h) suppressed hepatic glucose production, phosphoenolpyruvate carboxykinase mRNA, and plasma FFA to a similar extent in WT and C/EBPbeta(-/-) mice, suggesting that C/EBPbeta deletion does not alter the metabolic and gene regulatory response to insulin in liver and adipose tissue. In contrast, using submaximal (5 milliunits/kg/min) and maximal (20 milliunits/kg/min) insulin infusions, whole-body glucose disposal was 77% (p < 0.01) and 33% (p < 0.05) higher in C/EBPbeta(-/-) mice, respectively, compared with WT mice. Maximal insulin-stimulated 3-O-methylglucose uptake in isolated soleus muscle was 54% greater in C/EBPbeta(-/-) mice (p < 0.05). Furthermore, insulin-stimulated insulin receptor and Akt Ser(473) phosphorylation and phosphatidylinositol 3-kinase activity were 1.6-2.5-fold greater in skeletal muscle from C/EBPbeta(-/-) mice compared with WT mice. The level of insulin receptor substrate-1 protein was increased 2-fold in skeletal muscle from C/EBPbeta(-/-) mice. These results demonstrate that C/EBPbeta deletion decreases plasma FFA levels and increases insulin signal transduction specifically in skeletal muscle, and both contribute to increased whole-body insulin sensitivity.

CCAAT/enhancer-binding protein alpha is a physiological regulator of prolactin gene expression

The sequence -101/-92 of the PRL promoter has been shown to be essential for both basal and hormone-increased PRL gene transcription. It is important to identify transcription factors that bind to this sequence if we are to understand the regulation of the PRL gene. Nuclear proteins, metabolically labeled with 35S were used in gel mobility shift experiments to examine which protein(s) binds to this region of the PRL promoter. An abundant 43-kDa protein binds to the PRL promoter at -106/-87. Two 43-kDa transcription factors were identified in cytosolic extracts of GH4 cells, CCAAT enhancer-binding protein alpha (C/EBP alpha) and cAMP response element-binding protein. Both of these bind to the PRL promoter, and both were present in GH4 cell nuclear extract, but only C/EBP alpha was definitively identified in complexes with PRL promoter DNA. Expression of C/EBP alpha increased basal PRL gene expression almost 6-fold, whereas expression of Chop10 that can act as an inhibitor of C/EBP alpha reduced the basal activity of the PRL promoter 60-75%. Mutational analysis demonstrated that the ability of C/EBP alpha to increase basal expression of the PRL promoter was dependent on the sequence -101/-92. These data suggest that C/EBP alpha is an important transcription factor that regulates PRL gene expression.

The transcription factor CCAAT/enhancer-binding protein beta regulates gluconeogenesis and phosphoenolpyruvate carboxykinase (GTP) gene transcription during diabetes

CCAAT/enhancer-binding protein (C/EBP) beta and C/EBPalpha are members of the c/ebp gene family and are highly expressed in mammalian liver and adipose tissue. C/EBPalpha is essential for adipogenesis and neonatal gluconeogenesis, as shown by the C/EBPalpha knockout mouse. C/EBPbeta binds to several sequences of the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter with high affinity, and C/EBPbeta protein is increased 200% in the livers of streptozotocin-diabetic mice, concurrent with increased PEPCK mRNA. To elucidate the role of C/EBPbeta in the control of gluconeogenesis during diabetes, we studied the levels of plasma metabolites and hormones related to energy metabolism during diabetes in adult mice heterozygous and homozygous for a null mutation of the gene for C/EBPbeta. We also examined the expression of PEPCK and glucose 6-phosphatase mRNAs and regulation of blood glucose, including the contribution of gluconeogenesis to blood glucose in c/ebpbeta-/- mice. C/EBPbeta was not essential to basal PEPCK mRNA levels. However, C/EBPbeta deletion affected streptozotocin-diabetic response by: (a) delaying hyperglycemia, (b) preventing the increase of plasma free fatty acids, (c) limiting the full induction of PEPCK and glucose 6-phosphatase genes, and (d) preventing the increase in gluconeogenesis rate. Gel supershifts of transcription factor C/EBPalpha, bound to CRE, P3I, and AF-2 sites of the PEPCK promoter, was not increased in diabetic c/ebpbeta-/- mouse liver nuclei, suggesting that C/EBPalpha does not substitute for C/EBPbeta in the diabetic response of liver gene transcription. These results link C/EBPbeta to the metabolic and gene regulatory responses to diabetes and implicate C/EBPbeta as an essential factor underlying glucocorticoid-dependent activation of PEPCK gene transcription in the intact animal.

The C/EBP family of transcription factors in the liver and other organs

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are pivotal regulators of liver functions such as nutrient metabolism and its control by hormones, acute-phase response and liver regeneration. Recent progress in clarification of regulatory mechanisms for the C/EBP family members gives insight into understanding the liver functions at the molecular level.

Dietary energy tissue-specifically regulates endoplasmic reticulum chaperone gene expression in the liver of mice

A number of putative molecular chaperones seem to play essential roles in the correct folding, assembly and glycosylation of membrane and secreted proteins in the endoplasmic reticulum. We have shown that life span-extending dietary energy restriction significantly and specifically reduces GRP78 mRNA and protein by 50-75% in mice. Here, 5-mo-old female C3B10RF1 mice were given free access to food after being fed 50% less dietary energy since weaning. Hepatic GRP78 mRNA increased linearly, reaching the same level after 2 wk as was found in the liver of 20-mo-old mice with free access to food. This increase took place with no change in body weight. The mRNA levels of endoplasmic reticulum, cytosolic and mitochondrial chaperones were determined in young (7-mo-old) and old (21- or 28-mo-old) female C3B10RF1 mice. Each age group was either 50% energy restricted or was fed approximately 10% less energy than consumed by mice given free access to food. In young and old energy-restricted mice, hepatic expression of the endoplasmic reticulum chaperones ERp57 (37%), GRP170 (51%), ERp72 (43%), calreticulin (54%) and calnexin (23%) was significantly and specifically reduced. The GRP78, GRP94, GRP170, ERp57 and calnexin mRNA response to diet occurred reproducibly only in liver, and not in adipose, brain, heart, kidney, lung, muscle or small intestine. The mRNA for GRP75, a mitochondrial chaperone, HSC70, a cytoplasmic chaperone, protein disulfide isomerase, an endoplasmic reticulum chaperone, and C/EBPalpha, a transcription factor, was not regulated. Hepatic C/EBPbeta was 15% higher in old energy-restricted mice. Thus the expression of nearly all endoplasmic reticulum chaperones responded rapidly and specifically to dietary energy in mice.

Effects of diabetes mellitus on hepatocyte nuclear factor 1 decrease albumin gene transcription

We have previously reported that albumin gene transcription is reduced in diabetes mellitus (DM). The present study explored the mechanism by which albumin gene transcription is down-regulated in DM. Deletional studies and displacement of factors binding to site B of the albumin promoter indicated that the repressive effects of DM are mediated by nuclear factors binding to this site. Since hepatocyte nuclear factor 1 (HNF1) activates albumin promoter activity and is the predominant factor binding to site B, we examined HNF1. The abundance and binding activity of HNF1 were reduced in hepatonuclear extracts from diabetic compared to control rats. However, HNF1 mRNA levels were unchanged, suggesting that the effect of DM on HNF1 is at the post-transcriptional level. Extracts from diabetic animals also contained another protein, distinct from HNF1 and vHNF1, which bound to site B in gel retardation studies. In summary, our studies demonstrate that the reduced abundance and binding activity of HNF1 correlates with decreased albumin gene transcription in DM.