Sequence variation between the mouse and human glucose-6-phosphatase catalytic subunit gene promoters results in differential activation by peroxisome proliferator activated receptor gamma coactivator-1alpha

Identification of promoter response elements that mediate propionate induction of bovine cytosolic phosphoenolpyruvate carboxykinase (PCK1) gene transcription

Cytosolic phosphoenolpyruvate carboxykinase (PCK1) is a key enzyme for gluconeogenesis that is positively regulated by propionate in bovines at the transcription level. The specific elements that determine propionate responsiveness within the bovine PCK1 promoter are unknown. In silico promoter analysis of the bovine PCK1 gene revealed several clusters of transcription factor binding sites. In the present study, we determined the essentiality of the putative cyclic AMP response element (CRE) at -94 through -87 bp and the 2 putative hepatic nuclear factor 4α (HNF4α) binding elements at +68 through +72 and -1,078 through -1,074, respectively, in mediating bovine PCK1 promoter responses to propionate and other regulators, including butyrate, cyclic AMP (cAMP), and glucocorticoids. The wild-type bovine PCK1 promoter [PCK1_(WT)] was ligated to a luciferase reporter gene and transfected into rat hepatoma (H4IIE) cells. Activities of PCK1_(WT) were induced by approximately 2-, 2-, 4-, 8-, 9-, 18-, and 16-fold respectively when exposed to cAMP (as 1.0 mM 8-Br-cAMP), 5.0 μM dexamethasone, cAMP + dexamethasone, 2.5 mM propionate, cAMP + propionate, cAMP + dexamethasone + propionate, and 2.5 mM butyrate. Seven mutants lacking either one single site, 2 of the 3 sites, or all 3 sites, generated by site-directed mutagenesis, were tested. Responses to propionate and all other treatments were completely abolished when CRE at -94 through -87 bp and HNF4α at +68 through +72 bp were both deleted. Our data indicate that these 2 regulatory elements act synergistically to mediate the bovine PCK1 promoter responses to propionate as well as butyrate, cAMP, and dexamethasone. The activation of PCK1 through these regulatory elements serves to activate the metabolic potential of bovine toward gluconeogenesis when the primary substrate for gluconeogenesis, propionate, is also present.

Fluoxetine regulates glucose and lipid metabolism via the PI3K‑AKT signaling pathway in diabetic rats

Diabetes mellitus poses a major threat towards global heath due to a lack of effective treatment. Fluoxetine hydrochloride, a selective 5-hydroxytryptamine reuptake inhibitor, is the most commonly used antidepressant in clinical therapy; however, the potential molecular mechanisms of fluoxetine in diabetes remain unknown. In the present study, reduced glucose, total cholesterol and triglyceride levels and lipid metabolism, as well as upregulated proliferator-activated receptor γ, fatty acid synthase and lipoprotein lipase, and downregulated sterol regulatory element-binding protein 1-c were detected in rats with streptozotocin (STZ)-induced diabetes following treatment with fluoxetine. Furthermore, fluoxetine significantly inhibited the expression levels of glucose metabolism-associated proteins in liver tissues, including glycogen synthase kinase 3β (GSK-3β), glucose-6 phosphatase catalytic subunit (G6PC), phosphoenolpyruvate carboxykinase (PEPCK) and forkhead box protein O1 (FOXO1). In addition, fluoxetine treatment notably attenuated morphological liver damage in rats with STZ-induced diabetes. Additionally, fluoxetine could inhibit the phosphatidylinositol 3-kinase-protein kinase B (PI3K-AKT) signaling pathway, whereas LY294002, a specific inhibitor of PI3K, suppressed the function of PI3K-AKT signaling and suppressed the expression levels of glucose metabolism-associated proteins, including GSK-3β, G6PC, PEPCK and FOXO1 in BRL-3A cells. The results of the present study revealed that fluoxetine may regulate glucose and lipid metabolism via the PI3K-AKT signaling pathway in diabetic rats.

18β-Glycyrrhetinic acid acts through hepatocyte nuclear factor 4 alpha to modulate lipid and carbohydrate metabolism

Hepatocyte nuclear factor 4 alpha (HNF4α) regulates the expression of essential genes involved in very-low-density lipoprotein (VLDL) homeostasis and gluconeogenesis.　18β-glycyrrhetinic acid (GA) is an active ingredient of Glycyrrhiza uralensis an herbal medicine used for treating liver aliments. In this study, we established that GA functions as a partial antagonist of HNF4α through HNF4α-driven reporter luciferase assay and co-immunoprecipitation experiments with co-activator PGC1α. By virtual docking and site-directed mutagenesis analysis, we confirmed that serine 190 and arginine 235 of HNF4α are both essential for GA to exert its antagonistic action on HNF4α. Importantly, GA suppressed the expression of HNF4α target genes such as apolipoprotein B (ApoB), microsomal triglyceride transfer protein (MTP) and phospholipase A₂ G12B (PLA2G12B) modulating hepatic VLDL secretion in mice fed on a high fat diet. In addition, GA also suppressed gluconeogenesis and ameliorated glucose intolerance via down-regulating the expression of HNF4α target genes glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase (Pepck). Furthermore, GA significantly lowered blood glucose and improved insulin resistance in db/db mice. In all, we established that GA acts as a partial HNF4α antagonist modulating lipid and carbohydrate metabolism.

A lex naturalis delineates components of a human-specific, adrenal androgen-dependent, p53-mediated 'kill switch' tumor suppression mechanism

We have recently described in this journal our detection of an anthropoid primate-specific, adrenal androgen-dependent, p53-mediated, 'kill switch' tumor suppression mechanism that reached its fullest expression only in humans, as a result of human-specific exposure to polycyclic aromatic hydrocarbons caused by the harnessing of fire - but which has components reaching all the way back to the origin of the primate lineage. We proposed that species-specific mechanisms of tumor suppression are a generalized requirement for vertebrate species to increase in body size or lifespan beyond those of species basal to their lineage or to exploit environmental niches which increase exposure to carcinogenic substances. Using empirical dynamic modeling, we have also reported our detection of a relationship between body size, lifespan, and species-specific mechanism of tumor suppression (and here add carcinogen exposure), such that a change in any one of these variables requires an equilibrating change in one or more of the others in order to maintain lifetime cancer risk at a value of about 4%, as observed in virtually all larger, longer-lived species under natural conditions. Here we show how this relationship, which we refer to as the lex naturalis of vertebrate speciation, elucidates the evolutionary steps underlying an adrenal androgen-dependent, human-specific 'kill switch' tumor suppression mechanism; and further, how it prescribes a solution to 'normalize' lifetime cancer risk in our species from its current aberrant 40% to the 4% that characterized primitive humans. We further argue that this prescription writ by the lex naturalis represents the only tenable strategy for meaningful suppression of the accelerating impact of cancer upon our species.

Detection of a novel, primate-specific 'kill switch' tumor suppression mechanism that may fundamentally control cancer risk in humans: an unexpected twist in the basic biology of TP53

The activation of TP53 is well known to exert tumor suppressive effects. We have detected a primate-specific adrenal androgen-mediated tumor suppression system in which circulating DHEAS is converted to DHEA specifically in cells in which TP53 has been inactivated DHEA is an uncompetitive inhibitor of glucose-6-phosphate dehydrogenase (G6PD), an enzyme indispensable for maintaining reactive oxygen species within limits survivable by the cell. Uncompetitive inhibition is otherwise unknown in natural systems because it becomes irreversible in the presence of high concentrations of substrate and inhibitor. In addition to primate-specific circulating DHEAS, a unique, primate-specific sequence motif that disables an activating regulatory site in the glucose-6-phosphatase (G6PC) promoter was also required to enable function of this previously unrecognized tumor suppression system. In human somatic cells, loss of TP53 thus triggers activation of DHEAS transport proteins and steroid sulfatase, which converts circulating DHEAS into intracellular DHEA, and hexokinase which increases glucose-6-phosphate substrate concentration. The triggering of these enzymes in the TP53-affected cell combines with the primate-specific G6PC promoter sequence motif that enables G6P substrate accumulation, driving uncompetitive inhibition of G6PD to irreversibility and ROS-mediated cell death. By this catastrophic 'kill switch' mechanism, TP53 mutations are effectively prevented from initiating tumorigenesis in the somatic cells of humans, the primate with the highest peak levels of circulating DHEAS. TP53 mutations in human tumors therefore represent fossils of kill switch failure resulting from an age-related decline in circulating DHEAS, a potentially reversible artifact of hominid evolution.

Microcystis bloom containing microcystin-LR induces type 2 diabetes mellitus

Epidemiological data from Lake Taihu showed significantly higher incidences of type 2 diabetes mellitus (T2DM) than in other areas of China. This may be related to the occurrence of a Microcystis bloom in Lake Taihu in the summer and autumn every year. The objective of this study is to investigate whether the contaminated water from the Microcystis bloom and the derivative pollutant microcystin-LR (MC-LR) can explain the higher incidences of T2DM. Healthy male mice were fed with water from different regions of Lake Taihu, and were either acutely or chronically exposed to MC-LR through oral administration or intraperitoneal injection. Serum lipid profiles were determined, and the effects on T2DM-related gene expression and insulin receptor signaling pathway were investigated. Intraperitoneal glucose tolerance (IPGTT) and insulin resistance (IRT) tests were implemented, and the functions of pancreatic islet and β-cell were also evaluated. The results showed that both water sampled from the region with a Microcysis bloom and those containing MC-LR altered the serum glucide and lipid profiles in mice after exposure. The exposure to a Microcysis bloom water affected the expression T2DM-related genes: up-regulated the mRNA levels of FASn, ACACA, G6pc, LPL, and Insig2, and down-regulated the mRNA level of PEPCK and Gsk-3β. Both acute and chronic exposure of MC-LR, even at very low concentrations (1 μg/L), impaired the insulin receptor signalling pathway and induced hyperinsulinemia and insulin resistance in mice. In this study, the most important intracellular target of MC-LR was found to be hetapocellular mitochondria. Thus, exposure to Microcystis bloom water containing microcystin-LR can induce the incidence of T2DM, by impairing the function of mitochondria by microcystin-LR. The study suggests a review of the risk assessment concerning 1 μg/L MC-LR as the reference dose in surface water.

PGC-1α, glucose metabolism and type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a chronic disease characterized by glucose metabolic disturbance. A number of transcription factors and coactivators are involved in this process. Peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) is an important transcription coactivator regulating cellular energy metabolism. Accumulating evidence has indicated that PGC-1α is involved in the regulation of T2DM. Therefore, a better understanding of the roles of PGC-1α may shed light on more efficient therapeutic strategies. Here, we review the most recent progress on PGC-1α and discuss its regulatory network in major glucose metabolic tissues such as the liver, skeletal muscle, pancreas and kidney. The significant associations between PGC-1α polymorphisms and T2DM are also discussed in this review.

The cAMP-dependent protein kinase downregulates glucose-6-phosphatase expression through RORα and SRC-2 coactivator transcriptional activity

Fasting hormones activate the cAMP/PKA signaling pathway and stimulate expression of hepatic gluconeogenic enzymes including glucose-6-phosphatase (G6Pase). Previously it was shown that steroid receptor coactivator 2 (SRC-2) knock-out mice exhibit fasting hypoglycemia and that SRC-2 coactivates RAR-related orphan receptor alpha (RORα) at the proximal G6Pase promoter. We have investigated the upstream regulation and functional implications of this RORα/SRC-2 complex on G6Pase expression. In HepG2 cells, overexpression of the catalytic PKA subunit (PKA-Cα) reduced the SRC-2 protein level, recruitment to the G6Pase promoter, and its ability to coactivate RORα. Knock-down and transactivation experiments employing G6Pase promoter constructs demonstrated that RORα and SRC-2 are required for PGC-1α to stimulate G6Pase expression. These results suggest that PKA inhibits SRC-2 coactivation of RORα and in turn reduces PGC-1α dependent regulation of G6Pase. This indirect feedback mechanism may underlie the suppression of gluconeogenesis throughout long-term starvation.