Insulin regulation of gluconeogenesis.

The coordinated regulation between cellular glucose uptake and endogenous glucose production is indispensable for the maintenance of constant blood glucose concentrations. The liver contributes significantly to this process by altering the levels of hepatic glucose release, through controlling the processes of de novo glucose production (gluconeogenesis) and glycogen breakdown (glycogenolysis). Various nutritional and hormonal stimuli signal to alter hepatic gluconeogenic flux, and suppression of this metabolic pathway during the postprandial state can, to a significant extent, be attributed to insulin. Here, we review some of the molecular mechanisms through which insulin modulates hepatic gluconeogenesis, thus controlling glucose production by the liver to ultimately maintain normoglycemia. Various signaling pathways governed by insulin converge at the level of transcriptional regulation of the key hepatic gluconeogenic genes PCK1 and G6PC, highlighting this as one of the focal mechanisms through which gluconeogenesis is modulated. In individuals with compromised insulin signaling, such as insulin resistance in type 2 diabetes, insulin fails to suppress hepatic gluconeogenesis, even in the fed state; hence, an insight into these insulin-moderated pathways is critical for therapeutic purposes.

Corrigendum: Snf1-related kinase improves cardiac mitochondrial efficiency and decreases mitochondrial uncoupling.

This corrects the article DOI: 10.1038/ncomms14095.

Selective Chemical Inhibition of PGC-1α Gluconeogenic Activity Ameliorates Type 2 Diabetes.

Type 2 diabetes (T2D) is a worldwide epidemic with a medical need for additional targeted therapies. Suppression of hepatic glucose production (HGP) effectively ameliorates diabetes and can be exploited for its treatment. We hypothesized that targeting PGC-1α acetylation in the liver, a chemical modification known to inhibit hepatic gluconeogenesis, could be potentially used for treatment of T2D. Thus, we designed a high-throughput chemical screen platform to quantify PGC-1α acetylation in cells and identified small molecules that increase PGC-1α acetylation, suppress gluconeogenic gene expression, and reduce glucose production in hepatocytes. On the basis of potency and bioavailability, we selected a small molecule, SR-18292, that reduces blood glucose, strongly increases hepatic insulin sensitivity, and improves glucose homeostasis in dietary and genetic mouse models of T2D. These studies have important implications for understanding the regulatory mechanisms of glucose metabolism and treatment of T2D.

Adipose Tissue CLK2 Promotes Energy Expenditure during High-Fat Diet Intermittent Fasting.

A promising approach to treating obesity is to increase diet-induced thermogenesis in brown adipose tissue (BAT), but the regulation of this process remains unclear. Here we find that CDC-like kinase 2 (CLK2) is expressed in BAT and upregulated upon refeeding. Mice lacking CLK2 in adipose tissue exhibit exacerbated obesity and decreased energy expenditure during high-fat diet intermittent fasting. Additionally, tissue oxygen consumption and protein levels of UCP1 are reduced in CLK2-deficient BAT. Phosphorylation of CREB, a transcriptional activator of UCP1, is markedly decreased in BAT cells lacking CLK2 due to enhanced CREB dephosphorylation. Mechanistically, CREB dephosphorylation is rescued by the inhibition of PP2A, a phosphatase that targets CREB. Our results suggest that CLK2 is a regulatory component of diet-induced thermogenesis in BAT through increased CREB-dependent expression of UCP1.

Snf1-related kinase improves cardiac mitochondrial efficiency and decreases mitochondrial uncoupling.

Ischaemic heart disease limits oxygen and metabolic substrate availability to the heart, resulting in tissue death. Here, we demonstrate that the AMP-activated protein kinase (AMPK)-related protein Snf1-related kinase (SNRK) decreases cardiac metabolic substrate usage and mitochondrial uncoupling, and protects against ischaemia/reperfusion. Hearts from transgenic mice overexpressing SNRK have decreased glucose and palmitate metabolism and oxygen consumption, but maintained power and function. They also exhibit decreased uncoupling protein 3 (UCP3) and mitochondrial uncoupling. Conversely, Snrk knockout mouse hearts have increased glucose and palmitate oxidation and UCP3. SNRK knockdown in cardiac cells decreases mitochondrial efficiency, which is abolished with UCP3 knockdown. We show that Tribbles homologue 3 (Trib3) binds to SNRK, and downregulates UCP3 through PPARα. Finally, SNRK is increased in cardiomyopathy patients, and SNRK reduces infarct size after ischaemia/reperfusion. SNRK also decreases cardiac cell death in a UCP3-dependent manner. Our results suggest that SNRK improves cardiac mitochondrial efficiency and ischaemic protection.

Targeting hepatic glucose metabolism in the treatment of type 2 diabetes.

Type 2 diabetes mellitus is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycaemia. Although current diabetes treatments have exhibited some success in lowering blood glucose levels, their effect is not always sustained and their use may be associated with undesirable side effects, such as hypoglycaemia. Novel antidiabetic drugs, which may be used in combination with existing therapies, are therefore needed. The potential of specifically targeting the liver to normalize blood glucose levels has not been fully exploited. Here, we review the molecular mechanisms controlling hepatic gluconeogenesis and glycogen storage, and assess the prospect of therapeutically targeting associated pathways to treat type 2 diabetes.

Molecular pathophysiology of hepatic glucose production.

Maintaining blood glucose concentration within a relatively narrow range through periods of fasting or excess nutrient availability is essential to the survival of the organism. This is achieved through an intricate balance between glucose uptake and endogenous glucose production to maintain constant glucose concentrations. The liver plays a major role in maintaining normal whole body glucose levels by regulating the processes of de novo glucose production (gluconeogenesis) and glycogen breakdown (glycogenolysis), thus controlling the levels of hepatic glucose release. Aberrant regulation of hepatic glucose production (HGP) can result in deleterious clinical outcomes, and excessive HGP is a major contributor to the hyperglycemia observed in Type 2 diabetes mellitus (T2DM). Indeed, adjusting glycemia as close as possible to a non-diabetic range is the foremost objective in the medical treatment of patients with T2DM and is currently achieved in the clinic primarily through suppression of HGP. Here, we review the molecular mechanisms controlling HGP in response to nutritional and hormonal signals and discuss how these signals are altered in T2DM.

Adenosine activates thermogenic adipocytes.

Brown or beige fat activation can cause potent anti-obesity and anti-diabetic effects. In a study recently published in Nature, Gnad et al. show that adenosine is a novel activator of brown and beige fat that acts through the A2A receptor.

Transition metals and mitochondrial metabolism in the heart.

Transition metals are essential to many biological processes in almost all organisms from bacteria to humans. Their versatility, which arises from an ability to undergo reduction-oxidation chemistry, enables them to act as critical cofactors of enzymes throughout the cell. Accumulation of metals, however, can also lead to oxidative stress and cellular damage. The importance of metals to both enzymatic reactions and oxidative stress makes them key players in mitochondria. Mitochondria are the primary energy-generating organelles of the cell that produce ATP through a chain of enzymatic complexes that require transition metals, and are highly sensitive to oxidative damage. Moreover, the heart is one of the most mitochondrially-rich tissues in the body, making metals of particular importance to cardiac function. In this review, we focus on the current knowledge about the role of transition metals (specifically iron, copper, and manganese) in mitochondrial metabolism in the heart. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Snf1-related kinase inhibits colon cancer cell proliferation through calcyclin-binding protein-dependent reduction of ß-catenin.

Sucrose nonfermenting 1 (Snf1)-related kinase (SNRK) is a serine/threonine kinase with sequence similarity to AMP-activated protein kinase (AMPK); however, its function is not well characterized. We conducted a gene array to determine which genes are regulated by SNRK. The array demonstrated that SNRK overexpression increased the levels of genes involved in cell proliferation, including calcyclin-binding protein (CacyBP), a member of the ubiquitin ligase complex that targets nonphosphorylated ß-catenin for degradation. We confirmed that SNRK increased CacyBP mRNA and protein, and decreased ß-catenin protein in HCT116 and RKO colon cancer cells. Furthermore, SNRK inhibited colon cancer cell proliferation, and CacyBP down-regulation reversed the SNRK-mediated decrease in proliferation and ß-catenin. SNRK overexpression also decreased ß-catenin nuclear localization and target gene transcription, and ß-catenin down-regulation reversed the effects of SNRK knockdown on proliferation. SNRK transcript levels were reduced in human colon tumors compared to normal tissue by 35.82%, and stable knockdown of SNRK increased colon cancer cell tumorigenicity. Our results demonstrate that SNRK is down-regulated in colon cancer and inhibits colon cancer cell proliferation through CacyBP up-regulation and ß-catenin degradation, resulting in reduced proliferation signaling. These findings reveal a novel function for SNRK in the regulation of colon cancer cell proliferation and ß-catenin signaling.

Phosphorylation of contractile proteins in response to alpha- and beta-adrenergic stimulation in neonatal cardiomyocytes.

alpha- and beta-Adrenergic receptor agonists induce an inotropic response in the adult heart by promoting the phosphorylation of several regulatory proteins, including myosin-binding protein C (MyBP-C), cardiac troponin I (cTnI), and phospholamban (PLB). However, the adrenergic-induced phosphorylation of these proteins has not been characterized in the developing heart. Accordingly, we evaluated MyBP-C, cTnI, and PLB phosphorylation in cultured neonatal rat cardiomyocytes (NRCMs) after alpha- and beta-receptor activation with phenylephrine and isoproterenol. alpha-Receptor stimulation increased, whereas beta-receptor activation reduced MyBP-C phosphorylation. Isoelectric-focusing experiments indicated that the amount of monophosphorylated MyBP-C was sensitive to alpha-adrenergic activation, but diphosphorylated and triphosphorylated MyBP-C levels were largely unaffected. The phosphorylation of cTnI and PLB was consistent with the mechanism observed in adult hearts: alpha- and beta-Receptor stimulation phosphorylated both proteins. For cTnI, the greatest difference associated with beta-receptor activation was observed in the diphosphorylated state, whereas alpha-receptor activation was associated with a marked increase in the tetraphosphorylated protein and absence of the unphosphorylated state. Despite these apparent changes in cTnI and PLB phosphorylation, beta-receptor activation failed to alter calcium transients in NRCMs. Collectively, these findings suggest that, unlike cTnI and PLB, MyBP-C and inotropy are not coupled to beta-adrenergic stimulation in NRCMs. Therefore, cTnI and PLB probably play a more central role in modulating contractile function in NRCMs in response to catecholamines than does MyBP-C, and MyBP-C may have a structural role in stabilizing thick filament assembly rather than influencing cross-bridge formation in developing hearts.

Ubiquitin binding by a variant Jab1/MPN domain in the essential pre-mRNA splicing factor Prp8p.

The U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs) are components of the spliceosome, which catalyzes pre-mRNA splicing. One of the largest and the most highly conserved proteins in the spliceosome is Prp8p, a component of the U5 snRNP. Despite its size and conservation, very few motifs have been identified that suggest specific biochemical functions. A variant of the Jab1/MPN domain found in a class of deubiquitinating enzymes is present near the C terminus of Prp8p. Ubiquitination regulates a broad range of cellular pathways, and its functions generally require ubiquitin recognition by one or more ubiquitin-binding domains (UBDs). No precise role for ubiquitin has been defined in the pre-mRNA splicing pathway, and no known UBDs have been found within splicing proteins. Here we show that a Prp8p fragment containing the Jab1/MPN domain binds directly to ubiquitin with an affinity comparable to other known UBDs. Several mutations within this domain that compromise splicing also reduce interaction of the fragment with ubiquitin-Sepharose. Our results define a new UBD and suggest functional links between ubiquitin and the pre-mRNA splicing machinery.