The coactivator SRC-1 is an essential coordinator of hepatic glucose production

The multifaceted therapeutic value of targeting steroid receptor coactivator-1 in tumorigenesis

Steroid receptor coactivator-1 (SRC-1, also known as NCOA1) frequently functions as a transcriptional coactivator by directly binding to transcription factors and recruiting to the target gene promoters to promote gene transcription by increasing chromatin accessibility and promoting the formation of transcriptional complexes. In recent decades, various biological and pathological functions of SRC-1 have been reported, especially in the context of tumorigenesis. SRC-1 is a facilitator of the progression of multiple cancers, including breast cancer, prostate cancer, gastrointestinal cancer, neurological cancer, and female genital system cancer. The emerging multiorgan oncogenic role of SRC-1 is still being studied and may not be limited to only steroid hormone-producing tissues. Growing evidence suggests that SRC-1 promotes target gene expression by directly binding to transcription factors, which may constitute a novel coactivation pattern independent of AR or ER. In addition, the antitumour effect of pharmacological inhibition of SRC-1 with agents including various small molecules or naturally active compounds has been reported, but their practical application in clinical cancer therapy is very limited. For this review, we gathered typical evidence on the oncogenic role of SRC-1, highlighted its major collaborators and regulatory genes, and mapped the potential mechanisms by which SRC-1 promotes primary tumour progression.

PPARγ and C/EBPα response to acute cold stress in brown adipose tissue

Brown adipose tissue (BAT) has the ability to burn calories as heat. Utilizing BAT thermogenesis is thus an attractive way to combat obesity. However, the transcriptional network resulting in the lipid synthesis to oxidation shift during thermogenesis is not completely understood. Here, we report the regulation of two master regulators of adipogenesis, peroxisome proliferator-activated receptor gamma (PPARγ) and CCAAT/enhancer-binding protein alpha (C/EBPα), during acute cold stress in BAT. We found PPARγ dissociates from DNA in a fifth of its binding sites and these include Cebpa enhancers, leading to decreased C/EBPα expression. This dissociation requires PPARγ binding to activating ligands and is thus modulated by diet. Meanwhile, PPARα also detaches from DNA, and co-activator PGC1α associates with ERRα as part of a transcriptional network regulating lipid metabolism. Subsequent global replacement of C/EBPα by C/EBPβ and its associated transcriptional machinery is required for upregulation of structural lipid synthesis despite general upregulation of fatty acid oxidation.

Obesity Due to Steroid Receptor Coactivator-1 Deficiency Is Associated With Endocrine and Metabolic Abnormalities

CONTEXT

Genetic variants affecting the nuclear hormone receptor coactivator steroid receptor coactivator, SRC-1, have been identified in people with severe obesity and impair melanocortin signaling in cells and mice. As a result, obese patients with SRC-1 deficiency are being treated with a melanocortin 4 receptor agonist in clinical trials.

OBJECTIVE

Here, our aim was to comprehensively describe and characterize the clinical phenotype of SRC-1 variant carriers to facilitate diagnosis and clinical management.

METHODS

In genetic studies of 2462 people with severe obesity, we identified 23 rare heterozygous variants in SRC-1. We studied 29 adults and 18 children who were SRC-1 variant carriers and performed measurements of metabolic and endocrine function, liver imaging, and adipose tissue biopsies. Findings in adult SRC-1 variant carriers were compared to 30 age- and body mass index (BMI)-matched controls.

RESULTS

The clinical spectrum of SRC-1 variant carriers included increased food intake in children, normal basal metabolic rate, multiple fractures with minimal trauma (40%), persistent diarrhea, partial thyroid hormone resistance, and menorrhagia. Compared to age-, sex-, and BMI-matched controls, adult SRC-1 variant carriers had more severe adipose tissue fibrosis (46.2% vs 7.1% respectively, P = .03) and a suggestion of increased liver fibrosis (5/13 cases vs 2/13 in controls, odds ratio = 3.4), although this was not statistically significant.

CONCLUSION

SRC-1 variant carriers exhibit hyperphagia in childhood, severe obesity, and clinical features of partial hormone resistance. The presence of adipose tissue fibrosis and hepatic fibrosis in young patients suggests that close monitoring for the early development of obesity-associated metabolic complications is warranted.

Nuclear receptor coactivator SRC-1 promotes colorectal cancer progression through enhancing GLI2-mediated Hedgehog signaling

Overexpression of nuclear coactivator steroid receptor coactivator 1 (SRC-1) and aberrant activation of the Hedgehog (Hh) signaling pathway are associated with various tumorigenesis; however, the significance of SRC-1 in colorectal cancer (CRC) and its contribution to the activation of Hh signaling are unclear. Here, we identified a conserved Hh signaling signature positively correlated with SRC-1 expression in CRC based on TCGA database; SRC-1 deficiency significantly inhibited the proliferation, survival, migration, invasion, and tumorigenesis of both human and mouse CRC cells, and SRC-1 knockout significantly suppressed azoxymethane/dextran sodium sulfate (AOM/DSS)-induced CRC in mice. Mechanistically, SRC-1 promoted the expression of GLI family zinc finger 2 (GLI2), a major downstream transcription factor of Hh pathway, and cooperated with GLI2 to enhance multiple Hh-regulated oncogene expression, including Cyclin D1, Bcl-2, and Slug. Pharmacological blockages of SRC-1 and Hh signaling retarded CRC progression in human CRC cell xenograft mouse model. Together, our studies uncover an SRC-1/GLI2-regulated Hh signaling looping axis that promotes CRC tumorigenesis, offering an attractive strategy for CRC treatment.

Endothelium-specific depletion of LRP1 improves glucose homeostasis through inducing osteocalcin

The vascular endothelium is present within metabolic organs and actively regulates energy metabolism. Here we show osteocalcin, recognized as a bone-secreted metabolic hormone, is expressed in mouse primary endothelial cells isolated from heart, lung and liver. In human osteocalcin promoter-driven green fluorescent protein transgenic mice, green fluorescent protein signals are enriched in endothelial cells lining aorta, small vessels and capillaries and abundant in aorta, skeletal muscle and eye of adult mice. The depletion of lipoprotein receptor-related protein 1 induces osteocalcin through a Forkhead box O -dependent pathway in endothelial cells. Whereas depletion of osteocalcin abolishes the glucose-lowering effect of low-density lipoprotein receptor-related protein 1 depletion, osteocalcin treatment normalizes hyperglycemia in multiple mouse models. Mechanistically, osteocalcin receptor-G protein-coupled receptor family C group 6 member A and insulin-like-growth-factor-1 receptor are in the same complex with osteocalcin and required for osteocalcin-promoted insulin signaling pathway. Therefore, our results reveal an endocrine/paracrine role of endothelial cells in regulating insulin sensitivity, which may have therapeutic implications in treating diabetes and insulin resistance through manipulating vascular endothelium.

MicroRNA-185-5p inhibits hepatic gluconeogenesis and reduces fasting blood glucose levels by suppressing G6Pase

Aims/hypothesis: MicroRNAs (miRNAs) are known to contribute to many metabolic diseases, including type 2 diabetes. This study aimed to investigate the roles and molecular mechanisms of miR-185-5p in the regulation of hepatic gluconeogenesis. Methods: MicroRNA high-throughput sequencing was performed to identify differentially expressed miRNAs. High-fat diet-induced obese C57BL/6 mice and db/db mice, a genetic mouse model for diabetes, were used for examining the regulation of hepatic gluconeogenesis. Quantitative reverse transcriptase PCR and Western blotting were performed to measure the expression levels of various genes and proteins. Luciferase reporter assays were used to determine the regulatory roles of miR-185-5p on G6Pase expression. Results: Hepatic miR-185-5p expression was significantly decreased during fasting or insulin resistance. Locked nucleic acid (LNA)-mediated suppression of miR-185-5p increased blood glucose and hepatic gluconeogenesis in healthy mice. In contrast, overexpression of miR-185-5p in db/db mice alleviated blood hyperglycemia and decreased gluconeogenesis. At the molecular level, miR-185-5p directly inhibited G6Pase expression by targeting its 3'-untranslated regions. Furthermore, metformin, an anti-diabetic drug, could upregulate miR-185-5p expression to suppress G6Pase, leading to hepatic gluconeogenesis inhibition. Conclusions/interpretation: Our findings provided a novel insight into the role of miR-185-5p that suppressed hepatic gluconeogenesis and alleviated hyperglycemia by targeting G6Pase. We further identified that the /G6Pase axis mediated the inhibitory effect of metformin on hepatic gluconeogenesis. Thus, miR-185-5p might be a therapeutic target for hepatic glucose overproduction and fasting hyperglycemia.

Hepatic transcriptional responses to fasting and feeding

Mammals undergo regular cycles of fasting and feeding that engage dynamic transcriptional responses in metabolic tissues. Here we review advances in our understanding of the gene regulatory networks that contribute to hepatic responses to fasting and feeding. The advent of sequencing and -omics techniques have begun to facilitate a holistic understanding of the transcriptional landscape and its plasticity. We highlight transcription factors, their cofactors, and the pathways that they impact. We also discuss physiological factors that impinge on these responses, including circadian rhythms and sex differences. Finally, we review how dietary modifications modulate hepatic gene expression programs.

Loss of bone morphogenetic protein-binding endothelial regulator causes insulin resistance

Accumulating evidence suggests that chronic inflammation of metabolic tissues plays a causal role in obesity-induced insulin resistance. Yet, how specific endothelial factors impact metabolic tissues remains undefined. Bone morphogenetic protein (BMP)-binding endothelial regulator (BMPER) adapts endothelial cells to inflammatory stress in diverse organ microenvironments. Here, we demonstrate that BMPER is a driver of insulin sensitivity. Both global and endothelial cell-specific inducible knockout of BMPER cause hyperinsulinemia, glucose intolerance and insulin resistance without increasing inflammation in metabolic tissues in mice. BMPER can directly activate insulin signaling, which requires its internalization and interaction with Niemann-Pick C1 (NPC1), an integral membrane protein that transports intracellular cholesterol. These results suggest that the endocrine function of the vascular endothelium maintains glucose homeostasis. Of potential translational significance, the delivery of BMPER recombinant protein or its overexpression alleviates insulin resistance and hyperglycemia in high-fat diet-fed mice and Leprdb/db (db/db) diabetic mice. We conclude that BMPER exhibits therapeutic potential for the treatment of diabetes.

Decreased 11β-hydroxysteroid dehydrogenase 1 in lungs of steroid receptor coactivator (Src)-1/-2 double-deficient fetal mice is caused by impaired glucocorticoid and cytokine signaling

Our previous research revealed that steroid receptor coactivators (Src)-1 and -2 serve a critical cooperative role in production of parturition signals, surfactant protein A and platelet-activating factor, by the developing mouse fetal lung (MFL). To identify the global landscape of genes in MFL affected by Src-1/-2 double-deficiency, we conducted RNA-seq analysis of lungs from 18.5 days post-coitum (dpc) Src-1^-/- /-2^-/- (dKO) vs. WT fetuses. One of the genes most highly downregulated (~4.8 fold) in Src-1/-2 dKO fetal lungs encodes 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), which catalyzes conversion of inactive 11-dehydrocorticosterone to the glucocorticoid receptor (GR) ligand, corticosterone. Glucocorticoids were reported to upregulate 11β-HSD1 expression in various cell types via induction of C/EBP transcription factors. We observed that C/ebpα and C/ebpβ mRNA and protein were markedly reduced in Src-1/-2 double-deficient (Src-1/-2^d/d ) fetal lungs, compared to WT. Moreover, glucocorticoid induction of 11β-hsd1, C/ebpα and C/ebpβ in cultured MFL epithelial cells was prevented by the SRC family inhibitor, SI-2. Cytokines also contribute to the induction of 11β-HSD1. Expression of IL-1β and TNFα, which dramatically increased toward term in lungs of WT fetuses, was markedly reduced in Src-1/-2^d/d fetal lungs. Our collective findings suggest that impaired lung development and surfactant synthesis in Src-1/-2^d/d fetuses are likely caused, in part, by decreased GR and cytokine induction of C/EBP and NF-κB transcription factors. This results in reduced 11β-HSD1 expression and glucocorticoid signaling within the fetal lung, causing a break in the glucocorticoid-induced positive feedforward loop.

Altered Metabolome of Lipids and Amino Acids Species: A Source of Early Signature Biomarkers of T2DM

Diabetes mellitus, a disease of modern civilization, is considered the major mainstay of mortalities around the globe. A great number of biochemical changes have been proposed to occur at metabolic levels between perturbed glucose, amino acid, and lipid metabolism to finally diagnoe diabetes mellitus. This window period, which varies from person to person, provides us with a unique opportunity for early detection, delaying, deferral and even prevention of diabetes. The early detection of hyperglycemia and dyslipidemia is based upon the detection and identification of biomarkers originating from perturbed glucose, amino acid, and lipid metabolism. The emerging “OMICS” technologies, such as metabolomics coupled with statistical and bioinformatics tools, proved to be quite useful to study changes in physiological and biochemical processes at the metabolic level prior to an eventual diagnosis of DM. Approximately 300–400 such metabolites have been reported in the literature and are considered as predicting or risk factor-reporting metabolic biomarkers for this metabolic disorder. Most of these metabolites belong to major classes of lipids, amino acids and glucose. Therefore, this review represents a snapshot of these perturbed plasma/serum/urinary metabolic biomarkers showing a significant correlation with the future onset of diabetes and providing a foundation for novel early diagnosis and monitoring the progress of metabolic syndrome at early symptomatic stages. As most metabolites also find their origin from gut microflora, metabolism and composition of gut microflora also vary between healthy and diabetic persons, so we also summarize the early changes in the gut microbiome which can be used for the early diagnosis of diabetes.

Leptin decreases BC cell susceptibility to NK lysis via PGC1A pathway

Large prospective studies established a link between obesity and breast cancer (BC) development. Yet, the mechanisms underlying this association are not fully understood. Among the diverse adipocytokine secreted by hypertrophic adipose tissue, leptin is emerging as a key candidate molecule linking obesity and cancer, since it promotes proliferation and invasiveness of tumors. However, the potential implication of leptin on tumor escape mechanisms remains unknown. This study aims to explore the effect of leptin on tumor resistance to NK lysis and the underlying mechanism. We found that leptin promotes both BC resistance to NK92-mediated lysis and β oxidation on MCF-7, by the up-regulation of a master regulator of mitochondrial biogenesis, the peroxisome proliferator activated receptor coactivator-1 α (PGC1A). Using adenoviral approaches, we show that acute elevation of PGC1A enhances the fatty acid oxidation pathway and decreases the susceptibility of BC cells to NK92-mediated lysis. Importantly, we identified the involvement of PGC1A and leptin in the regulation of hypoxia inducible factor-1 alpha (HIF1A) expression by tumor cells. We further demonstrate that basal BC cells MDA-MB-231 and BT-20 exhibit an increased PGC1A mRNA level and an enhanced oxidative phosphorylation activity; in comparison with luminal BC cells MCF7 and MDA-361, which are associated with more resistance NK92 lysis. Altogether, our results demonstrate for the first time how leptin could promote tumor resistance to immune attacks. Reagents blocking leptin or PGC1A activity might aid in developing new therapeutic strategies to limit tumor development in obese BC patients.

Differential Proteomic Analysis of Chinese Giant Salamander Liver in Response to Fasting

Chinese giant salamander Andrias davidianus has strong tolerance to starvation. Fasting triggers a complex array of adaptive metabolic responses, a process in which the liver plays a central role. Here, a high-throughput proteomic analysis was carried out on liver samples obtained from adult A. davidianus after 3, 7, and 11 months of fasting. As a result, the expression levels of 364 proteins were significantly changed in the fasted liver. Functional analysis demonstrated that the expression levels of key proteins involved in fatty acid oxidation, tricarboxylic acid cycle, gluconeogenesis, ketogenesis, amino acid oxidation, urea cycle, and antioxidant systems were increased in the fasted liver, especially at 7 and 11 months after fasting. In contrast, the expression levels of vital proteins involved in pentose phosphate pathway and protein synthesis were decreased after fasting. We also found that fasting not only activated fatty acid oxidation and ketogenesis-related transcription factors PPARA and PPARGC1A, but also activated gluconeogenesis-related transcription factors FOXO1, HNF4A, and KLF15. This study confirms the central role of lipid and acetyl-CoA metabolism in A. davidianus liver in response to fasting at the protein level and provides insights into the molecular mechanisms underlying the metabolic response of A. davidianus liver to fasting.

SRC1 Deficiency in Hypothalamic Arcuate Nucleus Increases Appetite and Body Weight

Appetite is tightly controlled by neural and hormonal signals in animals. In general, steroid receptor co-activator 1 (SRC1) enhances steroid hormone signalling in energy balance and serves as a common co-activator of several steroid receptors, such as estrogen and glucocorticoid receptors. However, the key roles of SRC1 in energy balance remain largely unknown. We first confirmed that SRC1 is abundantly expressed in the hypothalamic arcuate nucleus (ARC), which is a critical centre for regulating feeding and energy balance; it is further co-localised with agouti-related protein and proopiomelanocortin neurons in the arcuate nucleus. Interestingly, local SRC1 expression changes with the transition between sufficiency and deficiency of food supply. To identify its direct role in appetite regulation, we repressed SRC1 expression in the hypothalamic ARC using lentivirus shRNA and found that SRC1 deficiency significantly promoted food intake and body weight gain, particularly in mice fed with a high-fat diet. We also found the activation of the AMP-activated protein kinase (AMPK) signalling pathway due to SRC1 deficiency. Thus, our results suggest that SRC1 in the ARC regulates appetite and body weight and that AMPK signalling is involved in this process. We believe that our study results have important implications for recognising the overlapping and integrating effects of several steroid hormones/receptors on accurate appetite regulation in future studies.

Development of an In Vitro Screening Platform for the Identification of Partial PPARγ Agonists as a Source for Antidiabetic Lead Compounds

Type 2 diabetes (T2D) is a metabolic disorder where insulin-sensitive tissues show reduced sensitivity towards insulin and a decreased glucose uptake (GU), which leads to hyperglycaemia. Peroxisome proliferator-activated receptor (PPAR)γ plays an important role in lipid and glucose homeostasis and is one of the targets in the discovery of drugs against T2D. Activation of PPARγ by agonists leads to a conformational change in the ligand-binding domain, a process that alters the transcription of several target genes involved in glucose and lipid metabolism. Depending on the ligands, they can induce different sets of genes that depends of their recruitment of coactivators. The activation of PPARγ by full agonists such as the thiazolidinediones leads to improved insulin sensitivity but also to severe side effects probably due to their behavior as full agonists. Partial PPARγ agonists are compounds with diminished agonist efficacy compared to full agonist that may exhibit the same antidiabetic effect as full agonists without inducing the same magnitude of side effects. In this review, we describe a screening platform for the identification of partial PPARγ agonists from plant extracts that could be promising lead compounds for the development of antidiabetic drugs. The screening platform includes a series of in vitro bioassays, such as GU in adipocytes, PPARγ-mediated transactivation, adipocyte differentiation and gene expression as well as in silico docking for partial PPARγ agonism.

Imbalanced expression pattern of steroid receptor coactivator-1 and -3 in liver cancer compared with normal liver: An immunohistochemical study with tissue microarray

Steroids affect normal and pathological functions of the liver through receptors, which require coactivators for their transcriptional activation. Steroid receptor coactivator (SRC)-1 and SRC-3 have been demonstrated to be regulated in numerous cancers; however, their expression profiles in liver cancer including hepatocellular carcinoma (HCC) and cholangiocellular carcinoma (CCC) remain unclear. Using tissue microarray immunohistochemistry, normal liver tissue and HCC tissue exhibited immunoreactivity of SRC-1, which were predominantly localized within extranuclear components; in CCC, they were detected within the cell nuclei; SRC-3 was also detected in the cell nuclei. Furthermore, no altered expression of SRC-1 and SRC-3 was observed in liver cancer compared with normal liver tissue; however, in CCC, the expression of SRC-3 was significantly increased compared with that detected in HCC. Importantly, although expression of SRC-1 and SRC-3 did not reveal any significant differences (30 vs. 40%) in normal liver tissue, HCC and CCC expression of SRC-1 was significantly decreased compared with that of SRC-3 (9.3 vs. 36%, and 6.7 vs. 67.7% for HCC and CCC, respectively). Further comparative analysis revealed that this discrepancy was detected in males with liver cancer, across all ages of HCC cases, younger CCC cases and all stages of liver cancer. The results suggested the presence of an imbalanced expression pattern of SRC-1 and SRC-3 from normal liver tissue to liver cancer (decreased SRC-1 and increased SRC-3), which may affect hepatic function and therefore promote liver carcinogenesis.

Metabolomics and Metabolic Diseases: Where Do We Stand?

Metabolomics, or the comprehensive profiling of small molecule metabolites in cells, tissues, or whole organisms, has undergone a rapid technological evolution in the past two decades. These advances have led to the application of metabolomics to defining predictive biomarkers for incident cardiometabolic diseases and, increasingly, as a blueprint for understanding those diseases' pathophysiologic mechanisms. Progress in this area and challenges for the future are reviewed here.

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.

Role of co-regulators in metabolic and transcriptional actions of thyroid hormone

Thyroid hormone (TH) controls a wide range of physiological processes through TH receptor (TR) isoforms. Classically, TRs are proposed to function as tri-iodothyronine (T3)-dependent transcription factors: on positively regulated target genes, unliganded TRs mediate transcriptional repression through recruitment of co-repressor complexes, while T3 binding leads to dismissal of co-repressors and recruitment of co-activators to activate transcription. Co-repressors and co-activators were proposed to play opposite roles in the regulation of negative T3 target genes and hypothalamic-pituitary-thyroid axis, but exact mechanisms of the negative regulation by TH have remained elusive. Important insights into the roles of co-repressors and co-activators in different physiological processes have been obtained using animal models with disrupted co-regulator function. At the same time, recent studies interrogating genome-wide TR binding have generated compelling new data regarding effects of T3, local chromatin structure, and specific response element configuration on TR recruitment and function leading to the proposal of new models of transcriptional regulation by TRs. This review discusses data obtained in various mouse models with manipulated function of nuclear receptor co-repressor (NCoR or NCOR1) and silencing mediator of retinoic acid receptor and thyroid hormone receptor (SMRT or NCOR2), and family of steroid receptor co-activators (SRCs also known as NCOAs) in the context of TH action, as well as insights into the function of co-regulators that may emerge from the genome-wide TR recruitment analysis.

Transcriptional and Chromatin Regulation during Fasting - The Genomic Era

An elaborate metabolic response to fasting is orchestrated by the liver and is heavily reliant on transcriptional regulation. In response to hormones (glucagon, glucocorticoids) many transcription factors (TFs) are activated and regulate various genes involved in metabolic pathways aimed at restoring homeostasis: gluconeogenesis, fatty acid oxidation, ketogenesis, and amino acid shuttling. We summarize recent discoveries regarding fasting-related TFs with an emphasis on genome-wide binding patterns. Collectively, the findings we discuss reveal a large degree of cooperation between TFs during fasting that occurs at motif-rich DNA sites bound by a combination of TFs. These new findings implicate transcriptional and chromatin regulation as major determinants of the response to fasting and unravels the complex, multi-TF nature of this response.

Programmed regulation of rat offspring adipogenic transcription factor (PPARγ) by maternal nutrition

We determined the protein expression of adipogenic transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ) and its co-repressor and co-activator complexes in adipose tissue from the obese offspring of under- and over-nourished dams. Female rats were fed either a high-fat (60% kcal) or control (10% kcal) diet before mating, and throughout pregnancy and lactation (Mat-OB). Additional dams were 50% food-restricted from pregnancy day 10 to term [intrauterine growth-restricted (IUGR)]. Adipose tissue protein expression was analyzed in newborn and adult male offspring. Normal birth weight Mat-OB and low birth weight IUGR newborns had upregulated PPARγ with variable changes in co-repressors and co-activators. As obese adults, Mat-OB and IUGR offspring had increased PPARγ with decreased co-repressor and increased co-activator expression. Nutritionally programmed increased PPARγ expression is associated with altered expression of its co-regulators in the newborn and adult offspring. Functional studies of PPARγ co-regulators are necessary to establish their role in PPARγ-mediated programmed obesity.

Minireview: nuclear receptor coregulators of the p160 family: insights into inflammation and metabolism

Nuclear receptor coactivators (NCOAs) are multifunctional transcriptional coregulators for a growing number of signal-activated transcription factors. The members of the p160 family (NCOA1/2/3) are increasingly recognized as essential and nonredundant players in a number of physiological processes. In particular, accumulating evidence points to the pivotal roles that these coregulators play in inflammatory and metabolic pathways, both under homeostasis and in disease. Given that chronic inflammation of metabolic tissues ("metainflammation") is a driving force for the widespread epidemic of obesity, insulin resistance, cardiovascular disease, and associated comorbidities, deciphering the role of NCOAs in "normal" vs "pathological" inflammation and in metabolic processes is indeed a subject of extreme biomedical importance. Here, we review the evolving and, at times, contradictory, literature on the pleiotropic functions of NCOA1/2/3 in inflammation and metabolism as related to nuclear receptor actions and beyond. We then briefly discuss the potential utility of NCOAs as predictive markers for disease and/or possible therapeutic targets once a better understanding of their molecular and physiological actions is achieved.

Energy metabolism in the liver

The liver is an essential metabolic organ, and its metabolic function is controlled by insulin and other metabolic hormones. Glucose is converted into pyruvate through glycolysis in the cytoplasm, and pyruvate is subsequently oxidized in the mitochondria to generate ATP through the TCA cycle and oxidative phosphorylation. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis. Long-chain fatty acids are incorporated into triacylglycerol, phospholipids, and/or cholesterol esters in hepatocytes. These complex lipids are stored in lipid droplets and membrane structures, or secreted into the circulation as very low-density lipoprotein particles. In the fasted state, the liver secretes glucose through both glycogenolysis and gluconeogenesis. During pronged fasting, hepatic gluconeogenesis is the primary source for endogenous glucose production. Fasting also promotes lipolysis in adipose tissue, resulting in release of nonesterified fatty acids which are converted into ketone bodies in hepatic mitochondria though β-oxidation and ketogenesis. Ketone bodies provide a metabolic fuel for extrahepatic tissues. Liver energy metabolism is tightly regulated by neuronal and hormonal signals. The sympathetic system stimulates, whereas the parasympathetic system suppresses, hepatic gluconeogenesis. Insulin stimulates glycolysis and lipogenesis but suppresses gluconeogenesis, and glucagon counteracts insulin action. Numerous transcription factors and coactivators, including CREB, FOXO1, ChREBP, SREBP, PGC-1α, and CRTC2, control the expression of the enzymes which catalyze key steps of metabolic pathways, thus controlling liver energy metabolism. Aberrant energy metabolism in the liver promotes insulin resistance, diabetes, and nonalcoholic fatty liver diseases.

Targeting steroid receptor coactivator 1 with antisense oligonucleotides increases insulin-stimulated skeletal muscle glucose uptake in chow-fed and high-fat-fed male rats

The steroid receptor coactivator 1 (SRC1) regulates key metabolic pathways, including glucose homeostasis. SRC1(-/-) mice have decreased hepatic expression of gluconeogenic enzymes and a reduction in the rate of endogenous glucose production (EGP). We sought to determine whether decreasing hepatic and adipose SRC1 expression in normal adult rats would alter glucose homeostasis and insulin action. Regular chow-fed and high-fat-fed male Sprage-Dawley rats were treated with an antisense oligonucleotide (ASO) against SRC1 or a control ASO for 4 wk, followed by metabolic assessments. SRC1 ASO did not alter basal EGP or expression of gluconeogenic enzymes. Instead, SRC1 ASO increased insulin-stimulated whole body glucose disposal by ~30%, which was attributable largely to an increase in insulin-stimulated muscle glucose uptake. This was associated with an approximately sevenfold increase in adipose expression of lipocalin-type prostaglandin D2 synthase, a previously reported regulator of insulin sensitivity, and an approximately 70% increase in plasma PGD2 concentration. Muscle insulin signaling, AMPK activation, and tissue perfusion were unchanged. Although GLUT4 content was unchanged, SRC1 ASO increased the cleavage of tether-containing UBX domain for GLUT4, a regulator of GLUT4 translocation. These studies point to a novel role of adipose SRC1 as a regulator of insulin-stimulated muscle glucose uptake.

Transcriptional coregulators: emerging roles of SRC family of coactivators in disease pathology

Transcriptional coactivators have evolved as an important new class of functional proteins that participate with virtually all transcription factors and nuclear receptors (NRs) to intricately regulate gene expression in response to a wide variety of environmental cues. Recent findings have highlighted that coactivators are important for almost all biological functions, and consequently, genetic defects can lead to severe pathologies. Drug discovery efforts targeting coactivators may prove valuable for treatment of a variety of diseases.

Hepatic SRC-1 activity orchestrates transcriptional circuitries of amino acid pathways with potential relevance for human metabolic pathogenesis

Disturbances in amino acid metabolism are increasingly recognized as being associated with, and serving as prognostic markers for chronic human diseases, such as cancer or type 2 diabetes. In the current study, a quantitative metabolomics profiling strategy revealed global impairment in amino acid metabolism in mice deleted for the transcriptional coactivator steroid receptor coactivator (SRC)-1. Aberrations were hepatic in origin, because selective reexpression of SRC-1 in the liver of SRC-1 null mice largely restored amino acids concentrations to normal levels. Cistromic analysis of SRC-1 binding sites in hepatic tissues confirmed a prominent influence of this coregulator on transcriptional programs regulating amino acid metabolism. More specifically, SRC-1 markedly impacted tyrosine levels and was found to regulate the transcriptional activity of the tyrosine aminotransferase (TAT) gene, which encodes the rate-limiting enzyme of tyrosine catabolism. Consequently, SRC-1 null mice displayed low TAT expression and presented with hypertyrosinemia and corneal alterations, 2 clinical features observed in the human syndrome of TAT deficiency. A heterozygous missense variant of SRC-1 (p.P1272S) that is known to alter its coactivation potential, was found in patients harboring idiopathic tyrosinemia-like disorders and may therefore represent one risk factor for their clinical symptoms. Hence, we reinforce the concept that SRC-1 is a central factor in the fine orchestration of multiple pathways of intermediary metabolism, suggesting it as a potential therapeutic target that may be exploitable in human metabolic diseases and cancer.

Steroid receptor coactivators: servants and masters for control of systems metabolism

Coregulator recruitment to nuclear receptors (NRs) and other transcription factors is essential for proper metabolic gene regulation, with coactivators enhancing and corepressors attenuating gene transcription. The steroid receptor coactivator (SRC) family is composed of three homologous members (SRC-1, SRC-2, and SRC-3), which are uniquely important for mediating steroid hormone and mitogenic actions. An accumulating body of work highlights the diverse array of metabolic functions regulated by the SRCs, including systemic metabolite homeostasis, inflammation, and energy regulation. We discuss here the cooperative and unique functions among the SRCs to provide a comprehensive atlas of systemic SRC metabolic regulation. Deciphering the fractional and synergistic contributions of the SRCs to metabolic homeostasis is crucial to understanding fully the networks underlying metabolic transcriptional regulation.

Glucagon regulates hepatic kisspeptin to impair insulin secretion

Early in the pathogenesis of type 2 diabetes mellitus (T2DM), dysregulated glucagon secretion from pancreatic α cells occurs prior to impaired glucose-stimulated insulin secretion (GSIS) from β cells. However, whether hyperglucagonemia is causally linked to β cell dysfunction remains unclear. Here we show that glucagon stimulates via cAMP-PKA-CREB signaling hepatic production of the neuropeptide kisspeptin1, which acts on β cells to suppress GSIS. Synthetic kisspeptin suppresses GSIS in vivo in mice and from isolated islets in a kisspeptin1 receptor-dependent manner. Kisspeptin1 is increased in livers and in serum from humans with T2DM and from mouse models of diabetes mellitus. Importantly, liver Kiss1 knockdown in hyperglucagonemic, glucose-intolerant, high-fat-diet fed, and Lepr(db/db) mice augments GSIS and improves glucose tolerance. These observations indicate a hormonal circuit between the liver and the endocrine pancreas in glycemia regulation and suggest in T2DM a sequential link between hyperglucagonemia via hepatic kisspeptin1 to impaired insulin secretion.

Regulation of pyruvate metabolism in metabolic-related diseases

Pyruvate is an obligatory intermediate in the oxidative disposal of glucose and a major precursor for the synthesis of glucose, glycerol, fatty acids, and non-essential amino acids. Stringent control of the fate of pyruvate is critically important for cellular homeostasis. The regulatory mechanisms for its metabolism are therefore of great interest. Recent advances include the findings that (a) the mitochondrial pyruvate carrier is sensitive to inhibition by thiazolidinediones; (b) pyruvate dehydrogenase kinases induce the Warburg effect in many disease states; and (c) pyruvate carboxylase is an important determinate of the rates of gluconeogenesis in humans with type 2 diabetes. These enzymes are potential therapeutic targets for several diseases.

Steroid receptor coactivator 1 is an integrator of glucose and NAD+/NADH homeostasis

Steroid receptor coactivator 1 (SRC-1) drives diverse gene expression programs necessary for the dynamic regulation of cancer metastasis, inflammation and gluconeogenesis, pointing to its overlapping roles as an oncoprotein and integrator of cell metabolic programs. Nutrient utilization has been intensely studied with regard to cellular adaptation in both cancer and noncancerous cells. Nonproliferating cells consume glucose through the citric acid cycle to generate NADH to fuel ATP generation via mitochondrial oxidative phosphorylation. In contrast, cancer cells undergo metabolic reprogramming to support rapid proliferation. To generate lipids, nucleotides, and proteins necessary for cell division, most tumors switch from oxidative phosphorylation to glycolysis, a phenomenon known as the Warburg Effect. Because SRC-1 is a key coactivator responsible for driving a hepatic gluconeogenic program under fasting conditions, we asked whether SRC-1 responds to alterations in nutrient availability to allow for adaptive metabolism. Here we show SRC-1 is stabilized by the 26S proteasome in the absence of glucose. RNA profiling was used to examine the effects of SRC-1 perturbation on gene expression in the absence or presence of glucose, revealing that SRC-1 affects the expression of complex I of the mitochondrial electron transport chain, a set of enzymes responsible for the conversion of NADH to NAD(+). NAD(+) and NADH were subsequently identified as metabolites that underlie SRC-1's response to glucose deprivation. Knockdown of SRC-1 in glycolytic cancer cells abrogated their ability to grow in the absence of glucose consistent with SRC-1's role in promoting cellular adaptation to reduced glucose availability.

ERK2-mediated phosphorylation of transcriptional coactivator binding protein PIMT/NCoA6IP at Ser298 augments hepatic gluconeogenesis

PRIP-Interacting protein with methyl transferase domain (PIMT) serves as a molecular bridge between CREB-binding protein (CBP)/ E1A binding protein p300 (Ep300) -anchored histone acetyl transferase and the Mediator complex sub-unit1 (Med1) and modulates nuclear receptor transcription. Here, we report that ERK2 phosphorylates PIMT at Ser(298) and enhances its ability to activate PEPCK promoter. We observed that PIMT is recruited to PEPCK promoter and adenoviral-mediated over-expression of PIMT in rat primary hepatocytes up-regulated expression of gluconeogenic genes including PEPCK. Reporter experiments with phosphomimetic PIMT mutant (PIMT(S298D)) suggested that conformational change may play an important role in PIMT-dependent PEPCK promoter activity. Overexpression of PIMT and Med1 together augmented hepatic glucose output in an additive manner. Importantly, expression of gluconeogenic genes and hepatic glucose output were suppressed in isolated liver specific PIMT knockout mouse hepatocytes. Furthermore, consistent with reporter experiments, PIMT(S298D) but not PIMT(S298A) augmented hepatic glucose output via up-regulating the expression of gluconeogenic genes. Pharmacological blockade of MAPK/ERK pathway using U0126, abolished PIMT/Med1-dependent gluconeogenic program leading to reduced hepatic glucose output. Further, systemic administration of T4 hormone to rats activated ERK1/2 resulting in enhanced PIMT ser(298) phosphorylation. Phosphorylation of PIMT led to its increased binding to the PEPCK promoter, increased PEPCK expression and induction of gluconeogenesis in liver. Thus, ERK2-mediated phosphorylation of PIMT at Ser(298) is essential in hepatic gluconeogenesis, demonstrating an important role of PIMT in the pathogenesis of hyperglycemia.

Acceleration of the glycolytic flux by steroid receptor coactivator-2 is essential for endometrial decidualization

Early embryo miscarriage is linked to inadequate endometrial decidualization, a cellular transformation process that enables deep blastocyst invasion into the maternal compartment. Although much of the cellular events that underpin endometrial stromal cell (ESC) decidualization are well recognized, the individual gene(s) and molecular pathways that drive the initiation and progression of this process remain elusive. Using a genetic mouse model and a primary human ESC culture model, we demonstrate that steroid receptor coactivator-2 (SRC-2) is indispensable for rapid steroid hormone-dependent proliferation of ESCs, a critical cell-division step which precedes ESC terminal differentiation into decidual cells. We reveal that SRC-2 is required for increasing the glycolytic flux in human ESCs, which enables rapid proliferation to occur during the early stages of the decidualization program. Specifically, SRC-2 increases the glycolytic flux through induction of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase 3 (PFKFB3), a major rate-limiting glycolytic enzyme. Similarly, acute treatment of mice with a small molecule inhibitor of PFKFB3 significantly suppressed the ability of these animals to exhibit an endometrial decidual response. Together, these data strongly support a conserved mechanism of action by which SRC-2 accelerates the glycolytic flux through PFKFB3 induction to provide the necessary bioenergy and biomass to meet the demands of a high proliferation rate observed in ESCs prior to their differentiation into decidual cells. Because deregulation of endometrial SRC-2 expression has been associated with common gynecological disorders of reproductive-age women, this signaling pathway, involving SRC-2 and PFKFB3, promises to offer new clinical approaches in the diagnosis and/or treatment of a non-receptive uterus in patients presenting idiopathic infertility, recurrent early pregnancy loss, or increased time to pregnancy.

Failure of an embryo to correctly implant into the endometrium is a common cause of pregnancy failure or early embryo miscarriage. Although advances in our understanding of oocyte and embryo development have significantly increased pregnancy success rates, these rates remain unacceptably low due in part to an endometrium that is unreceptive to embryo implantation. Using experimental mouse genetics and a primary human cell culture model, we show here that the development of a receptive endometrium requires steroid receptor coactivator-2, a factor which modulates the response of an endometrial cell to the pregnancy hormone, progesterone. Specifically, we show that SRC-2 increases progesterone-dependent glycolysis in the endometrial cell to provide energy and biomolecules for the next round of cell division. For an endometrium to be receptive to embryo implantation, specific endometrial cells (termed stromal cells) need to divide and numerically increase just prior to development of the receptive state. Therefore, SRC-2 is critical for the metabolic reprogramming of the endometrium to a receptive state, which provides the pretext for considering this factor and its metabolic targets in the design of future clinical approaches to diagnose and therapeutically treat those women at a high risk for early pregnancy loss.

Nuclear receptor coactivators: master regulators of human health and disease

Transcriptional coregulators (coactivators and corepressors) have emerged as the principal modulators of the functions of nuclear receptors and other transcription factors. During the decade since the discovery of steroid receptor coactivator-1 (SRC-1), the first authentic coregulator, more than 400 coregulators have been identified and characterized, and deciphering their function has contributed significantly to our understanding of their role in human physiology. Deregulated expression of coregulators has been implicated in diverse disease states and related pathologies. The advancement of molecular technologies has enabled us to better characterize the molecular associations of the SRC family of coactivators with other protein complexes in the context of gene regulation. These continuing discoveries not only expand our knowledge of the roles of coactivators in various human diseases but allow us to discover novel coactivator-targeting strategies for therapeutic intervention in these diseases.

Research resource: loss of the steroid receptor coactivators confers neurobehavioral consequences

Steroid receptor coactivators (SRCs) are important transcriptional modulators that regulate nuclear receptor and transcription factor activity to adjust transcriptional output to cellular demands. Highlighting their pleiotropic effects, dysfunction of the SRCs has been found in numerous pathologies including cancer, inflammation, and metabolic disorders. The SRC family is expressed strongly in the brain including the hippocampus, cortex, and hypothalamus. Studies focusing on the effect of SRC loss using congenic SRC knockout mice (SRC(-/-)) are limited in number, yet strongly indicate that the SRCs play important roles in regulating reproductive behavior, development, and motor coordination. To better understand the unique functions of the SRCs, we performed a neurobehavioral test battery focusing on anxiety and exploratory behaviors, motor coordination, sensorimotor gating, and nociception in both male and female null mice and compared them with their wild-type (WT) littermates. Results from the test battery reveal a role for SRC1 in motor coordination. Additionally, we found that SRC1 regulates anxiety responses in SRC1(-/-) male and female mice, and nociception sensitivity in SRC1(-/-) male but not female mice. By comparison, SRC2 regulates anxiety response with SRC2(-/-) females showing decreased anxiety in novel environments, as well as increased exploratory behavior in the open field compared with WT littermates. Additionally, SRC2(-/-) males were shown to have deficits in sensorimotor gating. Loss of SRC3 also shows sex differences in anxiety and exploratory behaviors. In particular, SRC3(-/-) female mice have increased anxiety and reduced exploratory activity and impairments in prepulse inhibition, whereas SRC3(-/-) male mice show no significant behavioral differences. In both genders, ablation of SRC3 decreases nocifensive behaviors. Collectively, these resource data suggest that loss of the SRCs results in behavioral phenotypes, underscoring the importance of understanding both the general and gender-based activity of SRCs in the brain.

MicroRNA 33 regulates glucose metabolism

Metabolic diseases are characterized by the failure of regulatory genes or proteins to effectively orchestrate specific pathways involved in the control of many biological processes. In addition to the classical regulators, recent discoveries have shown the remarkable role of small noncoding RNAs (microRNAs [miRNAs]) in the posttranscriptional regulation of gene expression. In this regard, we have recently demonstrated that miR-33a and miR33b, intronic miRNAs located within the sterol regulatory element-binding protein (SREBP) genes, regulate lipid metabolism in concert with their host genes. Here, we show that miR-33b also cooperates with SREBP1 in regulating glucose metabolism by targeting phosphoenolpyruvate carboxykinase (PCK1) and glucose-6-phosphatase (G6PC), key regulatory enzymes of hepatic gluconeogenesis. Overexpression of miR-33b in human hepatic cells inhibits PCK1 and G6PC expression, leading to a significant reduction of glucose production. Importantly, hepatic SREBP1c/miR-33b levels correlate inversely with the expression of PCK1 and G6PC upon glucose infusion in rhesus monkeys. Taken together, these results suggest that miR-33b works in concert with its host gene to ensure a fine-tuned regulation of lipid and glucose homeostasis, highlighting the clinical potential of miR-33a/b as novel therapeutic targets for a range of metabolic diseases.

Yin Yang 1 promotes hepatic gluconeogenesis through upregulation of glucocorticoid receptor

Gluconeogenesis is critical in maintaining blood glucose levels in a normal range during fasting. In this study, we investigated the role of Yin Yang 1 (YY1), a key transcription factor involved in cell proliferation and differentiation, in the regulation of hepatic gluconeogenesis. Our data showed that hepatic YY1 expression levels were induced in mice during fasting conditions and in a state of insulin resistance. Overexpression of YY1 in livers augmented gluconeogenesis, raising fasting blood glucose levels in C57BL/6 mice, whereas liver-specific ablation of YY1 using adenoviral shRNA ameliorated hyperglycemia in wild-type and diabetic db/db mice. At the molecular level, we further demonstrated that the major mechanism of YY1 in the regulation of hepatic glucose production is to modulate the expression of glucocorticoid receptor. Therefore, our study uncovered for the first time that YY1 participates in the regulation of hepatic gluconeogenesis, which implies that YY1 might serve as a potential therapeutic target for hyperglycemia in diabetes.

Research resource: tissue- and pathway-specific metabolomic profiles of the steroid receptor coactivator (SRC) family

The rapidly growing family of transcriptional coregulators includes coactivators that promote transcription and corepressors that harbor the opposing function. In recent years, coregulators have emerged as important regulators of metabolic homeostasis, including the p160 steroid receptor coactivator (SRC) family. Members of the SRC family have been ascribed important roles in control of gluconeogenesis, fat absorption and storage in the liver, and fatty acid oxidation in skeletal muscle. To provide a deeper and more granular understanding of the metabolic impact of the SRC family members, we performed targeted metabolomic analyses of key metabolic byproducts of glucose, fatty acid, and amino acid metabolism in mice with global knockouts (KOs) of SRC-1, SRC-2, or SRC-3. We measured amino acids, acyl carnitines, and organic acids in five tissues with key metabolic functions (liver, heart, skeletal muscle, brain, plasma) isolated from SRC-1, -2, or -3 KO mice and their wild-type littermates under fed and fasted conditions, thereby unveiling unique metabolic functions of each SRC. Specifically, SRC-1 ablation revealed the most significant impact on hepatic metabolism, whereas SRC-2 appeared to impact cardiac metabolism. Conversely, ablation of SRC-3 primarily affected brain and skeletal muscle metabolism. Surprisingly, we identified very few metabolites that changed universally across the three SRC KO models. The findings of this Research Resource demonstrate that coactivator function has very limited metabolic redundancy even within the homologous SRC family. Furthermore, this work also demonstrates the use of metabolomics as a means for identifying novel metabolic regulatory functions of transcriptional coregulators.

Minireview: Conversing with chromatin: the language of nuclear receptors

Nuclear receptors are transcription factors that are activated by physiological stimuli to bind DNA in the context of chromatin and regulate complex biological pathways. Major advances in nuclear receptor biology have been aided by genome scale examinations of receptor interactions with chromatin. In this review, we summarize the roles of the chromatin landscape in regulating nuclear receptor function. Chromatin acts as a central integrator in the nuclear receptor-signaling axis, operating in distinct temporal modalities. Chromatin effects nuclear receptor action by specifying its genomic localization and interactions with regulatory elements. On receptor binding, changes in chromatin operate as an effector of receptor signaling to modulate transcriptional events. Chromatin is therefore an integral component of the pathways that guide nuclear receptor action in cell-type-specific and cell state-dependent manners.

Ablation of steroid receptor coactivator-3 resembles the human CACT metabolic myopathy

Oxidation of lipid substrates is essential for survival in fasting and other catabolic conditions, sparing glucose for the brain and other glucose-dependent tissues. Here we show Steroid Receptor Coactivator-3 (SRC-3) plays a central role in long chain fatty acid metabolism by directly regulating carnitine/acyl-carnitine translocase (CACT) gene expression. Genetic deficiency of CACT in humans is accompanied by a constellation of metabolic and toxicity phenotypes including hypoketonemia, hypoglycemia, hyperammonemia, and impaired neurologic, cardiac and skeletal muscle performance, each of which is apparent in mice lacking SRC-3 expression. Consistent with human cases of CACT deficiency, dietary rescue with short chain fatty acids drastically attenuates the clinical hallmarks of the disease in mice devoid of SRC-3. Collectively, our results position SRC-3 as a key regulator of β-oxidation. Moreover, these findings allow us to consider platform coactivators such as the SRCs as potential contributors to syndromes such as CACT deficiency, previously considered as monogenic.

The function of steroid receptor coactivator-1 in normal tissues and cancer

In 1995, the steroid receptor coactivator-1 (SRC-1) was identified as the first authentic steroid receptor coactivator. Since then, the SRC proteins have remained at the epicenter of coregulator biology, molecular endocrinology and endocrine-related cancer. Cumulative works on SRC-1 have shown that it is primarily a nuclear receptor coregulator and functions to construct highly specific enzymatic protein complexes which can execute efficient and successful transcriptional activation of designated target genes. The versatile nature of SRC-1 enables it to respond to steroid dependent and steroid independent stimulation, allowing it to bind across many families of transcription factors to orchestrate and regulate complex physiological reactions. This review highlights the multiple functions of SRC-1 in the development and maintenance of normal tissue functions as well as its major role in mediating hormone receptor responsiveness. Insights from genetically manipulated mouse models and clinical data suggest SRC-1 is significantly overexpressed in many cancers, in particular, cancers of the reproductive tissues. SRC-1 has been associated with cellular proliferation and tumor growth but its major tumorigenic contributions are promotion and execution of breast cancer metastasis and mediation of resistance to endocrine therapies. The ability of SRC-1 to coordinate multiple signaling pathways makes it an important player in tumor cells' escape of targeted therapy.

Deletion of CaMKK2 from the liver lowers blood glucose and improves whole-body glucose tolerance in the mouse

Ca(2+)/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a member of the Ca(2+)/CaM-dependent protein kinase family that is expressed abundantly in brain. Previous work has revealed that CaMKK2 knockout (CaMKK2 KO) mice eat less due to a central nervous system -signaling defect and are protected from diet-induced obesity, glucose intolerance, and insulin resistance. However, here we show that pair feeding of wild-type mice to match food consumption of CAMKK2 mice slows weight gain but fails to protect from diet-induced glucose intolerance, suggesting that other alterations in CaMKK2 KO mice are responsible for their improved glucose metabolism. CaMKK2 is shown to be expressed in liver and acute, specific reduction of the kinase in the liver of high-fat diet-fed CaMKK2(floxed) mice results in lowered blood glucose and improved glucose tolerance. Primary hepatocytes isolated from CaMKK2 KO mice produce less glucose and have decreased mRNA encoding peroxisome proliferator-activated receptor γ coactivator 1-α and the gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, and these mRNA fail to respond specifically to the stimulatory effect of catecholamine in a cell-autonomous manner. The mechanism responsible for suppressed gene induction in CaMKK2 KO hepatocytes may involve diminished phosphorylation of histone deacetylase 5, an event necessary in some contexts for derepression of the peroxisome proliferator-activated receptor γ coactivator 1-α promoter. Hepatocytes from CaMKK2 KO mice also show increased rates of de novo lipogenesis and fat oxidation. The changes in fat metabolism observed correlate with steatotic liver and altered acyl carnitine metabolomic profiles in CaMKK2 KO mice. Collectively, these results are consistent with suppressed catecholamine-induced induction of gluconeogenic gene expression in CaMKK2 KO mice that leads to improved whole-body glucose homeostasis despite the presence of increased hepatic fat content.

Small molecule inhibition of the steroid receptor coactivators, SRC-3 and SRC-1

Overexpression of steroid receptor coactivator (SRC)-1 and SRC-3 is associated with cancer initiation, metastasis, advanced disease, and resistance to chemotherapy. In most of these cases, SRC-1 and SRC-3 have been shown to promote tumor cell growth by activating nuclear receptor and multiple growth factor signaling cascades that lead to uncontrolled tumor cell growth. Up until now, most targeted chemotherapeutic drugs have been designed largely to block a single pathway at a time, but cancers frequently acquire resistance by switching to alternative growth factor pathways. We reason that the development of chemotherapeutic agents against SRC coactivators that sit at the nexus of multiple cell growth signaling networks and transcriptional factors should be particularly effective therapeutics. To substantiate this hypothesis, we report the discovery of 2,2'-bis-(Formyl-1,6,7-trihydroxy-5-isopropyl-3-methylnaphthalene (gossypol) as a small molecule inhibitor of coactivator SRC-1 and SRC-3. Our data indicate that gossypol binds directly to SRC-3 in its receptor interacting domain. In MCF-7 breast cancer cells, gossypol selectively reduces the cellular protein concentrations of SRC-1 and SRC-3 without generally altering overall protein expression patterns, SRC-2, or other coactivators, such as p300 and coactivator-associated arginine methyltransferase 1. Gossypol reduces the concentration of SRC-3 in prostate, lung, and liver cancer cell lines. Gossypol inhibits cell viability in the same cancer cell lines where it promotes SRC-3 down-regulation. Additionally, gossypol sensitizes lung and breast cancer cell lines to the inhibitory effects of other chemotherapeutic agents. Importantly, gossypol is selectively cytotoxic to cancer cells, whereas normal cell viability is not affected. This data establish the proof-of-principle that, as a class, SRC-1 and SRC-3 coactivators are accessible chemotherapeutic targets. Given their function as integrators of multiple cell growth signaling systems, SRC-1/SRC-3 small molecule inhibitors comprise a new class of drugs that have potential as novel chemotherapeutics able to defeat aspects of acquired cancer cell resistance mechanisms.

Role of PGC-1α in exercise and fasting-induced adaptations in mouse liver

The transcriptional coactivator peroxisome proliferator-activated receptor (PPAR)-γ coactivator (PGC)-1α plays a role in regulation of several metabolic pathways. By use of whole body PGC-1α knockout (KO) mice, we investigated the role of PGC-1α in fasting, acute exercise and exercise training-induced regulation of key proteins in gluconeogenesis and metabolism in the liver. In both wild-type (WT) and PGC-1α KO mice liver, the mRNA content of the gluconeogenic proteins glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) was upregulated during fasting. Pyruvate carboxylase (PC) remained unchanged after fasting in WT mice, but it was upregulated in PGC-1α KO mice. In response to a single exercise bout, G6Pase mRNA was upregulated in both genotypes, whereas no significant changes were detected in PEPCK or PC mRNA. While G6Pase and PC protein remained unchanged, liver PEPCK protein content was higher in trained than untrained mice of both genotypes. The mRNA content of the mitochondrial proteins cytochrome c (Cyt c) and cytochrome oxidase (COX) subunit I was unchanged in response to fasting. The mRNA and protein content of Cyt c and COXI increased in the liver in response to a single exercise bout and prolonged exercise training, respectively, in WT mice, but not in PGC-1α KO mice. Neither fasting nor exercise affected the mRNA expression of antioxidant enzymes in the liver, and knockout of PGC-1α had no effect. In conclusion, these results suggest that PGC-1α plays a pivotal role in regulation of Cyt c and COXI expression in the liver in response to a single exercise bout and prolonged exercise training, which implies that exercise training-induced improvements in oxidative capacity of the liver is regulated by PGC-1α.

Hormonal regulation of hepatic glucose production in health and disease

We review mechanisms that regulate production of glucose by the liver, focusing on areas of budding consensus, and endeavoring to provide a candid assessment of lingering controversies. We also attempt to reconcile data from tracer studies in humans and large animals with the growing compilation of mouse knockouts that display changes in glucose production. A clinical hallmark of diabetes, excessive glucose production remains key to its treatment. Hence, we attempt to integrate emerging pathways into the broader goal to rejuvenate the staid antidiabetic pharmacopeia.

Functional characterization of the proximal promoter of the murine pyruvate carboxylase gene in hepatocytes: role of multiple gc boxes

Pyruvate carboxylase (PC) catalyzes the first committed step in gluconeogenesis in the liver. The murine PC gene possesses two promoters, the proximal (P1) and the distal (P2) which mediate production of distinct tissue-specific mRNA isoforms. By comparing the luciferase activities of 5'-nested deletions of the P1-promoter in the AML12 mouse hepatocyte cell line, the critical cis-acting elements required for maintaining basal transcription were located within the 166 nucleotides proximal to the transcription start site. Three GC boxes were identified within this region and shown by gel shift and ChIP assays to bind Sp1/Sp3. Over-expression of Sp1/Sp3 in AML12 and NIH3T3 cells increased P1-promoter activity, with Sp1 being a stronger activator than Sp3. Mutation of any one of the three GC boxes dramatically reduced basal promoter activity by 60-80% suggesting that all three boxes are equally strong regulatory elements. In AML12 cells, over-expression of Sp1/Sp3 restored the transcriptional activity of GC1 and GC2 but not GC3 mutants to levels similar to that of the WT construct, suggesting that GC3 is particularly critical for Sp1/Sp3-mediated induction. In NIH3T3 cells, however, the three boxes were equally important, indicating that the GC boxes differentially contribute to transcriptional regulation of the P1-promoter in the two cell lines. Mutants harboring two disrupted GC boxes showed a further decrease in promoter activity similar to the triple GC box mutant. Neither Sp1 nor Sp3 was able to fully restore the promoter activities of these mutants to that the WT level.