Protein Kinase B-α Inhibits Human Pyruvate Dehydrogenase Kinase-4 Gene Induction by Dexamethasone Through Inactivation of FOXO Transcription Factors

Abstract

Starvation and diabetes increase pyruvate dehydrogenase kinase-4 (PDK4) expression, which conserves gluconeogenic substrates by inactivating the pyruvate dehydrogenase complex. Mechanisms that regulate PDK4 gene expression, previously established to be increased by glucocorticoids and decreased by insulin, were studied. Treatment of HepG2 cells with dexamethasone increases the relative abundance of PDK4 mRNA, and insulin blocks this effect. Dexamethasone also increases human PDK4 (hPDK4) promoter activity in HepG2 cells, and insulin partially inhibits this effect. Expression of constitutively active PKB␣ abrogates dexamethasone stimulation of hPDK4 promoter activity, while coexpression of constitutively active FOXO1a or FOXO3a, which are mutated to alanine at the three phosphorylation sites for protein kinase B (PKB), disrupts the ability of PKB␣ to inhibit promoter activity. A glucocorticoid response element for glucocorticoid receptor (GR) binding and three insulin response sequences (IRSs) that bind FOXO1a and FOXO3a are identified in the hPDK4 promoter. Mutation of the IRSs reduces the ability of glucocorticoids to stimulate PDK4 transcription. Transfection studies with E1A, which binds to and inactivates p300/CBP, suggest that interactions between p300/CBP and GR as well as FOXO factors are important for glucocorticoid-stimulated hPDK4 expression. Insulin suppresses the hPDK4 induction by glucocorticoids through inactivation of the FOXO factors. Diabetes 53:899 -910, 2004 I n the well-fed state, the pyruvate dehydrogenanse complex (PDC) is relatively active and generates acetyl-CoA, the precursor for fatty acid synthesis and energy production by the Krebs cycle. In the starved state, PDC is relatively inactive, enabling the body to conserve three-carbon compounds for gluconeogenesis (1). Regulation of PDC activity is therefore important for control of glucose and lipid metabolism. PDC kinase (PDK) inactivates PDC by phosphorylating serine residues of the E1␣ component of the complex. PDC phosphatase (PDP) activates PDC by dephosphorylating E1␣. PDC activity, therefore, depends on the extent of E1␣ phosphorylation, which is determined by PDK and PDP activities (2). Four isoenzymes of PDK have been identified in the human genome (3-5). In most tissues the activity state of PDC is significantly decreased in starvation and diabetes (6,7), while PDK4 expression is dramatically increased (8 -12). The latter increases PDK activity, thereby contributing to the inactivation of PDC and the conservation of glucose and substrates (lactate, pyruvate, and alanine) that can be converted to glucose. This helps maintain euglycemia during starvation but exacerbates hyperglycemia in diabetes (2). A high-fat/low-carbohydrate diet also increases PDK4 protein and PDK activity in skeletal muscle (13,14). These findings suggest that the expression level of PDK4 is a major determinant of PDK activity. We have shown that glucocorticoids and peroxisome proliferator-activated receptor-␣ (PPAR␣) ligands induce PDK4 expression and that ...

Feedback regulation of hepatic gluconeogenesis through modulation of SHP/Nr0b2 gene expression by Sirt1 and FoxO1

“…To test whether Nr0b2 also represses other gluconeogenic genes, we generated a human PDK4 promoter reporter construct. As previously reported (22), FoxO1 acti- vated the human PDK-4 promoter 2.7-fold relative to GFP control (Fig. 6A).…”

Anti-inflammatory functions of glucocorticoid-induced genes

“…Such interactions may be particularly significant to the regulation of carbohydrate metabolism, where GCs and insulin have opposing actions. Glucose 6 phosphatase (Nakae et al, 2001) pyruvate dehydrogenase kinase 4 (Furuyama et al, 2003;Kwon et al, 2004), glucose 6 phosphate transporter (Kallwellis-Opara et al, 2003) and insulin-like growth factor binding protein 1 (Yeagley et al, 2001) are cooperatively regulated by GR and FoxO transcription factors. Insulin causes inactivation of FoxOs via Akt-mediated phosphorylation, thereby antagonizing GC-induced gene expression.…”

“…Composite GC responsive regions have been described in the promoters of tyrosine aminotransferase (Grange et al, 2001;Grange et al, 1989;Roux et al, 1995;Sassi et al, 1998) proliferin (Diamond et al, 1990), α1-acid glycoprotein (Alam et al, 1993;Nishio et al, 1993;Savoldi et al, 1997), the cyclin-dependent kinase inhibitor p21 waf1/cip1 Cram et al, 1998), β1-adrenergic receptor (Tseng et al, 2001), glucose-6-phosphate transporter (Kallwellis-Opara et al, 2003), pyruvate dehydrogenase kinase 4 (Kwon et al, 2004), monoamine oxidase (Manoli et al, 2005;Ou et al, 2006), β-casein (Rosen et al, 1998;Wyszomierski and Rosen, 2001) and others.…”

Overexpression of pyruvate dehydrogenase kinase 4 in heart perturbs metabolism and exacerbates calcineurin-induced cardiomyopathy

“…Expression of one of these isozymes, PDK4, is rapidly and markedly induced in heart and other tissues in response to various metabolic stimuli, including fasting and a high-fat diet (28,33,34,36). Transcription of the PDK4 gene is induced directly by the transcription factors FoxO1, which is repressed by insulin, and peroxisome proliferator-activated receptor (PPAR)-␣, which is activated by fatty acids (12,20,35). Thus PDK4 functions as a metabolic switch, altering substrate utilization to favor fatty acids under physiological conditions where fatty acid concentrations are elevated and insulin is reduced.…”