Increased hepatic pyruvate dehydrogenase kinase activity in fed hyperthyroid rats: studies in vivo and with cultured hepatocytes

The pivotal role of pyruvate dehydrogenase kinases in metabolic flexibility

Metabolic flexibility is the capacity of a system to adjust fuel (primarily glucose and fatty acids) oxidation based on nutrient availability. The ability to alter substrate oxidation in response to nutritional state depends on the genetically influenced balance between oxidation and storage capacities. Competition between fatty acids and glucose for oxidation occurs at the level of the pyruvate dehydrogenase complex (PDC). The PDC is normally active in most tissues in the fed state, and suppressing PDC activity by pyruvate dehydrogenase (PDH) kinase (PDK) is crucial to maintain energy homeostasis under some extreme nutritional conditions in mammals. Conversely, inappropriate suppression of PDC activity might promote the development of metabolic diseases. This review summarizes PDKs’ pivotal role in control of metabolic flexibility under various nutrient conditions and in different tissues, with emphasis on the best characterized PDK4. Understanding the regulation of PDC and PDKs and their roles in energy homeostasis could be beneficial to alleviate metabolic inflexibility and to provide possible therapies for metabolic diseases, including type 2 diabetes (T2D).

Transcriptional regulation of pyruvate dehydrogenase kinase

The pyruvate dehydrogenase complex (PDC) activity is crucial to maintains blood glucose and ATP levels, which largely depends on the phosphorylation status by pyruvate dehydrogenase kinase (PDK) isoenzymes. Although it has been reported that PDC is phosphorylated and inactivated by PDK2 and PDK4 in metabolically active tissues including liver, skeletal muscle, heart, and kidney during starvation and diabetes, the precise mechanisms by which expression of PDK2 and PDK4 are transcriptionally regulated still remains unclear. Insulin represses the expression of PDK2 and PDK4 via phosphorylation of FOXO through PI3K/Akt signaling pathway. Several nuclear hormone receptors activated due to fasting or increased fat supply, including peroxisome proliferator-activated receptors, glucocorticoid receptors, estrogen-related receptors, and thyroid hormone receptors, also participate in the up-regulation of PDK2 and PDK4; however, the endogenous ligands that bind those nuclear receptors have not been identified. It has been recently suggested that growth hormone, adiponectin, epinephrine, and rosiglitazone also control the expression of PDK4 in tissue-specific manners. In this review, we discuss several factors involved in the expressional regulation of PDK2 and PDK4, and introduce current studies aimed at providing a better understanding of the molecular mechanisms that underlie the development of metabolic diseases such as diabetes.

Regulation of pyruvate dehydrogenase kinase 4 (PDK4) by thyroid hormone: role of the peroxisome proliferator-activated receptor gamma coactivator (PGC-1 alpha)

PDK4 (pyruvate dehydrogenase kinase 4) regulates pyruvate oxidation through the phosphorylation and inhibition of the pyruvate dehydrogenase complex (PDC). PDC catalyzes the conversion of pyruvate to acetyl-CoA and is an important control point in glucose and pyruvate metabolism. PDK4 gene expression is stimulated by thyroid hormone (T(3)), glucocorticoids, and long chain fatty acids. The effects of T(3) on gene expression in the liver are mediated via the thyroid hormone receptor. Here, we have identified two binding sites for thyroid hormone receptor beta in the promoter of the rat PDK4 (rPDK4) gene. In addition, we have investigated the role of transcriptional coactivators and found that the PGC-1 alpha (peroxisome proliferator-activated receptor gamma coactivator) enhances the T(3) induction of rPDK4. Following T(3) administration, there is an increase in the association of PGC-1 alpha with the rPDK4 promoter. Interestingly, this increased association is with the proximal rPDK4 promoter rather than the distal region of the gene that contains the T(3) response elements. Administration of T(3) to hypothyroid rats elevated the abundance of PGC-1 alpha mRNA and protein in the liver. In addition, we observed greater association of PGC-1 alpha not only with the rPDK4 gene but also with phosphoenolpyruvate carboxykinase and CPT-1a (carnitine palmitoyltransferase 1a) genes. Knockdown of PGC-1 alpha in rat hepatocytes reduced the T(3) induction of PDK4, PEPCK, and CPT-1a genes. Our results indicate that T(3) regulates PGC-1 alpha abundance and association with hepatic genes, and in turn PGC-1 alpha is an important participant in the T(3) induction of selected genes.

Thyroid hormone controls myocardial substrate metabolism through nuclear receptor-mediated and rapid posttranscriptional mechanisms

Thyroid hormone regulates metabolism through transcriptional and posttranscriptional mechanisms. The integration of these mechanisms in heart is poorly understood. Therefore, we investigated control of substrate flux into the citric acid cycle (CAC) by thyroid hormone using retrogradely perfused isolated hearts (n = 20) from control (C) and age-matched thyroidectomized rats (T). We determined substrate flux and fractional contributions (Fc) to the CAC by 13C-NMR spectroscopy and isotopomer analyses in hearts perfused with [1,3-(13)C]acetoacetic acid (0.17 mM), L-[3-(13)C]lactic acid (LAC, 1.2 mM), [U-13C]long-chain mixed free fatty acids (FFA, 0.35 mM), and unlabeled glucose. Some T hearts were supplied triiodothyronine (T3, 10 nM; TT) for 60 min. Prolonged hypothyroid state reduced myocardial oxygen consumption, although T3 produced no significant change. Hypothyroidism reduced overall CAC(flux) but selectively altered only FFA(flux) among the individual substrates, though LAC(flux) trended upward. T3 rapidly decreased lactate Fc and flux. 13C labeling of glutamine through glutamate was increased in T with further enhancement in TT. The glutamate-to-glutamine ratio was significantly lower in T and TT. Immunoblots detected a decrease in hypothyroid hearts for muscle carnitine palmitoyltransferase I (CPT I) and a marked increase in pyruvate dehydrogenase kinase (PDK)-2 with no changes in liver CPT I, PDK-4, or hexokinase 2. TT, but not T, displayed elevated glutamine synthetase (GS) expression. These studies showed that T3 regulates cardiac metabolism through integration of several mechanisms, including changes in oxidative enzyme content and rapid modulation of individual substrates fluxes. T3 also moderates forward glutamine flux, possibly by increasing the overall activity of GS.

Diabetogenic impact of long-chain omega-3 fatty acids on pancreatic beta-cell function and the regulation of endogenous glucose production

In healthy individuals, peripheral insulin resistance evoked by dietary saturated lipid can be accompanied by increased insulin secretion such that glucose tolerance is maintained. Substitution of long-chain omega-3 fatty acids for a small percentage of dietary saturated fat prevents insulin resistance in response to high-saturated fat feeding. We substituted a small amount (7%) of dietary lipid with long-chain omega-3 fatty acids during 4 wk of high-saturated fat feeding to investigate the relationship between amelioration of insulin resistance and glucose-stimulated insulin secretion (GSIS). We demonstrate that, despite dietary delivery of saturated fat throughout, this manipulation prevents high-saturated fat feeding-induced insulin resistance with respect to peripheral glucose disposal and reverses insulin hypersecretion in response to glucose in vivo. Effects of long-chain omega-3 fatty acid enrichment to lower GSIS were also observed in perifused islets suggesting a direct effect on islet function. However, long-chain omega-3 fatty acid enrichment led to hepatic insulin resistance with respect to suppression of glucose output and impaired glucose tolerance in vivo. Our data demonstrate that the insulin response to glucose is suppressed to a greater extent than whole-body insulin sensitivity is enhanced by enrichment of a high-saturated fat diet with long-chain omega-3 fatty acids. Additionally, reduced GSIS despite glucose intolerance suggests that either long-chain omega-3 fatty acids directly impair the beta-cell response to saturated fat such that insulin secretion cannot be augmented to normalize glucose tolerance or beta-cell compensatory hypersecretion represents a response to insulin resistance at the level of peripheral glucose disposal but not endogenous glucose production.

Investigation of potential mechanisms regulating protein expression of hepatic pyruvate dehydrogenase kinase isoforms 2 and 4 by fatty acids and thyroid hormone

Liver contains two pyruvate dehydrogenase kinases (PDKs), namely PDK2 and PDK4, which regulate glucose oxidation through inhibitory phosphorylation of the pyruvate dehydrogenase complex (PDC). Starvation increases hepatic PDK2 and PDK4 protein expression, the latter occurring, in part, via a mechanism involving peroxisome proliferator-activated receptor-alpha (PPARalpha). High-fat feeding and hyperthyroidism, which increase circulating lipid supply, enhance hepatic PDK2 protein expression, but these increases are insufficient to account for observed increases in hepatic PDK activity. Enhanced expression of PDK4, but not PDK2, occurs in part via a mechanism involving PPAR-alpha. Heterodimerization partners for retinoid X receptors (RXRs) include PPARalpha and thyroid-hormone receptors (TRs). We therefore investigated the responses of hepatic PDK protein expression to high-fat feeding and hyperthyroidism in relation to hepatic lipid delivery and disposal. High-fat feeding increased hepatic PDK2, but not PDK4, protein expression whereas hyperthyroidism increased both hepatic PDK2 and PDK4 protein expression. Both manipulations decreased the sensitivity of hepatic carnitine palmitoyltransferase I (CPT I) to suppression by malonyl-CoA, but only hyperthyrodism elevated plasma fatty acid and ketone-body concentrations and CPT I maximal activity. Administration of the selective PPAR-alpha activator WY14,643 significantly increased PDK4 protein to a similar extent in both control and high-fat-fed rats, but WY14,643 treatment and hyperthyroidism did not have additive effects on hepatic PDK4 protein expression. PPARalpha activation did not influence hepatic PDK2 protein expression in euthyroid rats, suggesting that up-regulation of PDK2 by hyperthyroidism does not involve PPARalpha, but attenuated the effect of hyperthyroidism to increase hepatic PDK2 expression. The results indicate that hepatic PDK4 up-regulation can be achieved by heterodimerization of either PPARalpha or TR with the RXR receptor and that effects of PPARalpha activation on hepatic PDK2 and PDK4 expression favour a switch towards preferential expression of PDK4.

Studies of the long-term regulation of hepatic pyruvate dehydrogenase kinase

The administration of a low-carbohydrate/high-saturated-fat (LC/HF) diet for 28 days or starvation for 48 h both increased pyruvate dehydrogenase kinase (PDHK) activity in extracts of rat hepatic mitochondria, by approx. 2.1-fold and 3.5-fold respectively. ELISAs of extracts of hepatic mitochondria, conducted over a range of pyruvate dehydrogenase (PDH) activities, revealed that mitochondrial immunoreactive PDHKII (the major PDHK isoform in rat liver) was significantly increased by approx. 1.4-fold after 28 days of LC/HF feeding and by approx. 2-fold after 48 h of starvation. The effect of LC/HF feeding to increase hepatic PDHK activity was retained through hepatocyte preparation, but was decreased on 21 h culture with insulin (100 micro-i.u./ml). A sustained (24 h) 2-4-fold elevation in plasma insulin concentration in vivo (achieved by insulin infusion via an osmotic pump) suppressed the effect of LC/HF feeding so that hepatic PDHK activities did not differ significantly from those of (insulin-infused) control rats. The increase in hepatic PDHK activity evoked by 28 days of LC/HF feeding was prevented and reversed (within 24 h) by the replacement of 7% of the dietary lipid with long-chain omega-3 fatty acids. Analysis of hepatic membrane lipid revealed a 1.9-fold increase in the ratio of total polyunsaturated omega-3 fatty acids to total mono-unsaturated fatty acids. The results indicate that the increased hepatic PDHK activities observed in livers of LC/HF-fed or 48 h-starved rats are associated with long-term actions to increase hepatic PDHKII concentrations. The long-term regulation of hepatic PDHK by LC/HF feeding might be achieved through an impaired action of insulin to suppress PDHK activity. In addition, the fatty acid composition of the diet, rather than the fat content, is a key influence.

Different mechanisms underlie the long-term regulation of pyruvate dehydrogenase kinase (PDHK) by tri-iodothyronine in heart and liver

Antibodies to purified recombinant PDHKII were used for ELISAs of PDHKII in mitochondrial extracts. In liver, hyperthyroidism elicited a 2.3-fold increase in PDHK activity (P < 0.01) which was accompanied by a significant 1.5-fold (P < 0.001) increase in the amount of mitochondrial immunoreactive PDHKII. In contrast, despite a stable 2.0-fold increase in cardiac PDHK activity (P < 0.001), the amount of mitochondrial immunoreactive PDHKII in heart was unaffected by hyperthyroidism. The mechanisms for long-term regulation of PDHK activity by thyroid hormones therefore differ fundamentally between heart and liver.