Control of pyruvate dehydrogenase kinase gene expression

Several tissues, but most notably the brain, depend upon a continuous supply of glucose to meet their need for energy. Exhaustion of endogenous glycogen stores during starvation renders such tissues dependent upon hepatic gluconeogenesis. Effective use of compounds that can be converted to glucose is therefore essential for survival during starvation. Since no pathway exists for the conversion of the two-carbon compound acetate to glucose, the body can ill afford the conversion of pyruvate to acetyl-CoA by the pyruvate dehydrogenase complex. During starvation, therefore, the activity of this complex must be effectively inhibited in all tissues that are not dependent upon complete oxidation of glucose carbon to meet their energy. This is accomplished through the action of mitochondrial protein kinases that inactivate the E1 component of the pyruvate dehydrogenase complex. Five mitochondrial protein kinases are now known, each encoded by a different nuclear gene. Four of them are pyruvate dehydrogenase kinase (PDK) isoenzymes. Of these, isoenzymes 2 and 4 (PDK2 and PDK4) are the most interesting. Starvation and diabetes alter expression of PDK2 and PDK4 in a tissue-dependent manner, suggesting that they are regulated by induction/repression mechanisms to control the activity state of the pyruvate dehydrogenase complex. For example, PDK4, measured at both the protein and transcript level, is greatly increased in the heart and skeletal muscle in response to starvation. Re-feeding decreases PDK activity and also reverses the increase in PDK4 protein and message. Administration of WY-14,643, a selective agonist for the peroxisome proliferator-activated receptor α (PPARα), also induces large increases in PDK activity, PDK4 protein, and PDK4 mRNA. This is particularly exciting since it suggests that free fatty acids, believed to be endogenous PPARα ligands, may function as signals for increased PDK4 gene expression. In contrast to heart and skeletal muscle, starvation does not increase expression of PDK isoenzymes in brain, a finding consistent with the continuous need for glucose oxidation in this tissue. However, three other tissues, namely liver, kidney, and lactating mammary gland, respond to starvation with increased expression of both PDK2 and PDK4. From these findings we propose that increased expression of PDK isoenzymes in several mammalian tissues causes an imbalance in the relative kinase and phosphatase activities in the starved state. This promotes hyper-phosphorylation and inactivation of pyruvate dehydrogenase complex in order to conserve glucose as well as the three carbon compounds that can be converted to glucose. Factors that regulate expression of PDK4 have been studied with the Morris 7800Cl hepatoma cell line. These relatively slow-growing cells are rich in mitochondria, and like liver, express relatively large amounts of PDK2 but very little PDK4. Treatment of these cells with dexamethasone has the same effect as starvation in the intact animal, i.e., causes a remarkable increase in PDK4 message level. The response to the steroid is rapid, concentration dependent, blocked by antagonists to the glucocorticoid receptor, and disappears rapidly upon removal of the steroid from the culture medium. WY-14,643 exerts effects similar to dexamethasone, i.e., incubation of 7800Cl cells with this PPARα ligand greatly increases PDK4 message. The response is again rapid and dependent on concentration of the ligand. The half-life of the PDK4 message is not altered by either dexamethasone or WY-14,643, suggesting that PDK4 gene expression is most likely increased by both compounds. Insulin very effectively prevents and also rapidly reverses the stimulatory effect of dexamethasone on PDK4 gene expression. On the other hand, insulin is not very effective in opposing and reversing the changes in PDK4 message induced by WY-14,643. The high basal level of the PDK2 message in 7800Cl cells is not altered by dexamethasone and WY-14,643. Nevertheless, insulin effectively down -regulates PDK2 message levels, albeit with a time course much slower than insulin's negative effect on PDK4 gene expression. It appears from these studies that glucocorticoids, insulin, and PPARα ligands (e.g., long-chain fatty acids) are likely regulators of PDK4 gene expression. Insulin appears to be important in regulating PDK2 gene expression. Changes in concentration of these factors during starvation are likely responsible for up-regulation of PDK activity and inactivation of the pyruvate dehydrogenase complex.