Is the p29 protein involved in the rapid regulation of phosphoenolpyruvate carboxykinase (GTP)?

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: has it been overlooked?

BACKGROUND

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

SCOPE OF REVIEW

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

MAJOR CONCLUSIONS

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

GENERAL SIGNIFICANCE

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

The inhibition of phosphoenolpyruvate carboxykinase following in vivo chronic phenobarbital treatment in the rat is due to a post-translational event

Chronic treatment of rats with phenobarbital has been reported to decrease gluconeogenesis in rat hepatocytes by a 50% inhibition of phosphoenolpyruvate (P-pyruvate) carboxykinase activity [Argaud, D., Halimi, S., Catelloni, F. & Leverve, X. (1991) Biochem. J. 280, 663-669]. Contrary to the current knowledge of P-pyruvate carboxykinase regulation, we failed to find a diminution of either P-pyruvate carboxykinase protein (by using a polyclonal antibody) or P-pyruvate carboxykinase mRNA, in the liver of rats treated with phenobarbital for 2 weeks. Kinetic studies of P-pyruvate carboxykinase activity, measured by either carboxylation of P-pyruvate or decarboxylation of oxaloacetate, revealed a decrease in both V(max) and Km after phenobarbital treatment, whereas the nutritional state affected only the V(max), as expected. Assessment of P-pyruvate carboxykinase specificity was confirmed by the full inhibition of the enzyme with its specific inhibitor 3-mercaptopicolinate in the micromolar range. P-Pyruvate carboxykinase, purified either by ammonium sulfate fractionation or by immunoprecipitation, exhibited a similar decrease in affinity after phenobarbital treatment. Although the molecular mass does not appear to be altered, the pH sensitivity to 3-mercaptopicolinate inhibition and the enzyme recovery after immunoprecipitation both seemed to be affected. This leads us to propose that the effect of chronic phenobarbital treatment on P-pyruvate carboxykinase activity is not the result of transcriptional regulation but is exerted at the post-translational level.