Glucose decreases Na+,K+-ATPase activity in pancreatic beta-cells. An effect mediated via Ca2+-independent phospholipase A2 and protein kinase C-dependent phosphorylation of the alpha-subunit

Stimulation of insulin secretion and associated nuclear accumulation of iPLA(2)beta in INS-1 insulinoma cells

Accumulating evidence suggests that the cytosolic calcium-independent phospholipase A(2) (iPLA(2)beta) manifests a signaling role in insulin-secreting (INS-1) beta-cells. Earlier, we reported that insulin-secretory responses to cAMP-elevating agents are amplified in iPLA(2)beta-overexpressing INS-1 cells (Ma Z, Ramanadham S, Bohrer A, Wohltmann M, Zhang S, and Turk J. J Biol Chem 276: 13198-13208, 2001). Here, immunofluorescence, immunoaffinity, and enzymatic activity analyses are used to examine distribution of iPLA(2)beta in stimulated INS-1 cells in greater detail. Overexpression of iPLA(2)beta in INS-1 cells leads to increased accumulation of iPLA(2)beta in the nuclear fraction. Increasing glucose concentrations alone results in modest increases in insulin secretion, relative to parental cells, and in nuclear accumulation of the iPLA(2)beta protein. In contrast, cAMP-elevating agents induce robust increases in insulin secretion and in time-dependent nuclear accumulation of iPLA(2)beta fluorescence, which is reflected by increases in nuclear iPLA(2)beta protein content and specific enzymatic activity. The stimulated effects are significantly attenuated in the presence of cell-permeable inhibitors of protein phosphorylation and glycosylation. These findings suggest that conditions that amplify insulin secretion promote translocation of beta-cell iPLA(2)beta to the nuclei, where it may serve a crucial signaling role.

Studies of phospholipid metabolism, proliferation, and secretion of stably transfected insulinoma cells that overexpress group VIA phospholipase A2

A cytosolic 84 kDa Group VIA phospholipase A2 (iPLA2beta) that does not require Ca2+ for catalysis was cloned from Chinese hamster ovary (CHO) cells, murine P388D1 cells, pancreatic islet beta-cells, and other sources. Proposed iPLA2beta functions include participation in phosphatidylcholine (PC) homeostasis by degrading excess PC generated in CHO cells that overexpress CTP:phosphocholine cytidylyltransferase (CT), which catalyzes the rate-limiting step in PC biosynthesis; participation in biosynthesis of arachidonate-containing PC species in P388D1 cells by generating lysophosphatidylcholine (LPC) acceptors for arachidonate incorporation; and participation in signaling events in insulin secretion from islet beta-cells. To further examine iPLA2beta functions in beta-cells, we prepared stably transfected INS-1 insulinoma cell lines that overexpress iPLA2beta activity eightfold compared to parental INS-1 cells or to INS-1 cells transfected with an empty retroviral vector that did not contain iPLA2beta cDNA. The iPLA2beta-overexpressing cells exhibit a twofold increase in CT activity compared to parental cells but little change in rates of [3H]choline incorporation into or disappearance from PC. Electrospray ionization (ESI) tandem mass spectrometric measurements indicate that iPLA2beta-overexpressing cells have 1.5-fold higher LPC levels than parental INS-1 cells but do not exhibit increased rates of [3H]arachidonate incorporation into phospholipids, and incorporation is unaffected by a bromoenol lactone (BEL) suicide substrate inhibitor of iPLA2beta. The rate of appearance of arachidonate-containing phosphatidylethanolamine species visualized by ESI mass spectrometry is also similar in iPLA2beta-overexpressing and parental INS-1 cells incubated with supplemental arachidonic acid, and this process is unaffected by BEL. Compared to parental INS-1 cells, iPLA2beta-overexpressing cells proliferate more rapidly and exhibit amplified insulin secretory responses to a protein kinase C-activating phorbol ester, glucose, and a cAMP analog. These findings suggest that iPLA2beta plays a signaling role in beta-cells that differs from housekeeping functions in PC biosynthesis and degradation in P388D1 and CHO cells.

A new pathway for glucose-dependent insulinotropic polypeptide (GIP) receptor signaling: evidence for the involvement of phospholipase A2 in GIP-stimulated insulin secretion

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca(2+) levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic beta-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nm) and ATP (5 microm) significantly increased [(3)H]arachidonic acid ([(3)H]AA) efflux from transfected Chinese hamster ovary K1 cells expressing the GIP receptor (basal, 128 +/- 11 cpm/well; GIP, 212 +/- 32 cpm/well; ATP, 263 +/- 35 cpm/well; n = 4; p < 0.05). In addition, GIP receptors were shown for the first time to be capable of functionally coupling to AA production through Gbetagamma dimers in Chinese hamster ovary K1 cells. In a beta-cell model (betaTC-3), GIP was found to elicit [(3)H]AA release, independent of glucose, in a concentration-dependent manner (EC(50) value of 1.4 +/- 0.62 nm; n = 3). Although GIP did not potentiate insulin release under extracellular Ca(2+)-free conditions, it was still capable of elevating intracellular cAMP and stimulating [(3)H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca(2+)-independent phospholipase A(2). Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the beta-cell.

Studies of insulin secretory responses and of arachidonic acid incorporation into phospholipids of stably transfected insulinoma cells that overexpress group VIA phospholipase A2 (iPLA2beta ) indicate a signaling rather than a housekeeping role for iPLA2beta

A cytosolic 84-kDa group VIA phospholipase A(2) (iPLA(2)beta) that does not require Ca(2+) for catalysis has been cloned from several sources, including rat and human pancreatic islet beta-cells and murine P388D1 cells. Many potential iPLA(2)beta functions have been proposed, including a signaling role in beta-cell insulin secretion and a role in generating lysophosphatidylcholine acceptors for arachidonic acid incorporation into P388D1 cell phosphatidylcholine (PC). Proposals for iPLA(2)beta function rest in part on effects of inhibiting iPLA(2)beta activity with a bromoenol lactone (BEL) suicide substrate, but BEL also inhibits phosphatidate phosphohydrolase-1 and a group VIB phospholipase A(2). Manipulation of iPLA(2)beta expression by molecular biologic means is an alternative approach to study iPLA(2)beta functions, and we have used a retroviral construct containing iPLA(2)beta cDNA to prepare two INS-1 insulinoma cell clonal lines that stably overexpress iPLA(2)beta. Compared with parental INS-1 cells or cells transfected with empty vector, both iPLA(2)beta-overexpressing lines exhibit amplified insulin secretory responses to glucose and cAMP-elevating agents, and BEL substantially attenuates stimulated secretion. Electrospray ionization mass spectrometric analyses of arachidonic acid incorporation into INS-1 cell PC indicate that neither overexpression nor inhibition of iPLA(2)beta affects the rate or extent of this process in INS-1 cells. Immunocytofluorescence studies with antibodies directed against iPLA(2)beta indicate that cAMP-elevating agents increase perinuclear fluorescence in INS-1 cells, suggesting that iPLA(2)beta associates with nuclei. These studies are more consistent with a signaling than with a housekeeping role for iPLA(2)beta in insulin-secreting beta-cells.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Calcium-stimulated insulin secretion in diffuse and focal forms of congenital hyperinsulinism

OBJECTIVES

To identify infants with hyperinsulinism caused by defects of the beta-cell adenosine triphosphate-dependent potassium channel complex and to distinguish focal and diffuse forms of hyperinsulinism caused by these mutations.

STUDY DESIGN

The acute insulin response to intravenous calcium stimulation (CaAIR) was determined in 9 patients <20 years with diffuse hyperinsulinism caused by defective beta-cell sulfonylurea receptor (SUR1(-/-)), 3 patients with focal congenital hyperinsulinism (6 weeks to 18 months), a 10-year-old with insulinoma, 5 with hyperinsulinism/hyperammonemia syndrome caused by defective glutamate dehydrogenase (6 months to 28 years), 4 SUR1(+/-) heterozygotes with no symptoms, and 9 normal adults. Three infants with congenital focal disease, 1 with diffuse hyperinsulinism, and the child with insulinoma underwent selective pancreatic intra-arterial calcium stimulation with hepatic venous sampling.

RESULTS

Children with diffuse SUR1(-/-) disease and infants with congenital focal hyperinsulinism responded to CaAIR, whereas the normal control group, patients with hyperinsulinism/hyperammonemia syndrome, and SUR1(+/-) carriers did not. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling revealed selective, significant step-ups in insulin secretion that correlated anatomically with the location of solitary lesions confirmed surgically in 2 of 3 infants with congenital focal disease and in the child with insulinoma. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling demonstrated markedly elevated baseline insulin levels throughout the pancreas of the infant with diffuse hyperinsulinism.

CONCLUSIONS

The intravenous CaAIR is a safe and simple test for identifying infants with diffuse SUR1(-/-) hyperinsulinism or with focal congenital hyperinsulinism. Preoperative selective arterial calcium stimulation of the pancreas with hepatic venous sampling can localize focal lesions causing hyperinsulinism in children. The combination of these calcium stimulation tests may help distinguish focal lesions suitable for cure by local surgical resection.