okadaic-acid and Insulinoma

Rab GTPases and their effectors play important roles in membrane trafficking between cellular compartments in eukaryotic cells. In the present study, we examined the roles of Rab11B and its effectors in insulin secretion in pancreatic beta-cells. In the mouse insulin-secreting cell line MIN6, Rab11 was co-localized with insulin-containing granules, and over-expression of the GTP- or the GDP-bound form of Rab11B significantly inhibited regulated secretion, indicating involvement of Rab11B in regulated insulin secretion. To determine the downstream signal of Rab11-mediated insulin secretion, we examined the effects of various Rab11-interacting proteins on insulin secretion, and found that Rip11 is involved in cAMP-potentiated insulin secretion but not in glucose-induced insulin secretion. Analyses by immunocytochemistry and subcellular fractionation revealed Rip11 to be co-localized with insulin granules. The inhibitory effect of the Rip11 mutant was not altered in MIN6 cells lacking Epac2, which mediates protein kinase A (PKA)-independent potentiation of insulin secretion, compared with wild-type MIN6 cells. In addition, Rip11 was found to be phosphorylated by PKA in MIN6 cells. The present study shows that both Rab11 and its effector Rip11 participate in insulin granule exocytosis and that Rip11, as a substrate of PKA, regulates the potentiation of exocytosis by cAMP in pancreatic beta-cells.

Topics: 1-Methyl-3-isobutylxanthine; Animals; Carrier Proteins; Cell Line, Tumor; Colforsin; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Exocytosis; Glucose; Green Fluorescent Proteins; Immunoblotting; Insulin; Insulin Secretion; Insulin-Secreting Cells; Insulinoma; Isoquinolines; Mice; Microscopy, Fluorescence; Microscopy, Immunoelectron; Mitochondrial Proteins; Okadaic Acid; Phosphorylation; Protein Transport; rab GTP-Binding Proteins; Recombinant Fusion Proteins; Reverse Transcriptase Polymerase Chain Reaction; Sulfonamides; Transfection

The sphingolipid ceramide (CER) and its metabolites have been recognized as important mediators of signal transduction processes leading to a variety of cellular responses, including survival and demise via apoptosis. Accumulating evidence implicates key regulatory roles for intracellularly generated CER in metabolic dysfunction of the islet beta cell. We have previously reported localization of an okadaic (OKA)-sensitive CER-activated protein phosphatase (CAPP) in the islet beta cell. We have also reported immunological identification of the structural A subunit, the regulatory B56alpha subunit, and the catalytic C subunit for CAPP holoenzyme complex in insulin-secreting INS-1 cells. Herein, we provide the first evidence to suggest that siRNA-mediated knockdown of the alpha isoform of the catalytic subunit of PP2Ac (PP2Acalpha) markedly reduces the CAPP activity in INS 832/13 cells. Potential significance of the functional activation of CAPP holoenzyme in the context of lipid-and glucose-induced metabolic dysfunction of the islet beta cell is discussed.

Topics: Animals; Cell Line, Tumor; Ceramides; Insulinoma; Okadaic Acid; Pancreatic Neoplasms; Phosphoprotein Phosphatases; Protein Phosphatase 2; Rats; RNA, Small Interfering

We have previously demonstrated that high concentrations of glucose stimulate insulin gene expression by causing hyperacetylation of histone H4 at the insulin gene promoter. Furthermore, we have shown that the glucose-mediated hyperacetylation of histone H4 depends on the recruitment of the histone acetyltransferase p300 by the beta cell-specific transcription factor Pdx-1. In this study, we demonstrate that the histone deacetylases Hdac-1 and Hdac-2 are rapidly recruited to the insulin promoter in the mouse insulinoma cell line MIN6 when cells are switched from high to low glucose media. Moreover, we demonstrate that the beta cell-specific homeodomain protein Pdx-1 interacts with histone deacetylases Hdac-1 and Hdac-2 at low levels of glucose. In vitro studies indicate that the interaction between Pdx-1 and Hdac-1 or Hdac-2 is direct and requires the C terminus of Pdx-1. Treatment of MIN6 cells with okadaic acid, which inhibits the activity of protein phosphatases, abolishes the interaction of Pdx-1 with Hdac-1 and Hdac-2 on low levels of glucose, suggesting the requirement of a dephosphorylation event for this interaction to occur. These data indicate that insulin gene expression is decreased on low levels of glucose by recruitment of Hdac-1 and Hdac-2 to the insulin promoter by the transcription factor Pdx-1.

Topics: Acetylation; Animals; Culture Media; DNA; Gene Expression; Glucose; Histone Deacetylase 1; Histone Deacetylase 2; Histone Deacetylases; Histones; Homeodomain Proteins; Immunosorbent Techniques; Insulin; Insulinoma; Mice; Okadaic Acid; Peptide Fragments; Phosphorylation; Promoter Regions, Genetic; Recombinant Fusion Proteins; Repressor Proteins; Structure-Activity Relationship; Trans-Activators; Transfection; Tumor Cells, Cultured

In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion is an early pathogenetic event. Glucose is the cardinal physiological stimulator of insulin secretion from the pancreatic beta-cell, but the mechanisms involved in glucose sensing are not fully understood. Specific ser/thr protein phosphatase (PPase) inactivation by okadaic acid promotes Ca(2+) entry and insulin exocytosis in the beta-cell. We now show that glycolytic and Krebs cycle intermediates, whose concentrations increase upon glucose stimulation, not only dose dependently inhibit ser/thr PPase enzymatic activities, but also directly promote insulin exocytosis from permeabilized beta-cells. Thus, fructose-1,6-bisphosphate, phosphoenolpyruvate, 3-phosphoglycerate, citrate, and oxaloacetate inhibit PPases and significantly enhance insulin exocytosis, nonadditive to that of okadaic acid, at micromolar Ca2+ concentrations. In contrast, the effect of GTP is potentiated by okadaic acid, suggesting that the action of GTP does not require PPase inactivation. We conclude that specific glucose metabolites and GTP inhibit beta-cell PPase activities and directly stimulate Ca2+-independent insulin exocytosis. The glucose metabolites, but not GTP, seem to require PPase inactivation for their stimulatory effect on exocytosis. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by metabolic intermediates, may be a novel regulatory mechanism linking glucose sensing to insulin exocytosis in the beta-cell.

Topics: Animals; Calcium; Citric Acid; Drug Synergism; Enzyme Inhibitors; Exocytosis; Fructosediphosphates; Glucose; Glyceric Acids; Guanosine Triphosphate; Insulin; Insulinoma; Islets of Langerhans; Okadaic Acid; Oxaloacetic Acid; Pancreatic Neoplasms; Phosphoenolpyruvate; Phosphoprotein Phosphatases; Phosphorylation; Rats; Tumor Cells, Cultured

Reversible protein phosphorylation is an important mechanism by which cells transduce external signals into biologic responses. Levels of protein phosphorylation are determined by the balanced actions of both protein kinases and protein phosphatases (PPases). However, compared with protein kinases, regulation of PPases has been relatively neglected. The insulin secretagogue L-arginine, an immediate metabolic precursor to polyamines, causes a rapid and transient decrease in PPase-1 activity in insulin-secreting RINm5F cells. We here show that polyamines dose-dependently suppress PPase-1-like activity when added to RINm5F cell homogenates at physiologic concentrations (spermine > spermidine > putrescine), while having minor and inconsistent effects on PPase-2A-like activity. The IC50 value for spermine on PPase-1-like activity was approximately 4 mM. The inhibitory effect was reproduced and of comparable magnitude on purified PPases types 1 and 2A. On the other hand, when endogenous polyamine pools were exhausted by 4 days of exposure to the specific L-ornithine decarboxylase inhibitor DL-alpha-difluoromethylornithine, there was an increase in PPase-2A-like activity. Quantitative Western analysis revealed that the amount of PPase-2A protein did not change after this treatment. It is concluded that polyamines cause time-and concentration-dependent inhibitory effects on RINm5F cell PPase activities, which may contribute to the increase in phosphorylation state that occurs after secretory stimulation.

Topics: Arginine; Eflornithine; Enzyme Activation; Enzyme Inhibitors; Histones; Humans; Insulin; Insulin Secretion; Insulinoma; Islets of Langerhans; Okadaic Acid; Pancreatic Neoplasms; Phosphoprotein Phosphatases; Phosphorylation; Polyamines; Protein Processing, Post-Translational; Protein Serine-Threonine Kinases; Putrescine; Spermidine; Spermine; Tumor Cells, Cultured

Intracellular movement of secretory granules is a proximal stage in the secretory cascade that ends in the release product from cells. We investigated mechanisms underlying the control of this movement by acetylcholine using an insulinoma cell line, MIN6, in which acetylcholine increases both insulin secretion and granule movement. The peak activation of movement was observed 3 min after an acetylcholine challenge. The effects were nullified by the muscarinic inhibitor atropine, phospholipase C (PLC) inhibitors (D 609 and compound 48/80), and pretreatment with the Ca2+ pump inhibitor, thapsigargin. Inhibitors of Ca2+-dependent phospholipase A2 (arachidonyl trifluoromethyl ketone and methyl arachidonyl fluorophosphate) also partially inhibited the movement caused by acetylcholine, but downregulation of protein kinase C by overnight incubation with the phorbol ester 12-o-tetradecanoylphorbol-13-acetate failed to exert any influence. Acetylcholine stimulation of granule movement was not reproduced by membrane depolarization with high K+. Phosphorylation of the endogenous myosin light chain in MIN6 cells was increased by addition of acetylcholine and decreased by the Ca2+ chelator BAPTA (1,2-bis[2-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid). The calmodulin inhibitor W-7 and the myosin light-chain kinase inhibitor ML-9 decreased the motile events in the beta-cells under both nonstimulated and acetylcholine-stimulated conditions. These findings led us to conclude that inositol trisphosphate [corrected] causes Ca2+ mobilization by muscarinic activation of PLC, leading to intracellular translocation of insulin granules to the ready-releasable pool in pancreatic beta-cells via Ca2+/calmodulin-dependent phosphorylation of myosin light chains.

Topics: Acetylcholine; Animals; Atropine; Calcium; Calcium-Transporting ATPases; Cytoplasmic Granules; Enzyme Inhibitors; Inositol Phosphates; Insulin; Insulin Secretion; Insulinoma; Islets of Langerhans; Kinetics; Mice; Microscopy, Electron; Microscopy, Fluorescence; Muscarinic Antagonists; Okadaic Acid; Pancreatic Neoplasms; Phosphorylation; Rats; Tumor Cells, Cultured; Type C Phospholipases

It has been shown that okadaic acid (OA) diminishes insulin secretion of rat pancreatic islets in response to glucose, glyceraldehyde and KCl. Glucose, glyceraldehyde and KCl cause release of insulin by depolarization and subsequent opening of L-type calcium channels. Calcium entry into cells is thought to be related to protein phosphorylation. To evaluate whether or not OA mediated inhibition of insulin secretion in response to depolarization might be due to an interference with calcium uptake, we studied its effect on KCl (30 mM)-induced increases of cytosolic calcium and discharge of insulin in the insulin secreting clonal tumor cell line RINm5F. OA inhibited KCl-stimulated insulin release in concentrations > or = 1 microM. In intact RINm5F cells similar concentrations of OA decreased the activity of protein phosphates PP-1/PP-2A and inhibited the depolarization-induced rise of cytosolic calcium ([Ca2+]i). The latter action could also be achieved with the protein phosphatase inhibitor calyculin A, whereas the OA analogue 1-nor-okadaone, which is without effect on phosphatases, did not affect [Ca2+]i or insulin release. It is concluded that depression of depolarization-induced insulin secretion by OA is due to inhibition of calcium entry along voltage dependent calcium channels. The data also suggest that in RINm5F cells protein phosphatases PP-1/PP-2A are related to the function of voltage-dependent calcium channels.

Topics: Animals; Calcium; Enzyme Inhibitors; Insulin; Insulin Secretion; Insulinoma; Membrane Potentials; Okadaic Acid; Pancreatic Neoplasms; Phosphoprotein Phosphatases; Phosphorylase Phosphatase; Potassium Chloride; Rats; Tumor Cells, Cultured

Stimulus-induced insulin secretion involves the activation of several protein kinases within the beta cell. Most prominent are protein kinase A, protein kinase C and calcium/calmodulin-dependent protein kinases. Protein kinase action is functionally antagonized by protein phosphatases. The four ubiquious serine/threonine protein phosphatases are termed PP-1, PP-2A, -2B and -2C. PP-1 and PP-2A are in vivo parts of major protein complexes. These complexes presumably regulate the phosphatase activity and direct the enzyme to its site of action. Therefore, PP-1 and -2A could play an important role in controlling intracellular signal transmission. Two different toxins, okadaic acid and calyculin A, both from marine invertebrates, were recently discovered and identified as potent and highly specific inhibitors of PP-1 and PP-2A. Both compounds emerged as very useful tools for studying intracellular phosphorylation events. We took advantage of these substances to investigate the significance of protein phosphatase action in stimulus-induced insulin secretion. To avoid major complexity, we confined our study to the cAMP and the phosphoinositide signal pathway. Okadaic acid alone evoked virtually no secretory response. cAMP-dependent secretion was markedly enhanced by 1 microM okadaic acid. The stimulatory effect of okadaic acid was strongly dependent on the concentration of cAMP analoga. In contrast, insulin release caused by the cholinergic agonist carbachol was not influenced by okadaic acid. Calyculin A (10 nM) slightly increased cAMP-induced secretion, but its high toxicity prohibited accurate interpretation of the data. Our findings support the idea that serine/threonine phosphatases act as important regulators in stimulus response coupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Bucladesine; Carbachol; Colforsin; Cyclic AMP; Ethers, Cyclic; Insulin; Insulinoma; Marine Toxins; Okadaic Acid; Oxazoles; Phosphoprotein Phosphatases; Rats; Tumor Cells, Cultured

This study investigates the occurrence and regulation of serine/threonine protein phosphatases (PPases) in insulin-secreting RINm5F insulinoma cells. PPases types 1 and 2A were identified in crude RINm5F cell homogenates by both enzymatic assay and Western blot analysis. We then characterized and compared the inhibitory actions of several compounds isolated from cyanobacteria, marine dinoflagellates and marine sponges, (viz. okadaic acid, microcystin-LR, calyculin-A and nodularin) cation-independent PPase activities in RINm5F cell homogenates. It was found that okadaic acid was the least potent inhibitor (IC50 approximately 10(-9) M, IC100 approximately 10(-6) M), while the other compounds exhibited IC50 values of approximately 5 x 10(-10) M and IC100 approximately 5 x 10(-9) M. The findings indicate that the inhibitory substances employed in this study may be used pharmacologically to investigate the role of serine/threonine PPases in RINm5F cell insulin secretion, a process that is likely to be regulated to a major extent by protein phosphorylation.

Topics: Animals; Ethers, Cyclic; In Vitro Techniques; Insulinoma; Marine Toxins; Microcystins; Okadaic Acid; Oxazoles; Pancreatic Neoplasms; Peptides, Cyclic; Phosphoprotein Phosphatases; Rabbits; Rats; Tumor Cells, Cultured