Increases in phosphorylation of the myosin II heavy chain, but not regulatory light chains, correlate with insulin secretion in rat pancreatic islets and RINm5F cells

Although cytoskeletal proteins such as myosin II are implicated in the control of insulin secretion, their precise role is poorly understood. In other secretory cells, myosin II is predominantly regulated via the phosphorylation of the regulatory light chains (RLC). The current study was aimed at investigating RLC phosphorylation in beta-cells. In both the insulin-secreting cell line RINm5F and rat pancreatic islets, the RLC was basally phosphorylated on the myosin light chain kinase sites (Ser19/Thr18). Phosphorylation at these sites was not consistently increased by either metabolic stimuli (glyceraldehyde/glucose) or the depolarizing agent KCl. The RLC sites phosphorylated by protein kinase C (PKC) (Ser1/Ser2) were unphosphorylated in the basal state, not affected by nutrients or KCl, and only slightly increased by the PKC activator phorbol 12-myristate 13-acetate (PMA). Like the other insulin secretagogues, however, PMA did promote serine phosphorylation of the myosin heavy chain (MHC) in RINm5F cells. Phosphopeptide mapping suggested that the same peptide was phosphorylated under both PMA and glyceraldehyde stimulation, which further extends our previous study of the Ca2+-dependent phosphorylation of this protein (Wilson JR, Ludowyke RI, Biden TJ: Nutrient stimulation results in a rapid Ca2+-dependent threonine phosphorylation of myosin heavy chain in rat pancreatic islets and RINm5F cells. J Biol Chem 273:22729-22737, 1998). Overall, our results demonstrate that in RINm5F cells and rat pancreatic islets, MHC phosphorylation correlates better with insulin secretion than phosphorylation of the RLC. We therefore propose that in beta-cells, in contrast to other secretory cells, phosphorylation of the MHC is more important than that of the RLC for regulation of the myosin II protein complex during insulin secretion.

Effects of alginate composition on the metabolic, secretory, and growth characteristics of entrapped beta TC3 mouse insulinoma cells

The effects of alginate composition on cell growth as well as the metabolic and secretory profile of transformed beta-cells entrapped in alginate/poly-L-lysine/alginate (APA) solid beads were investigated following entrapment of beta TC3 mouse insulinoma cells in alginate composed of either high mannuronic acid or high guluronic acid residues. Entrapped cultures were maintained in spinner flasks for 40-60 days. The pattern of cell growth and the overall rates of glucose consumption and insulin secretion were investigated. Cultures of beta TC3 cells entrapped in alginate composed predominantly of mannuronic acid units (77%) displayed a linear increase in the rates of glucose consumption and insulin secretion concomitant with an increase in cell population in the periphery of the beads. Conversely, cultures of beta TC3 cells entrapped in alginate composed predominantly of high guluronic acid units (69%) displayed a decrease in the rates of glucose consumption and insulin secretion during the first three weeks of culture, followed by a rapid recovery that surpassed the initial rates by day 40. This biphasic pattern was concomitant to a decrease in viable cells during the first three weeks as ascertained by histology, followed by an increase in cell proliferation. Cell growth in high guluronic acid alginate took place at random locations throughout the solid bead and not in the periphery, as was the case in high mannuronic acid alginate preparations. Possible reasons for these differences and the significance of these findings in the context of a bioartificial pancreas composed of APA entrapped transformed cells are discussed.

Prolonged STAT1 activation is associated with interferon-gamma priming for interleukin-1-induced inducible nitric-oxide synthase expression by islets of Langerhans

In this study, the ability of interferon-gamma (IFN-gamma) to prime rat and nonobese diabetic (NOD) mouse islets for interleukin-1 (IL-1)-stimulated expression of inducible nitric-oxide synthase (iNOS) has been examined. IL-1-induced iNOS expression by rat islets is concentration-dependent with maximal expression occurring in response to 1.0 unit/ml. Individually, neither 0.1 unit/ml IL-1 nor 150 units/ml IFN-gamma stimulates iNOS expression or nitrite production by rat islets. However, a 30-60-min pulse of rat islets with IFN-gamma, followed by washing to remove the cytokine and continued culture with 0.1 unit/ml IL-1 for 40 h, results in iNOS expression and nitrite production to levels similar in magnitude to the individual effects of 1.0 unit/ml IL-1. A 1-h pulse with IFN-gamma primes for IL-1-induced islet degeneration that is mediated by the expression of iNOS and increased production of nitric oxide. IFN-gamma also primes for IL-1-induced iNOS expression and nitrite formation by NOD mouse islets. The priming actions of IFN-gamma appear to be selective for beta-cells, as IFN-gamma primes for IL-1-induced nitrite formation by primary beta-cells and RINm5F insulinoma cells, but not primary alpha-cells. The priming actions of IFN-gamma for IL-1-induced iNOS expression do not require de novo protein synthesis as preincubation of RINm5F cells with cycloheximide does not inhibit iNOS mRNA accumulation under priming conditions. The priming actions of IFN-gamma on IL-1-induced iNOS expression persists for extended periods of up to 7 days and are associated with persistent signal transducers and activators of transcription (STAT)-1 activation. A 30-min pulse of rat islets with IFN-gamma stimulates STAT1 phosphorylation, and STAT1 remains phosphorylated for up to 7 days following IFN-gamma removal. In addition, STAT1 remains nuclear for up to 7 days after IFN-gamma removal. These results indicate that IFN-gamma primes for IL-1-induced islet degeneration via a nitric oxide-dependent mechanism. These findings also provide evidence that the priming actions of IFN-gamma for IL-1-induced iNOS expression by islets are associated with the prolonged phosphorylation and activation of STAT1.

Glucagon-like peptide-1 regulates the beta cell transcription factor, PDX-1, in insulinoma cells

Glucagon-like peptide-1 (GLP-1) enhances insulin biosynthesis and secretion as well as transcription of the insulin, GLUT2 and glucokinase genes. The latter are also regulated by the PDX-1 homeoprotein. We investigated the possibility that GLP-1 may be having its long-term pleiotropic effects through a hitherto unknown regulation of PDX-1. We found that PDX-1 mRNA level was significantly increased (p<0.01) after 2 hours and insulin mRNA level was subsequently increased (p<0.01) after 3 hours of treatment with GLP-1 (10 nM) in RIN 1046-38 insulinoma cells. Under these experimental conditions, there was also a 1.6-fold increase in the expression of PDX-1 protein in whole cell and nuclear extracts. Overexpression of PDX-1 in these cells confirmed the finding of the wild type cells such that GLP-1 induced a 2-fold increase in whole cell extracts and a 3-fold increase in nuclear extracts of PDX-1 protein levels. The results of electrophoretic mobility shift experiments showed that PDX-1 protein binding to the Al element of the rat insulin II promoter was also increased 2 h post treatment with GLP-1. In summary, we have uncovered a previously unknown aspect to the regulation of PDX-1 in beta cells. This has important implications in the physiology of adult beta cells and the treatment of type 2 diabetes mellitus with GLP-1 or its analogs.

Interferon-alpha-2a is a potent inhibitor of hormone secretion by cultured human pituitary adenomas

Interferon-alpha (IFN alpha) may exert direct inhibitory effects on cell proliferation and on the production of different peptide hormones. We investigated the effect of IFN alpha on hormone production by 15 GH-secreting pituitary adenomas, 4 clinically nonfunctioning or gonadotroph pituitary adenomas, and 4 prolactinomas in vitro. In the GH-secreting pituitary adenoma cultures, a short term (72-h) incubation with IFN alpha (50-100 U/mL) significantly inhibited GH secretion in 3 of 7 cases and PRL secretion in 6 of 7 cultures. During prolonged incubation (14 days) with IFN alpha, GH and/or PRL secretion was significantly inhibited in 7 of 8 cultures (GH, 17-78% inhibition; PRL, 39-88% inhibition). In the clinically nonfunctioning or gonadotroph cultures, incubation with IFN alpha resulted in inhibition of the secretion of gonadotropins and/or alpha-subunit in all cases (27-62%), whereas in the prolactinoma cultures PRL secretion was inhibited by IFN alpha in all cases (37-76%). The effect of IFN alpha was additive to the inhibitory effects of the dopamine agonist bromocriptine (10 nmol/L) or the somatostatin analog octreotide (10 nmol/L). The inhibition of hormone secretion by IFN alpha was accompanied by inhibition of the intracellular hormone concentrations. The effect of IFN alpha was dose dependent, with an IC50 for inhibition of hormone secretion of 2.3 +/- 0.3 U/mL (n = 5), which is relatively low compared with the concentrations that are reached in patients treated with IFN alpha for various malignancies. In conclusion, the potent antihormonal effect of IFN alpha on cultured pituitary adenomas suggests that this drug might be of benefit in the treatment of selected patients with secreting pituitary adenomas. As treatment with IFN alpha is associated with considerable adverse reactions, studies with this drug should only be considered in inoperable, invasive aggressive, and dopamine agonist- and/or somatostatin analog-resistant functioning pituitary macroadenomas.

An insulin-like growth factor-mediated, phosphatidylinositol 3' kinase-independent survival signaling pathway in beta tumor cells

Hyperproliferation of tumor cells usually coincides with increased tumor cell apoptosis. To overcome apoptosis, tumor cells frequently induce the expression of growth factors that mediate cell survival. In nontransformed cells, including fibroblasts and neurons, survival factor-mediated signal transduction involves the activation of phosphatidylinositol 3' kinase (PI-3K) and protein kinase B/c-Akt (PKB). Here we demonstrate that tumor cell lines derived from a transgenic mouse model of pancreatic beta cell carcinogenesis use insulin-like growth factors to repress apoptosis independently of PI-3K and PKB. The results indicate that tumor cells can use additional survival signal transduction pathways.

Immunohistochemical detection of alpha1E voltage-gated Ca(2+) channel isoforms in cerebellum, INS-1 cells, and neuroendocrine cells of the digestive system

Polyclonal antibodies were raised against a common and a specific epitope present only in longer alpha1E isoforms of voltage-gated Ca(2+) channels, yielding an "anti-E-com" and an "anti-E-spec" serum, respectively. The specificity of both sera was established by immunocytochemistry and immunoblotting using stably transfected HEK-293 cells or membrane proteins derived from them. Cells from the insulinoma cell line INS-1, tissue sections from cerebellum, and representative regions of gastrointestinal tract were stained immunocytochemically. INS-1 cells expressed an alpha1E splice variant with a longer carboxy terminus, the so-called alpha1Ee isoform. Similarily, in rat cerebellum, which was used as a reference system, the anti-E-spec serum stained somata and dendrites of Purkinje cells. Only faint staining was seen throughout the cerebellar granule cell layer. After prolonged incubation times, neurons of the molecular layer were stained by anti-E-com, suggesting that a shorter alpha1E isoform is expressed at a lower protein density. In human gastrointestinal tract, endocrine cells of the antral mucosa (stomach), small and large intestine, and islets of Langerhans were stained by the anti-E-spec serum. In addition, staining by the anti-E-spec serum was observed in Paneth cells and in the smooth muscle cell layer of the lamina muscularis mucosae. We conclude that the longer alpha1Ee isoform is expressed in neuroendocrine cells of the digestive system and that, in pancreas, alpha1Ee expression is restricted to the neuroendocrine part, the islets of Langerhans. alpha1E therefore appears to be a common voltage-gated Ca(2+) channel linked to neuroendocrine and related systems of the body.

The Gly972-->Arg amino acid polymorphism in IRS-1 impairs insulin secretion in pancreatic beta cells

Recent studies have identified several polymorphisms in the human insulin receptor substrate-1 (IRS-1) gene. The most prevalent IRS-1 variant, a Gly-->Arg change at the codon 972, has been reported to be increased in prevalence among patients with type 2 diabetes. Carriers of the Arg(972) substitution are characterized by lower fasting insulin and C-peptide levels compared with non-carriers, suggesting that the Arg(972) IRS-1 variant may contribute to impairment of insulin secretion. In this study, we stably overexpressed both wild-type IRS-1 (RIN-WT) and Arg(972) IRS-1 variant (RIN-Arg(972)) in RIN beta cells to investigate directly whether the polymorphism in codon 972 of IRS-1 impairs insulin secretion. The Arg(972) IRS-1 variant did not affect expression or function of endogenous IRS-2. RIN-WT showed a marked increase in both glucose- and insulin-stimulated tyrosine phosphorylation of IRS-1 compared with control RIN cells. The Arg(972) IRS-1 variant did not alter the extent of either glucose- or insulin-stimulated tyrosine phosphorylation of recombinant IRS-1. However, RIN-Arg(972) showed a significant decrease in binding of the p85 subunit of phosphatidylinositol-3-kinase (PI 3-kinase) with IRS-1, compared with RIN-WT. Compared with control RIN cells, insulin content was reduced to the same extent in RIN-WT or RIN-Arg(972) at both the protein and mRNA levels. Both glucose- and sulfonylurea-induced insulin secretion was increased in RIN-WT compared with control RIN cells. By contrast, RIN cells expressing Arg(972) IRS-1 exhibited a marked decrease in both glucose- and sulfonylurea-stimulated insulin secretion compared with RIN-WT. These data suggest that the insulin signaling pathway involving the IRS-1/PI 3-kinase may play an important role in the insulin secretory process in pancreatic beta cells. More importantly, the results suggest that the common Arg(972) IRS-1 polymorphism may impair glucose-stimulated insulin secretion, thus contributing to the relative insulin deficiency observed in carriers of this variant.

Tumor necrosis factor-alpha-activated cell death pathways in NIT-1 insulinoma cells and primary pancreatic beta cells

Tumor necrosis factor-alpha (TNFalpha) is a potential mediator of beta cell destruction in insulin-dependent diabetes mellitus. We have studied TNF-responsive pathways leading to apoptosis in beta cells. Primary beta cells express low levels of the type I TNF receptor (TNFR1) but do not express the type 2 receptor (TNFR2). Evidence for TNFR1 expression on beta cells came from flow cytometry using monoclonal antibodies specific for TNFR1 and TNFR2 and from RT-PCR of beta cell RNA. NIT-1 insulinoma cells similarly expressed TNFR1 (at higher levels than primary beta cells) as detected by flow cytometry and radio-binding studies. TNF induced NF-kappaB activation in both primary islet cells and NIT-1 cells. Apoptosis in response to TNFalpha was observed in NIT-1 cells whereas apoptosis of primary beta cells required both TNFalpha and interferon-gamma (IFNgamma). Apoptosis could be prevented in NIT-1 cells by expression of dominant negative Fas-associating protein with death domain (dnFADD). Apoptosis in NIT-1 cells was increased by coincubation with IFNgamma, which also increased caspase 1 expression. These data show that TNF-activated pathways capable of inducing apoptotic cell death are present in beta cells. Caspase activation is the dominant pathway of TNF-induced cell death in NIT-1 cells and may be an important mechanism of beta cell damage in insulin-dependent diabetes mellitus.

Molecular basis for the inactivation of Ca2+- and voltage-dependent BK channels in adrenal chromaffin cells and rat insulinoma tumor cells

Large-conductance Ca2+- and voltage-dependent potassium (BK) channels exhibit functional diversity not explained by known splice variants of the single Slo alpha-subunit. Here we describe an accessory subunit (beta3) with homology to other beta-subunits of BK channels that confers inactivation when it is coexpressed with Slo. Message encoding the beta3 subunit is found in rat insulinoma tumor (RINm5f) cells and adrenal chromaffin cells, both of which express inactivating BK channels. Channels resulting from coexpression of Slo alpha and beta3 subunits exhibit properties characteristic of native inactivating BK channels. Inactivation involves multiple cytosolic, trypsin-sensitive domains. The time constant of inactivation reaches a limiting value approximately 25-30 msec at Ca2+ of 10 microM and positive activation potentials. Unlike Shaker N-terminal inactivation, but like native inactivating BK channels, a cytosolic channel blocker does not compete with the native inactivation process. Finally, the beta3 subunit confers a reduced sensitivity to charybdotoxin, as seen with native inactivating BK channels. Inactivation arises from the N terminal of the beta3 subunit. Removal of the beta3 N terminal (33 amino acids) abolishes inactivation, whereas the addition of the beta3 N terminal onto the beta1 subunit confers inactivation. The beta3 subunit shares with the beta1 subunit an ability to shift the range of voltages over which channels are activated at a given Ca2+. Thus, the beta-subunit family of BK channels regulates a number of critical aspects of BK channel phenotype, including inactivation and apparent Ca2+ sensitivity.

[Development of insulinoma cells as therapy in insulin-dependent diabetes mellitus]

In healthy persons insulin secreting beta-cells of the pancreas regulate blood glucose levels within a narrow physiological range. Since the detection of insulin in 1922 by Banting and Best, subcutaneous insulin replacement has remained the sole treatment modality for insulin-dependent diabetes mellitus (IDDM). However, even trained patients undergoing intensive insulin therapy fail to achieve normal function of the pancreatic beta-cells. One approach to solve this problem is pancreas and islet cell transplantation. Because of technical problems, limited number of transplantable organs and toxicity of immunosuppressive therapy, both are still in an experimental state. An alternative approach is the development of genetically modified insulin secreting cell lines for replacement of islet cells. So far, experiments support the expectation to develop genetically manipulated cell lines who imitate the function of islet beta-cells and are protected from the immune response. The ultimate goal is the development of an engineered human beta-cell line and, after animal experiments, to use it for treatment of patients with IDDM.

Overexpression of Fas ligand does not confer immune privilege to a pancreatic beta tumor cell line (betaTC-3)

BACKGROUND

Fas ligand (Fas-L) is thought to provide immune privilege to specific tissues and tumors by inducing an apoptotic signal of cytotoxic T cells expressing its Fas receptor. Purpose. The purpose of this work was to evaluate whether an immortalized insulin-secreting cell line (betaTC-3) gains immune privilege by inducing overexpression of Fas-L.

METHODS

A lipofection technique was used to transfect a betaTC-3 tumor cell line with a plasmid (pcDNA3.1/Zeo) carrying the Fas-L gene and a zeocin resistance gene. Insertion of Fas-L into betaTC was characterized by reverse transcription polymerase chain reaction (RT-PCR) and the ability of transfectants (betaTC-3/Fas-L) to induce apoptosis of Fas-sensitive T cells. Transfectants and control cells were tested for insulin secretion following which 1 x 10(6) insulin-secreting betaTC-3 and betaTC-3/Fas-L cells were subcutaneously implanted into syngeneic, allogeneic, and Fas mutant (lpr) syngeneic mice. Survival of the insulin-secreting cells was then determined by monitoring serum glucose levels in recipients.

RESULTS

Successful transfection of vector resistance gene was achieved in the transfected betaTC-3 cells, which was confirmed by zeocin resistance. RT-PCR in resistant Fas-L clones confirmed the transcription of Fas-L, which was absent in controls. Fas-L transfectants induced 20 +/- 4.2% apoptosis of Fas-sensitive T cells, while controls induced 3.47 +/- 2.3% by flow cytometry (P = 0.04, n = 3). Insulin secretion was equivalent in both betaTC-3 and betaTC-3/Fas-L cells. Syngeneic mice implanted with control betaTC-3 cells died within 3 weeks from hypoglycemia due to overgrowth of betaTC-3 tumor. Implanted Fas-L transfected betaTC-3 cells were killed and had no effect on glycemic status except in Fas mutant hosts, where tumors formed in two of three mice.

CONCLUSIONS

Despite the ability of transfected betaTC-3 cells to induce apoptosis of T cells in vitro, expression of Fas-L provided no immune privilege to these cells in vivo, but paradoxically induced killing of betaTC-3 cells even in syngeneic hosts.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

Overexpression of G11alpha and isoforms of phospholipase C in islet beta-cells reveals a lack of correlation between inositol phosphate accumulation and insulin secretion

It has been suggested that insulin secretion from pancreatic islets may be mediated in part by activation of phospholipases C (PLCs) and phosphoinositide hydrolysis. The purpose of this study was to determine whether the relatively modest fuel-stimulated insulin secretion responses of rodent beta-cell lines might be explained by inadequate expression or activation of PLC isoforms. We have found that two insulinoma cell lines, INS-1 and betaG 40/110, completely lack PLC-delta1 expression but have levels of expression of PLC-beta1, -beta2, -beta3, -delta2, and -gamma1 that are similar to or slightly reduced from those found in fresh rat islets. Adenovirus-mediated overexpression of PLC-delta1, -beta1, or -beta3 in INS-1 or betaG 40/110 cells results in little or no enhancement in inositol phosphate (IP) accumulation and no improvement in insulin secretion when the cells are stimulated with glucose or carbachol, despite the fact that the overexpressed proteins are fully active in cell extracts. Overexpression of PLC-beta1 or -beta3 in normal rat islets elicits a larger increase in IP accumulation but, again, has no effect on insulin secretion. Because the effect of carbachol on insulin secretion is thought to be mediated through muscarinic receptors that link to the Gq/11 class of heterotrimeric G proteins, we also overexpressed G11alpha in INS-1 cells, either alone or in concert with overexpression of PLC-beta1 or -beta3. Overexpression of G11alpha enhances IP accumulation, an effect slightly potentiated by co-overexpression of PLC-beta1 or -beta3, but these maneuvers do not affect glucose or carbachol-stimulated insulin secretion. In sum, our studies show a lack of correlation between IP accumulation and insulin secretion in INS-1 cells, betaG 40/110 cells, or cultured rat islets. We conclude that overexpression of PLC isoforms and/or G11alpha is not an effective means of enhancing fuel responsiveness in the insulinoma cell lines studied.

Beta-cell gene expression and functional characterisation of the human insulinoma cell line CM

Animal insulinoma cell lines are widely used to study physiological and pathophysiological mechanisms involved in glucose metabolism and to establish in vitro models for studies on beta-cells. In contrast, human insulinoma cell lines are rarely used because of difficulties in obtaining and culturing them for long periods. The aim of our study was to investigate, under different experimental conditions, the capacity of the human insulinoma cell line CM to retain beta-cell function, particularly the expression of constitutive beta-cell genes (insulin, the glucose transporters GLUT1 and GLUT2, glucokinase), intracellular and secreted insulin, beta-cell granules, and cAMP content. Results showed that CM cells from an early-passage express specific beta-cell genes in response to glucose stimulation, in particular the insulin and GLUT genes. Such capacity is lost at later passages when cells are cultured at standard glucose concentrations. However, if cultured at lower glucose concentration (0.8 mM) for a longer time, CM cells re-acquire the capacity to respond to glucose stimulation, as shown by the increased expression of beta-cell genes (insulin, GLUT2, glucokinase). Nonetheless, insulin secretion could not be restored under such experimental conditions despite the presence of intracellular insulin, although cAMP response to a potent activator of adenylate cyclase, forskolin, was present indicating a viable system. In conclusion, these data show that the human insulinoma cell line CM, at both early-passage and late-passage, posseses a functional glucose-signalling pathway and insulin mRNA expression similar to normal beta-cells, representing, therefore, a good model for studies concerning the signalling and expression of beta-cells. Furthermore, we have previously shown that it is also a good model for immunological studies. In this respect it is important to note that the CM cell line is one of the very few existing human beta-cell lines in long-term culture.

Reduced expression of neural cell adhesion molecule induces metastatic dissemination of pancreatic beta tumor cells

As in the development of many human cancers, in a transgenic mouse model of beta-cell carcinogenesis (Rip1Tag2), expression of neural cell adhesion molecule (NCAM) changes from the 120-kDa isoform in normal tissue to the 140/180-kDa isoforms in tumors. NCAM-deficient RiplTag2 mice, generated by crossing Rip1Tag2 mice with NCAM knockout mice, develop metastases, a tumor stage that is not seen in normal Rip1Tag2 mice. In contrast, overexpression of NCAM 120 in NCAM-deficient Rip1Tag2 mice prevents tumor metastasis. The results indicate that the loss of NCAM-mediated cell adhesion is one rate-limiting step in the actual metastatic dissemination of beta tumor cells.

Molecular cloning of a pancreatic islet-specific glucose-6-phosphatase catalytic subunit-related protein

A pancreatic islet-specific glucose-6-phosphatase-related protein (IGRP) was cloned using a subtractive cDNA expression cloning procedure from mouse insulinoma tissue. Two alternatively spliced variants that differed by the presence or absence of a 118-bp exon (exon IV) were detected in normal balb/c mice, diabetic ob/ob mice, and insulinoma tissue. The longer, 1901-bp full-length cDNA encoded a 355-amino acid protein (molecular weight 40,684) structurally related (50% overall identity) to the liver glucose-6-phosphatase and exhibited similar predicted transmembrane topology, conservation of catalytically important residues, and the presence of an endoplasmic reticulum retention signal. The shorter transcript encoded two possible open reading frames (ORFs), neither of which possessed His174, a residue thought to be the phosphoryl acceptor (Pan CJ, Lei KJ, Annabi B, Hemrika W, Chou JY: Transmembrane topology of glucose-6-phosphatase. J Biol Chem 273:6144-6148, 1998). Northern blot and reverse transcription-polymerase chain reaction analysis showed that the mRNA was highly expressed in pancreatic islets and expressed more in beta-cell lines than in an alpha-cell line. It was notably absent in tissues and cell lines of non-islet neuroendocrine origin, and no other major tissue source of the mRNA was found. During development, it was expressed in parallel with insulin mRNA. The mRNA was efficiently translated and glycosylated in an in vitro translation/membrane translocation system and readily transcribed into COS 1, HIT, and CHO cells using cytomegalovirus or Rous sarcoma virus promoters. Whereas the liver glucose-6-phosphatase showed activity in these transfection systems, the IGRP failed to show glucose phosphotransferase or phosphatase activity with p-nitrophenol phosphate, inorganic pyrophosphate, or a range of sugar phosphates hydrolyzed by the liver enzyme. While the metabolic function of the enzyme is not resolved, its remarkable tissue-specific expression warrants further investigation, as does its transcriptional regulation in conditions where glucose responsiveness of the pancreatic islet is altered.

RINm5f cells express inactivating BK channels whereas HIT cells express noninactivating BK channels

Large-conductance Ca2+- and voltage-activated BK-type K+ channels are expressed abundantly in normal rat pancreatic islet cells and in the clonal rat insulinoma tumor (RINm5f) and hamster insulinoma tumor (HIT) beta cell lines. Previous work has suggested that the Ca2+ sensitivity of BK channels in RIN cells is substantially less than that in HIT cells, perhaps contributing to differences between the cell lines in responsiveness to glucose in mediating insulin secretion. In both RIN cells and normal pancreatic beta cells, BK channels are thought to play a limited role in responses of beta cells to secretagogues and in the electrical activity of beta cells. Here we examine in detail the properties of BK channels in RIN and HIT cells using inside-out patches and whole cell recordings. BK channels in RIN cells exhibit rapid inactivation that results in an anomalous steady-state Ca2+ dependence of activation. In contrast, BK channels in HIT cells exhibit the more usual noninactivating behavior. When BK inactivation is taken into account, the Ca2+ and voltage dependence of activation of BK channels in RIN and HIT cells is essentially indistinguishable. The properties of BK channel inactivation in RIN cells are similar to those of inactivating BK channels (termed BKi channels) previously identified in rat chromaffin cells. Inactivation involves multiple, trypsin-sensitive cytosolic domains and exhibits a dependence on Ca2+ and voltage that appears to arise from coupling to channel activation. In addition, the rates of inactivation onset and recovery are similar to that of BKi channels in chromaffin cells. The charybdotoxin (CTX) sensitivity of BKi currents is somewhat less than that of the noninactivating BK variant. Action potential voltage-clamp waveforms indicate that BK current is activated only weakly by Ca2+ influx in RIN cells but more strongly activated in HIT cells even when Ca2+ current magnitude is comparable. Concentrations of CTX sufficient to block BKi current in RIN cells have no effect on action potential activity initiated by glucose or DC injection. Despite its abundant expression in RIN cells, BKi current appears to play little role in action potential activity initiated by glucose or DC injection in RIN cells, but BK current may play an important role in action potential repolarization in HIT cells.

Cloning from insulinoma cells of synapsin I associated with insulin secretory granules

Synapsin I is a synaptic vesicle-associated protein involved in neurotransmitter release. The functions of this protein are apparently regulated by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). We reported evidence for CaM kinase II and a synapsin I-like protein present in mouse insulinoma MIN6 cells (Matsumoto, K., Fukunaga, K., Miyazaki, J., Shichiri, M., and Miyamoto, E. (1995) Endocrinology 136, 3784-3793). Phosphorylation of the synapsin I-like protein in these cells correlated with the activation of CaM kinase II and insulin secretion. In the present study, we screened the MIN6 cDNA library with the full-length cDNA probe of rat brain synapsin Ia and obtained seven positive clones; the largest one was then sequenced. The largest open reading frame deduced from the cDNA sequence of 3695 base pairs encoded a polypeptide of 670 amino acids, which exhibited significant sequence similarity to rat synapsin Ib. The cDNA contained the same sequence as the first exon of the mouse synapsin I gene. These results indicate that synapsin Ib is present in MIN6 cells. Synapsin I was expressed in normal rat islets, as determined by reverse transcriptase-polymerase chain reaction analysis. Immunoblot analysis after subcellular fractionation of MIN6 cells demonstrated that synapsin Ib and delta subunit of CaM kinase II co-localized with insulin secretory granules. By analogy concerning regulation of neurotransmitter release, our results suggest that phosphorylation of synapsin I by CaM kinase II may induce the release of insulin from islet cells.

Enhanced phosphoinositide hydrolysis via overexpression of phospholipase C beta1 or delta1 inhibits stimulus-induced insulin release in insulinoma MIN6 cells

To study the effects of enhanced phosphoinositide hydrolysis on insulin secretion, phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) or PLCdelta1 was overexpressed in insulinoma MIN6 cells via adenoviral vectors. Inositol phosphate production stimulated by NaF (with AlCl3) in PLCbeta1-overexpressing cells and that stimulated by KCl or glucose in both PLCbeta1- and PLCdelta1-overexpressing cells were greater than that in control cells. In addition, reduced phosphatidylinositol-4,5-bisphosphate levels were observed in these cells stimulated by NaF or KCl. The greater phosphoinositide hydrolysis was accompanied by 25-45% inhibition of insulin secretion. These data suggest that excessive phosphoinositide hydrolysis inhibits secretagogue-induced insulin release in MIN6 cells.

Islet amyloid polypeptide/amylin messenger RNA and protein expression in human insulinomas in relation to amyloid formation

OBJECTIVE

Islet amyloid polypeptide (IAPP), also named amylin, is the predominant protein component of amyloid deposits in human islet beta cell tumours of the pancreas (insulinomas). IAPP is co-produced with insulin by islet beta cells. We investigated IAPP expression in relation to insulin expression and to amyloid formation in eleven insulinomas.

DESIGN AND METHODS

RNA and protein extracts were prepared from the same pieces of tumour tissue, and from specimens of two normal human pancreata. IAPP and insulin mRNA and peptide content were quantified using Northern blot analysis and radioimmunoassay (RIA) respectively. Molecular forms of IAPP immunoreactivity were analysed by reversed-phase high-performance liquid chromatography (HPLC). The presence of islet hormones and of amyloid was assessed by (immuno)histochemical staining of paraffin sections. Plasma levels of IAPP and insulin prior to tumour resection were determined by RIA.

RESULTS

IAPP and insulin mRNA and peptide content varied widely between the tumour specimens, and there was considerable intratumour heterogeneity of peptide content. HPLC analysis indicated correct proteolytic processing of the IAPP precursor protein. Amyloid deposits were detected only in the three tumours with the highest IAPP content. In contrast to insulin, plasma levels of IAPP were not elevated in the insulinoma patients.

CONCLUSIONS

The spectrum of hormone production by insulinomas cannot be inferred from only a few tissue sections due to intratumour heterogeneity. Expression of the IAPP and insulin genes is not coupled in insulinomas, which produce properly processed mature IAPP. In addition to IAPP overproduction, additional factors such as intracellular accumulation of IAPP are involved in amyloidogenesis in insulinomas.

Ca2+ channel inhibition induced by nitric oxide in rat insulinoma RINm5F cells

The effect of nitric oxide (NO) donors on high-voltage-activated Ca2+ channels in insulin-secreting RINm5F cells was investigated using the patch-clamp technique in the whole-cell configuration. Sodium nitroprusside (SNP, 2-400 microM) induced a dose-dependent reduction in Ba2+ currents with maximal inhibition of 58%. The IC50 for SNP was 45 microM. A different NO donor, (+/-)S-nitroso-N-acetylpenicillamine (SNAP, 500 microM), also produced a 50% decrease in current amplitude. When 200 microM SNP was administered together with the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidozoline-1-oxyl-3-oxide (carboxy-PTIO, 300 microM), the Ba2+ current inhibition was lowered to 7%. Administration of 500 microM 8-bromoguanosine 3':5'-cyclic monophosphate sodium salt (8-Br-cGMP) mimicked the effects of SNP, causing a comparable decrease (56%) in peak-current amplitude. When soluble guanylyl cyclase was blocked by 10 microM 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), the inhibitory effect of 200 microM SNP was reduced from 39% to 15%. The SNP-induced current decrease was 36% of controls after the blockade of L-type Ca2+ channels and 30% in the presence of 2.5 microM omega-conotoxin-MVIIC. These data indicate that NO inhibits both L-type and P/Q-type Ca2+ channels in RINm5F cells, probably by an increase in the intracellular levels of cGMP. NO may then significantly influence the Ca2+-dependent release of hormones from secretory cells as well as that of neurotransmitters from nerve terminals.

Evaluation of immunoisolated insulin-secreting beta TC6-F7 cells as a bioartificial pancreas

OBJECTIVE

To evaluate the growth and insulin secretion from microencapsulated beta TC6-F7 cells in vitro and to assess the in vivo function of microencapsulated cells transplanted in rats with steptozotocin (STZ)-induced diabetes.

METHOD

Alginate-poly-L-lysine encapsulated beta TC6-F7 cells were exposed to glucose, isobutylmethylxanthine (IBMX) and glucagon-like peptide I (7-36 amide) in a static in vitro challenge. In vivo, 2.5-3.5 x 10(7) encapsulated cells were implanted into diabetic rats. Graft function was evaluated by monitoring blood glucose concentrations and by an intraperitoneal glucose tolerance test.

RESULTS

The cell density (number of cells per capsule) of cultured microencapsulated beta TC6-F7 cells increased almost 35-fold over a 55 day observation period to reach a plateau of approximately 3500 cells/capsule. While insulin secretion per capsule remained unchanged over the first 21 days of culture, a 7-fold increase was observed during the last 14 days of the 55 day observation period. Intraperitoneal transplantation of 3.5 x 10(7) encapsulated cells into diabetic rats resulted, within 24 hours, in reversal of hyperglycemia for up to 60 days. Post-transplantation blood glucose concentrations varied between 2 and 4 mM. Glucose clearance rates evaluated by an intraperitoneal glucose tolerance test at 30 days post-transplantation resulted in a markedly flat glucose clearance curve with blood glucose never rising above 4 mM. The glucose challenge of microencapsulated cells recovered 30 days post-transplantation resulted in a 2-fold increase in insulin response at glucose concentrations greater than 5.5 mM as compared to glucose-free media. In addition, immunostaining of recovered grafted tissue for insulin, reveals a strong presence of the peptide within the cell population.

CONCLUSIONS

These data demonstrate the potential use of an immunoisolated beta-cell line for the treatment of diabetes.