The amphicrine pancreatic cell line, AR42J, secretes GABA and amylase by separate regulated pathways.

Treatment of AR42J cells with dexamethasone leads to an enhanced formation of amylase-containing granules and facilitates their regulated secretion. Besides the exocrine properties, AR42J cells possess a specific uptake system for [3H]GABA. The stored GABA can be released upon potassium depolarisation in a Ca(2+)-dependent manner. After treatment with dexamethasone, potassium depolarisation fails to release GABA, but instead causes a Ca(2+)-dependent secretion of amylase. Since vesicles similar to small synaptic vesicles of neurons have been identified in AR42J cells, we suggest that the regulated GABA release is mediated by this vesicle type. It is tentatively speculated that other epithelial cells, which also contain small synaptic vesicles and amino acid neurotransmitters, may release them in a similar fashion.     

Antibodies directed against the peroxisomal targeting signal of firefly luciferase recognize multiple mammalian peroxisomal proteins

We have previously shown that the peroxisomal targeting signal in firefly luciferase consists of the COOH-terminal three amino acids of the protein, serine-lysine-leucine (Gould, S.J., G.A. Keller, N. Hosken, J. Wilkinson, and S. Subramani, 1989. J. Cell Biol. 108:1657-1664). Antibodies were raised against a synthetic peptide that contained this tripeptide at its COOH terminus. Immunofluorescence and immunocryoelectron microscopy revealed that the anti-peptide antibodies specifically detected peroxisomes in mammalian cells. Further characterization revealed that the antibodies were primarily directed against the COOH-terminal three amino acids of the peptide. In Western blot experiments, the antibodies recognized 15-20 rat liver peroxisomal proteins, but reacted with only a few proteins from other subcellular compartments. These results provide independent immunological evidence that the peroxisomal targeting signal identified in firefly luciferase is present in many peroxisomal proteins.     

NADPH oxidase and apoptosis in cerulein-stimulated pancreatic acinar AR42J cells

Apoptosis linked to oxidative stress has been implicated in pancreatitis. We investigated whether NADPH oxidase mediates apoptosis in cerulein-stimulated pancreatic acinar AR42J cells. We report here that cerulein treatment resulted in the activation of NADPH oxidase, as determined by ROS production, translocation of cytosolic subunits p 47(phox) and p 67(phox) to the membrane, and interaction between NADPH oxidase subunits. Cerulein induced Ca(2+) oscillation, the expression of apoptotic genes p53 and bax, and apoptotic indices (DNA fragmentation, TUNEL staining, caspase 3 activity, decrease in cell viability) in AR42J cells. Treatment with a Ca(2+) chelator, BAPTA-AM, or transfection with antisense oligonucleotides for NADPH oxidase subunits p22(phox) and p 47(phox) inhibited cerulein-induced ROS production, translocation of NADPH oxidase cytosolic subunits p 47(phox) and p 67(phox) to the membrane, and the expression of apoptotic genes and apoptotic indices, as compared to the cells without treatment and those transfected with the corresponding sense oligonucleotides. These results indicate that NADPH oxidase may mediate ROS-induced apoptosis in pancreatic acinar cells in a Ca(2+)-dependent manner.     

Roles of CTPL/Sfxn3 and Sfxn family members in pancreatic islet

Pancreatic AR42J cells have the feature of pluripotency of the precursor cells of the gut endoderm. Betacellulin (BTC) and activin A (Act) convert them into insulin-secreting cells. Using mRNA differential display techniques, we have identified a novel mitochondrial transporter, which is highly expressed during the course of differentiation, and have designated it citrate transporter protein-like protein (CTPL). Recently sideroflexin 1 (Sfxn1) was shown to be a susceptible gene of flexed-tail (f/f) mice, and CTPL has turned out to be a rat orthologous protein of Sfxn3, a member of sideroflexin family. CTPL/Sfxn3 was targeted to mitochondrial membrane like Sfxn1. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were upregulated in the early phase of differentiation into insulin-secreting cells but the expression levels of Sfxn1 and Sfxn3 did not change. All Sfxn family members were expressed in rat pancreatic islet. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were also upregulated in islets of streptozotocin-induced diabetic rats compared to normal rats. The downregulation of CTPL/Sfxn3 in a rat insulinoma cell line, INS-1, with the antisense oligonucleotide did not affect the insulin secretion. Taken together, CTPL/Sfxn3 and some other family members might be important in the differentiation of pancreatic beta-cells as a channel or a carrier molecule and be related to the regeneration of pancreatic endocrine cells.     

Ca2+-dependent hydrophobic-interaction chromatography. Isolation of a novel Ca2+-binding protein and protein kinase C from bovine brain.

Several bovine brain proteins have been found to interact with a hydrophobic chromatography resin (phenyl-Sepharose CL-4B) in a Ca2+-dependent manner. These include calmodulin, the Ca2+/phospholipid-dependent protein kinase (protein kinase C) and a novel Ca2+-binding protein that has now been purified to electrophoretic homogeneity. This latter protein is acidic (pI 5.1) and, like calmodulin and some other high-affinity Ca2+-binding proteins, exhibits a Ca2+-dependent mobility shift on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, with an apparent Mr of 22 000 in the absence of Ca2+ and Mr 21 000 in the presence of Ca2+. This novel calciprotein is distinct from known Ca2+-binding proteins on the basis of Mr under denaturing conditions, Cleveland peptide mapping and amino acid composition analysis. It may be a member of the calmodulin superfamily of Ca2+-binding proteins. This calciprotein does not activate two calmodulin-dependent enzymes, namely cyclic nucleotide phosphodiesterase and myosin light-chain kinase, nor does it have any effect on protein kinase C. It may be a Ca2+-dependent regulatory protein of an as-yet-undefined enzymic activity. The Ca2+/phospholipid-dependent protein kinase is also readily purified by Ca2+-dependent hydrophobic-interaction chromatography followed by ion-exchange chromatography, during which it is easily separated from calmodulin. A preparation of protein kinase C that lacks contaminating kinase or phosphatase activities is thereby obtained rapidly and simply. Such a preparation is ideal for the study of phosphorylation reactions catalysed in vitro by protein kinase C.     

Cross-talk between bone morphogenetic protein and transforming growth factor-beta signaling is essential for exendin-4-induced insulin-positive differentiation of AR42J cells

A key goal of cellular engineering is to manipulate progenitor cells to become beta-cells, allowing cell replacement therapy to cure diabetes mellitus. As a paradigm for cell engineering, we have studied the molecular mechanisms by which AR42J cells become beta-cells. Bone morphogenetic proteins (BMPs), implicated in a myriad of developmental pathways, have not been well studied in insulin-positive differentiation. We found that the canonical intracellular mediators of BMP signaling, Smad-1 and Smad-8, were significantly elevated in AR42J cells undergoing insulin-positive differentiation in response to exendin-4 treatment, suggesting a role for BMP signaling in beta-cell formation. Similarly, endogenous BMP-2 ligand and ALK-1 receptor (activin receptor-like kinase-1; known to activate Smads 1 and 8) mRNAs were specifically up-regulated in exendin-4-treated AR42J cells. Surprisingly, Smad-1 and Smad-8 levels were suppressed by the addition of BMP-soluble receptor inhibition of BMP ligand binding to its receptor. Here, insulin-positive differentiation was also ablated. BMP-2 ligand antisense also strongly inhibited Smad-1 and Smad-8 expression, again with the abolition of insulin-positive differentiation. These results demonstrate a previously unrecognized key role for BMP signaling in mediating insulin-positive differentiation through the intracellular Smad signaling pathway. In short, BMP signaling may represent a novel downstream target of exendin-4 (glucagon-like peptide 1) signaling and potentially serve as an upstream regulator of transforming growth factor-beta isoform signaling to differentiate the acinar-like AR42J cells into insulin-secreting cells.     

p66SHC promotes T cell apoptosis by inducing mitochondrial dysfunction and impaired Ca2+ homeostasis

p66Shc, a redox enzyme that enhances reactive oxygen species (ROS) production by mitochondria, promotes T cell apoptosis. We have addressed the mechanisms regulating p66Shc-dependent apoptosis in T cells exposed to supraphysiological increases in [Ca2+]c. p66Shc expression resulted in profound mitochondrial dysfunction in response to the Ca2+ ionophore A23187, as revealed by dissipation of mitochondrial transmembrane potential, cytochrome c release and decreased ATP levels. p66Shc expression also caused a dramatic alteration in the cells' Ca2+-handling ability, which resulted in Ca2+ overload after A23187 treatment. The impairment in Ca2+ homeostasis was ROS dependent and caused by defective Ca2+ extrusion due at least in part to decreased plasma membrane ATPase (PMCA) expression. Both effects of p66Shc required Ca2+-dependent serine-36 phosphorylation. The mitochondrial effects of p66Shc were potentiated by but not strictly dependent on the rise in [Ca2+]c. Thus, Ca2+-dependent p66Shc phosphorylation causes both mitochondrial dysfunction and impaired Ca2+ homeostasis, which synergize in promoting T cell apoptosis.     

Identification of a novel human subfamily of mitochondrial carriers with calcium-binding domains

Aralar1 and citrin were identified as calcium binding aspartate/glutamate carriers (AGC) in mitochondria. The presence of calcium binding motifs facing the extramitochondrial space allows the regulation of the transport activity of these carriers by cytosolic calcium and provides a new mechanism to transduce calcium signals in mitochondria without the requirement of calcium entry in the organelle. We now report the complete characterization of a second subfamily of human calcium binding mitochondrial carriers named SCaMC (short calcium-binding mitochondrial carriers). We have identified three SCaMC genes in the human genome. All code for highly conserved proteins (about 70-80% identity), of about 500 amino acids with a characteristic mitochondrial carrier domain at the C terminus, and an N-terminal extension harboring four EF-hand binding motifs with high similarity to calmodulin. All SCaMC proteins were found to be located exclusively in mitochondria, and their N-terminal extensions were dispensable for the correct mitochondrial targeting of the polypeptides. SCaMC-1 is the human orthologue of the rabbit Efinal protein, which was reported to be located in peroxisomes, and SCaMC-2 is the human orthologue of the rat MCSC protein, described as up-regulated by dexamethasone in AR42J cells. One of the SCaMC genes, SCaMC-2, has four variants generated by alternative splicing, resulting in proteins with a common C terminus but with variations in their N-terminal halves, including the loss of one to three EF-hand motifs. These results make SCaMC one of most complex subfamilies of mitochondrial carriers and suggest that the large number of isoforms and splice variants may confer different calcium sensitivity to the transport activity of these carriers.     

Snare protein expression and adenoviral transfection of amphicrine AR42J.

The amphicrine AR42J acinar cell line is an excellent model to study both exocrine and neuroendocrine exocytotic mechanisms. As a first step toward this goal, we determined the specific isoforms of the v- and t-SNARE and Munc18 families expressed in these cells. In addition, we show that dexamethasone-induced differentiation toward the exocrine phenotype causes an upregulation of several of these proteins. AR42J is notoriously difficult to transfect, limiting its usefulness as a model. However, we have now overcome this obstacle by acheiving high efficiency expression of a beta-galactosidase reporter gene and truncated SNAP-25 gene using adenoviral infection techniques. The AR42J cells can now be used to pursue and elucidate the distinct functions of individual SNARE isoforms used in endocrine and exocrine secretion within a single cell line.     

Modulating zymogen granule formation in pancreatic AR42J cells

Zymogen granules (ZG) are specialized organelles in the exocrine pancreas which allow digestive enzyme storage and regulated secretion. To investigate ZG biogenesis, cargo sorting and packaging, suitable cellular model systems are required. Here, we demonstrate that granule formation in pancreatic AR42J cells, an acinar model system, can be modulated by altering the growth conditions in cell culture. We find that cultivation of AR42J cells in Panserin^? 401, a serum-free medium, enhances the induction of granule formation in the presence or absence of dexamethasone when compared to standard conditions including serum. Biochemical and morphological studies revealed an increase in ZG markers on the mRNA and protein level, as well as in granule size compared to standard conditions. Our data indicate that this effect is related to pronounced differentiation of AR42J cells. To address if enhanced expression of ZG proteins promotes granule formation, we expressed several zymogens and ZG membrane proteins in unstimulated AR42J cells and in constitutively secreting COS-7 cells. Neither single expression nor co-expression was sufficient to initiate granule formation in AR42J cells or the formation of granule-like structures in COS-7 cells as described for neuroendocrine cargo proteins. The importance of our findings for granule formation in exocrine cells is discussed.     

Inhibition of inositol 1,4,5-trisphosphate-mediated Ca2+ release by Ca2+ in cells from peripheral tissues

Permeabilized cells attached to culture plates were used to evaluate the inhibition of inositol 1,4,5-trisphosphate-mediated release (IPMCR) by Ca2+. In AR42J cells, a pancreatic acinar cell line, when permeabilization and Ca2+ uptake were carried out at low ionized Ca2+ (0.06 microM), Ca2+ had little effect on IPMCR. On the other hand, when permeabilization and Ca2+ uptake were performed at 5 microM Ca2+, IPMCR was inhibited by Ca2+ with an apparent affinity of 0.24 microM. This inhibition could be modified by exposing the cytosol of permeabilized cells to low Ca2+. Hence, permeabilizing the cells in the presence of 5 microM Ca2+ and then exposing them to Ca2+ concentrations between 0.01 and 5 microM before washing and Ca2+ uptake in the presence of 5 microM Ca2+ resulted in a Ca2(+)-dependent loss of inhibitory activity. The loss of inhibitory activity occurred with an apparent affinity for Ca2+ of 0.21 microM. A similar phenomenon with a comparable apparent dissociation constant for Ca2+ was found with three other cell types from peripheral tissues: the osteosarcoma cell line UMR-106-01, the kidney inner medullary cell line IMCD, and primary culture of urinary bladder smooth muscle cells. The properties of inhibition of IPMCR by Ca2+ in cells from peripheral tissues differ from those previously described in neuronal tissues and suggest that a different factor(s) mediates the inhibition of IPMCR by Ca2+ in cells from peripheral and neuronal tissues.     

Amino acid sequence of the Ca2(+)-triggered luciferin binding protein of Renilla reniformis

The complete amino acid sequence of the Ca2(+)-triggered luciferin binding protein (LBP) of Renilla reniformis has been determined. The apoprotein has an unblocked amino terminus and contains 184 residues with a calculated Mr of 20,541. LBP is a member of the EF-hand superfamily of Ca2(+)-binding proteins and bears three predicted EF-hand domains. The sequence and organization of EF-hand domains are similar to those of the Ca2(+)-dependent photoprotein, aequorin.     

Ghrelin system in pancreatic AR42J cells: its ligand stimulation evokes calcium signalling through ghrelin receptors

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor. The presence of ghrelin in pancreatic islet cells has been previously reported and it is known to increase the [Ca2+]i in (-cells, affecting insulin secretion. However, evidence for the existence of the ghrelin system and its calcium signalling pathway in the exocrine pancreas remains unclear. Thus this study aims, first, to investigate the expression of ghrelin and its receptor in pancreatic AR42J cells and, secondly, to elucidate its calcium signalling pathway. Our results showed that ghrelin and ghrelin receptor were consistently expressed in AR42J cells. Moreover, fluorescence imaging showed that cholecystokinin-8, ghrelin and growth hormone-releasing hexapeptide stimulate [Ca2+]i in AR42J cells in a dose-dependent manner. Ghrelin and the hexapeptide produced a biphasic elevation in [Ca2+]i with an initial transient increase, followed by a sustained plateau. In the presence of (D-Lys3)-GHRP-6, the [Ca2+]i evoked by ghrelin was suppressed. In the absence of extracellular Ca2+, the transient phase of the ghrelin response was maintained but greatly diminished while the plateau phase was completely abolished. Pretreatment with 2-aminoethoxydiphenyl borate and xestospongin C abolished the transient phase and inhibited the sustained phase of the ghrelin response. The stimulatory effect of ghrelin was also blocked by nifedipine. These results indicate that ligand stimulation of the ghrelin receptor could lead to a biphasic [Ca2+]i mobilization in these cells. These data suggests the presence of a ghrelin system in pancreatic AR42J cells. In addition, its roles in exocrine function are implicated in the pancreas.     

A betacellulin mutant promotes differentiation of pancreatic acinar AR42J cells into insulin-producing cells with low affinity of binding to ErbB1

Betacellulin (BTC) is one of the members of the epidermal growth factor (EGF) ligand family of ErbB receptor tyrosine kinases. It is a differentiation factor as well as a potent mitogen. BTC promotes the differentiation of pancreatic acinar-derived AR42J cells into insulin-producing cells. It independently and preferentially binds to two type I tyrosine kinase receptors, the EGF receptor (ErbB1) and ErbB4. However, the physiochemical characteristics of BTC that are responsible for its preferential binding to these two receptors have not been fully defined. In this study, to investigate the essential amino acid residues of BTC for binding to the two receptors, we introduced point mutations into the EGF domain of BTC employing error-prone PCR. The receptor binding abilities of 190 mutants expressed in Escherichia coli were assessed by enzyme immunoassay. Replacement of the glutamic acid residue at position 88 with a lysine residue in BTC was found to produce a significant loss of affinity for binding to ErbB1, while the affinity of binding to ErbB4 was unchanged. In addition, the mutant of BTC-E/88/K showed less growth-promoting activity on BALB/c 3T3 cells compared with that of the wild-type BTC protein. Interestingly, the BTC mutant protein promoted differentiation of pancreatic acinar AR42J cells at a high frequency into insulin-producing cells compared with AR42J cells that were treated with wild-type BTC protein. These results indicate the possibility of designing BTC mutants, which have an activity of inducing differentiation only, without facilitating growth promotion.     

Increase of heat shock protein gene expression by melatonin in AR42J cells.

Heat shock proteins (HSPs) have been reported to protect the pancreatic cells from the acute damage produced by caerulein overstimulation. However the effects of caerulein, melatonin or hyperthermia preconditioning on mRNA signal for HSP60 in the pancreatic acinar cells has not been examined yet. The aims of this study were: 1. To investigate the gene expression for HSP60 in the pancreatic AR42J cells stimulated by melatonin, caerulein or combination of both these substances. 2. To compare above changes with mRNA signal for HSP60 in pancreatic AR42J cells subjected to hyperthermia preconditioning. AR42J cells were incubated in standard medium at 37 degrees C for: 0, 1, 3, 5, 12 or 24 h, under basal conditions. Above cells were then subjected to heat shock (42 degrees C) for 0, 1 or 3 h. In the next part of the study AR42J cells were incubated in presence of caerulein (10(-11), 10(-9) or 10( -7) M), melatonin (10(-8) or 10(-6) M), or combination of above under basal conditions or following heat shock pretreatment. Gene expression for HSP60 was determined by RT-PCR. The mRNA signal for HSP60 has been observed in AR42J cells under basal conditions, and this signal was markedly and time-dependently increased in these cells subjected to hyperthermia preconditioning. Incubation of AR42J cells in presence of melatonin (10(-8) or 10(-6) M) resulted in the significant and dose-dependent increase of gene expression for HSP60 in both groups of AR42J cells: preconditioned and in those, which were not subjected to hyperthermia. Caerulein stimulation reduced mRNA signal for HSP60. The strongest signal has been observed after the exposition of AR42J cells to hyperthermia preconditioning, combined with melatonin and caerulein. We conclude that: 1. Gene expression for HSP60 has been detected in pancreatic AR42J cells under basal conditions. 2. Hyperthermia preconditioning resulted in a significant and time-dependent increase of HSP60 signal in pancreatic AR42J cells. 3. HSP60 gene expression was significantly increased in pancreatic AR42J cells stimulated by melatonin whereas caerulein reduced this signal. 4. The strongest gene expression for HSP60 has been found in the cells subjected to the combination of hyperthermia preconditioning, caerulein and melatonin.     

Expression of cell adhesion molecule E-cadherin in Xenopus embryos begins at gastrulation and predominates in the ectoderm

The expression of the Ca2+-dependent epithelial cell adhesion molecule E-cadherin (also known as uvomorulin and L-CAM) in the early stages of embryonic development of Xenopus laevis was examined. E-Cadherin was identified in the Xenopus A6 epithelial cell line by antibody cross-reactivity and several biochemical characteristics. Four independent mAbs were generated against purified Xenopus E-cadherin. All four mAbs recognized the same polypeptides in A6 cells, adult epithelial tissues, and embryos. These mAbs inhibited the formation of cell contacts between A6 cells and stained the basolateral plasma membranes of A6 cells, hepatocytes, and alveolar epithelial cells. The time of E-cadherin expression in early Xenopus embryos was determined by immunoblotting. Unlike its expression in early mouse embryos, E-cadherin was not present in the eggs or early blastula of Xenopus laevis. These findings indicate that a different Ca2+-dependent cell adhesion molecule, perhaps another member of the cadherin gene family, is responsible for the Ca2+-dependent adhesion between cleavage stage Xenopus blastomeres. Detectable accumulation of E-cadherin started just before gastrulation at stage 9 1/2 and increased rapidly up to the end of gastrulation at stage 15. In stage 15 embryos, specific immunofluorescence staining of E-cadherin was discernible only in ectoderm, but not in mesoderm and endoderm. The ectoderm at this stage consists of two cell layers. The outer cell layer of ectoderm was stained intensely, and staining was localized to the basolateral plasma membrane of these cells. Lower levels of staining were observed in the inner cell layer of ectoderm. The coincidence of E-cadherin expression with the process of gastrulation and its restriction to the ectoderm indicate that it may play a role in the morphogenetic movements of gastrulation and resulting segregation of embryonic germ layers.