| 1. |
- Arkhammar, P., et al.
(author)
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Protein kinase C modulates the insulin secretory process by maintaining a proper function of the beta-cell voltage-activated Ca2+ channels
- 1994
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In: Journal of Biological Chemistry. - : Baishideng Publishers. - 0021-9258 .- 1083-351X. ; 269:4, s. 2743-2749
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Journal article (peer-reviewed)abstract
- In the present study an attempt was made to further elucidate the molecular mechanisms whereby protein kinase C (PKC) modulates the beta-cell stimulus-secretion coupling. Regulation of Ca2+ channel activity, [Ca2+]i, and insulin release were investigated in both normal pancreatic mouse beta-cells and in similar beta-cells deprived of PKC activity. [Ca2+]i was measured with the intracellular fluorescent Ca2+ indicator fura-2 and the Ca2+ channel activity was estimated by the whole cell configuration of the patch-clamp technique. To reveal the various isoenzymes of PKC present in the mouse beta-cell, proteins were separated by one-dimensional gel electrophoresis and Western blotting was performed. The production of inositol phosphates was measured by ion-exchange chromatography and insulin release was measured radioimmunologically. Acute stimulation with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate resulted in suppression of both the carbamylcholine-induced increase in [Ca2+]i and production of inositol 1,4,5-trisphosphate. Under these conditions the increase in [Ca2+]i in response to glucose was similar to that found in control cells. When beta-cells were deprived of PKC, by exposure to 200 nM 12-O-tetradecanoylphorbol-13-acetate for 24-48 h, there was an enhanced response to carbamylcholine. This response constituted increases in both the [Ca2+]i signal and production of inositol 1,4,5-trisphosphate. Interestingly, cells with down-regulated PKC activity responded more slowly to glucose stimulation, when comparing the initial increase in [Ca2+]i, than control cells. On the other hand, the maximal increase in [Ca2+]i was similar whether or not PKC was present. Moreover, PKC down-regulated cells exhibited a significant reduction of maximal whole cell Ca2+ currents, a finding that may explain the altered kinetics with regard to the [Ca2+]i increase in response to the sugar. Both the alpha and beta 1 forms of the PKC isoenzymes were present in the mouse beta-cell and were also subjected to PKC down-regulation. Hence, either of these isoenzymes or both may be involved in the modulation of phospholipase C and Ca2+ channel activity. Since insulin release under physiological conditions is critically dependent on Ca(2+)-influx through the voltage-gated L-type Ca2+ channels, the kinetics of hormone release was expected to demonstrate a similar delay as that of the [Ca2+]i increase. Although not as pronounced, such a delay was indeed also observed in the onset of insulin release. There was, however, no effect on the total amounts of hormone released. There was,h owever, no effect on thet otal amounts of hormone released. The present study con- firms that PKC has multiple roles and thereby interacta at different sites in the complex series of events consti- tuting the #?-cell signal-transduction pathway. It is sug- gested that PKC may be tonically active and effective in the maintenance of the phosphorylation state of the voltage-gated L-type Ca2+ channel, enabling an appro- priate function of this channel in the insulin secretory process.
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| 2. |
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| 3. |
- Oberg, C, et al.
(author)
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Expression of protein tyrosine kinases in islet cells : possible role of the Flk-1 receptor for beta-cell maturation from duct cells.
- 1994
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In: Growth Factors. - 0897-7194 .- 1029-2292. ; 10:2, s. 115-26
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Journal article (peer-reviewed)abstract
- To elucidate the expression of genes of importance for beta-cell replication and the production of insulin, single-stranded cDNAs from different preparations of insulin producing cells were used as template for the polymerase chain reaction (PCR) using primers specific for protein tyrosine kinases (PTKs). In RINm5F cells, as well as in fetal rat islets, the receptor PTK fetal liver kinase-1 (Flk-1) was expressed among other receptor and cytoplasmic tyrosine kinases. To elucidate the putative effects of stimulation of the Flk-1 receptor, fetal rat islet-like structures were cultured in the presence of the ligand for this receptor, vascular endothelial growth factor (VEGF). VEGF was found to stimulate both the insulin content/islet DNA ratio and the accumulation of insulin in the culture medium without affecting the rates of beta-cell replication. To investigate the localization of expression of the Flk-1 receptor in the pancreas, serial sections of fetal pancreata were immunostained for Flk-1 and insulin. Expression of Flk-1 was detected in endothelial-like cells and cells lining pancreatic ducts. The latter are considered to contain precursor cells for the endocrine pancreas. In conclusion, specific protein tyrosine kinases are expressed in islet cells, and are presumably participating in the regulation of islet function. Specifically, the receptor PTK Flk-1 may play a role of beta-cell maturation from pancreatic duct cells.
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| 4. |
- Welsh, M, et al.
(author)
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Shb is a ubiquitously expressed Src homology 2 protein.
- 1994
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In: Oncogene. - 0950-9232 .- 1476-5594. ; 9:1, s. 19-27
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Journal article (peer-reviewed)abstract
- To identify serum-inducible genes in the insulin-producing cell line beta TC-1, a library subtraction screening procedure was performed on serum-deprived (G0) and serum-restimulated (G1) insulin-producing beta TC-1 cells. A cDNA containing a motif with strong homology to Src homology 2 (SH2) domains was found using this procedure and called Shb. The Shb cDNA contains two methionine codons in its N-terminus and thus may code for two proteins of 67 and 56 kDa, each with one SH2 domain in its C-terminus. No other structural similarity to proteins with catalytic activity could be detected, suggesting that Shb is a so called adaptor. Shb contains the proline-rich sequence PPPGPGR between the two proposed initiator methionines which resembles a sequence for binding to Src homology 3 (SH3) domains. A second proline-rich sequence was detected after the second methionine codon. The Shb cDNA hybridized to a similar or identical mRNA of 3.1 kb expressed in mouse brain, liver, kidney, heart, NIH3T3 fibroblasts and beta TC-1 cells. Western blot analysis of the same tissues using an antiserum directed against a synthetic peptide corresponding to a part of the SH2 domain of Shb, revealed reactivity with two proteins of 56 and 67 kDa. In addition, a third reactive component of 40 kDa was detected in most tissues. Transfection and transient expression of the Shb cDNA in COS-1 cells yielded increased expression of the 67, 56 and 40 kDa proteins. Transfection and stable expression of the Shb cDNA in pig aortic endothelial cells showed increased expression primarily of the 67 kDa protein. A fusion protein consisting of the SH2 domain of Shb linked to glutathione S-transferase showed increased binding to glycoproteins of cells stimulated with platelet-derived growth factor (PDGF-BB). Furthermore, the autophosphorylated PDGF beta-receptor but not the autophosphorylated epidermal growth factor (EGF) receptor bound specifically to immobilized fusion protein. It is concluded that Shb is a novel SH2-containing protein with proline-rich domains and therefore probably involved in the signal-transduction of some ligand-activated tyrosine kinase receptors.
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| 5. |
- Blume-Jensen, Peter, et al.
(author)
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Activation of the human c-kit product by ligand-induced dimerization mediates circular actin reorganization and chemotaxis
- 1991
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In: EMBO Journal. - 0261-4189 .- 1460-2075. ; 10:13, s. 4121-4128
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Journal article (peer-reviewed)abstract
- The proto-oncogene c-kit is allelic with the murine white spotting (W) locus and encodes a transmembrane protein tyrosine kinase that is structurally related to the receptors for platelet-derived growth factor (PDGF) and colony-stimulating factor-1 (CSF-1). Recently the ligand for the c-kit product, stem cell factor (SCF), was identified in both transmembrane and soluble forms. In order to examine the mechanism for receptor activation by SCF and biological properties of the activated c-kit product, we transfected the wild-type human c-kit cDNA into porcine aortic endothelial cells. We found that the receptor was down-regulated and transmitted a mitogenic signal in response to stimulation with soluble SCF. We also demonstrate that SCF induces dimerization of the c-kit product in intact cells, and that dimerization of the receptor is correlated with activation of its kinase. Activation of the c-kit product by SCF was found to induce circular actin reorganization indistinguishable from that mediated by the PDGF beta-receptor in response to PDGF-BB. Furthermore, soluble SCF was a potent chemotactic agent for cells expressing the c-kit product, a property which might be of importance during embryonic development.
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