Signal transduction in adipocytes: Regulation by insulin versus other growth factors.
Item
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Title
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Signal transduction in adipocytes: Regulation by insulin versus other growth factors.
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Identifier
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AAI9707099
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identifier
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9707099
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Creator
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Han, Yuan-Ping.
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Contributor
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Adviser: Ronald A. Kohanski
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Date
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1996
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Language
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English
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Publisher
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City University of New York.
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Subject
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Biology, Cell | Biology, Molecular
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Abstract
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Protein tyrosine phosphatases (PTP) can play either negative or positive roles in signal transduction initiated by receptor tyrosine kinases. In adipocytes, stimulation of glucose uptake is the most prominent physiologically important signaling pathway under the acute regulation of insulin and its receptor. This pathway is inhibited by phenylarsine oxide (PAO), which is known to block PTP action. We have found that a 67kDa phosphotyrosyl-protein, pp67, present in the cytosol of 3T3-L1 adipocytes, became dephosphorylated in response to insulin, insulin-like growth factor-1, and epidermal growth factor. The kinetics of insulin-dependent pp67 dephosphorylation were similar to those of insulin-stimulated glucose transport in these cells. Dephosphorylation of pp67 was inhibited by phenylarsine oxide (PAO), but not by wortmannin, an inhibitor of phosphatidylinositol 3{dollar}\sp\prime{dollar}-kinase and glucose transport activation. These findings are consistent with activation of a PTP by insulin as a potential mediator of glucose transport. In addition to showing that stimulation of PI3{dollar}\sp\prime{dollar}-kinase was limiting for growth factor activation of glucose transport, we identified PI3{dollar}\sp\prime{dollar}-kinase as a target for inhibition by PAO. Therefore, the PI3{dollar}\sp\prime{dollar}-kinase dependent pathway is adequate to explain the known effectors and inhibitors of glucose transport. While these data do not exclude the possibility, it is not necessary to invoke a PI3{dollar}\sp\prime{dollar}-kinase independent, PTP-directed pathway, such as the one reported here by pp67 dephosphorylation.;In contrast to the persistent activation of a PTP for pp67, the insulin-stimulated MAP kinase pathway was transiently activated in 3T3-L1 adipocytes. We observed the transient formation of a membrane-associated complex including phosphotyrosyl-insulin receptor substrate-1 (IRS-1), SOS1, and possibly Grb2. We also found that this binary--and possibly ternary--complex was present at higher levels and for longer times in the cytosolic compartment. Based upon the known requirement for SOS1 activation in the membrane compartment, we conclude that the transient formation of the SOS1-IRS-1 complex in the membrane is the basis for the transient activation of MAP kinase tyrosine phosphorylation. Electrophoretic changes in SOS1, which occur following growth factor stimulation, were demonstrated to take place after MAP kinase activation had passed its peak, and therefore could not be the cause for the transient. Furthermore, we found a novel electrophoretic form of SOS1 which was induced by insulin and to a lesser extent by other growth factors. Unlike the known growth-factor induced shifts in electrophoretic mobilities of several signaling molecules, this novel SOS1 form was insensitive to phosphatase treatment. However, the underlying modification of SOS1 was not identified.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
