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Section of Gastroenterology, Boston Veterans Administration Medical Center, and Boston University School of Medicine, Boston, Massachusetts 02118
Address all correspondence and requests for reprints to: Chi-Chuan Tseng, M.D., Ph.D., Section of Gastroenterology, Boston University School of Medicine, Boston, Massachusetts 02118.
The glucose-dependent insulinotropic peptide receptor (GIP-R) is a
member of the G protein-coupled receptors. Recent studies have
indicated that elevated serum GIP concentrations in type II diabetic
patients might induce desensitization of the GIP-R, and this mechanism
could contribute to impaired insulin secretion. The cellular and
molecular mechanisms governing GIP desensitization are unknown. Here,
we report the results of studies on a new family of proteins known as
regulators of G protein signaling (RGS) that have been shown to mediate
the desensitization process of other receptors. GIP-R and RGS1, -2, -3,
and -4 complementary DNAs were cotransfected into human embryonic
kidney cells (L293). GIP-stimulated cAMP generation in control cells
and in those coexpressing RGS1, -3, and -4 displayed a dose-dependent
increase 10 min after GIP treatment. In contrast, RGS2 expression
inhibited the GIP-induced cAMP response by 50%, a response similar to
that of cells desensitized by preincubation with 10-7
M GIP. In ßTC3 cells, preincubation of GIP attenuated
GIP-induced insulin release by 45% at 15 min and by 55% at 30 min.
Expression of RGS2 in the ßTC3 cells significantly decreased
GIP-stimulated insulin secretion, whereas glucose-induced insulin
release was not affected. RGS2 messenger RNA was identified by Northern
blot analysis to be expressed endogenously in ßTC3 and L293 cells,
and its level was significantly induced by GIP treatment in ßTC3
cells. Moreover, RGS2 bound Gs
protein in an in
vitro system, suggesting that RGS2 attenuated the
Gs-adenylate cyclase signaling pathway. These results
suggest a potential role for RGS2 in modulating GIP-mediated insulin
secretion in pancreatic islet cells.
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