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Adipose tissue expression, alternative splicing, desensitization and internalization of the glucose-dependent insulinotropic polypeptide receptor Gill, Ruth Ellen
Abstract
Glucose dependent insulinotropic polypeptide (GIP) is a gut hormone which plays an important role as an incretin. Thus, the GIP receptor, a member of the G-protein coupled receptor (GPCR) family, is expressed in the β-cells of the endocrine pancreas. However, it is also expressed in other tissues, including adipose tissue, where GIP exerts effects on fat metabolism. Non-insulin dependent diabetes mellitus (NIDDM), a disease characterized by impaired insulin secretion and insulin resistance, is associated with obesity. Therefore, the possibility of altered GIP receptor expression was examined in the Zucker fatty (fa/fa) rat, a model of obesity. In contrast to previous studies, 11 week old obese Zucker rats did not show higher plasma glucose concentrations in response to oral glucose than did their lean littermates, but this is probably due to differences in statistical analysis. However, the 16 week old obese animals showed fasting hyperglycemia, severe glucose intolerance and greater plasma GIP responses to oral glucose than did their lean littermates, suggesting that they had become diabetic. The 11 week old lean and obese Zucker rats displayed nearly identical levels of adipose tissue GIP receptor mRNA, therefore, the adipose cell GIP receptor expression appears not to be involved in the phenotype of the animal at this stage of development. The GIP receptor is a member of the secretin/VIP subfamily of GPCRs, several of whose members exist in alternatively spliced forms. The presence of the GIP receptor in tissues other than the endocrine pancreas suggests the possibility of different signal transduction pathways. The third intracellular (i3) loop of many GPCRs is important for G-protein recognition and coupling, so this area of the GIP receptor was examined for alternative splicing. An alternatively spliced variant of the GIP receptor expressed predominantly in islet tissue was detected and predicted to encode a truncated form of the GIP receptor. A second splice variant expressed predominantly in adipose tissue was predicted to encode a 28 amino acid insert located at the C-terminal end of the i3 loop. The second and third intracellular loops of the GIP receptor contain several serine and threonine residues. As potential phosphorylation sites, these residues were examined for their ability to mediate receptor desensitization and internalization. The mutation of Ser-248, Ser-320, Thr-324 and Ser-339 to alanines resulted in a reduced ability to stimulate cyclic-adenosine monophosphate (cAMP) production in response to GIP stimulation as compared to the wild type GIP receptor (wtGIPR). The mutation of Thr- 342 to alanine resulted in an increased ability to stimulate cAMP production in response to GIP stimulation in low expressing subclones. The wild type GIP receptor did not show significant desensitization in response to preincubation with GIP, so it was not possible to determine the result of mutations on receptor desensitization. GIPR/S248A was the only mutant receptor to show a decrease in internalization rate and maximum internalization as compared to the wtGIPR. Neither GIPR/S320A nor GIPR/T324A showed changes in internalization rate or maximum internalization compared to the wtGIPR, while GIPR/S339A and GIPR/T342A showed an increase in both these parameters. Therefore, Ser-339 and Thr-342 may act as negative regulators of receptor internalization. The above studies have added to the growing knowledge about GIP's signaling pathways, its role in adipose tissue and its potential role in obesity and NIDDM.
Item Metadata
Title |
Adipose tissue expression, alternative splicing, desensitization and internalization of the glucose-dependent insulinotropic polypeptide receptor
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2000
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Description |
Glucose dependent insulinotropic polypeptide (GIP) is a gut hormone which plays
an important role as an incretin. Thus, the GIP receptor, a member of the G-protein
coupled receptor (GPCR) family, is expressed in the β-cells of the endocrine pancreas.
However, it is also expressed in other tissues, including adipose tissue, where GIP exerts
effects on fat metabolism. Non-insulin dependent diabetes mellitus (NIDDM), a disease
characterized by impaired insulin secretion and insulin resistance, is associated with
obesity. Therefore, the possibility of altered GIP receptor expression was examined in
the Zucker fatty (fa/fa) rat, a model of obesity.
In contrast to previous studies, 11 week old obese Zucker rats did not show higher
plasma glucose concentrations in response to oral glucose than did their lean littermates,
but this is probably due to differences in statistical analysis. However, the 16 week old
obese animals showed fasting hyperglycemia, severe glucose intolerance and greater
plasma GIP responses to oral glucose than did their lean littermates, suggesting that they
had become diabetic. The 11 week old lean and obese Zucker rats displayed nearly
identical levels of adipose tissue GIP receptor mRNA, therefore, the adipose cell GIP
receptor expression appears not to be involved in the phenotype of the animal at this
stage of development.
The GIP receptor is a member of the secretin/VIP subfamily of GPCRs, several of
whose members exist in alternatively spliced forms. The presence of the GIP receptor in
tissues other than the endocrine pancreas suggests the possibility of different signal
transduction pathways. The third intracellular (i3) loop of many GPCRs is important for
G-protein recognition and coupling, so this area of the GIP receptor was examined for
alternative splicing.
An alternatively spliced variant of the GIP receptor expressed predominantly in
islet tissue was detected and predicted to encode a truncated form of the GIP receptor. A
second splice variant expressed predominantly in adipose tissue was predicted to encode
a 28 amino acid insert located at the C-terminal end of the i3 loop.
The second and third intracellular loops of the GIP receptor contain several serine
and threonine residues. As potential phosphorylation sites, these residues were examined
for their ability to mediate receptor desensitization and internalization. The mutation of
Ser-248, Ser-320, Thr-324 and Ser-339 to alanines resulted in a reduced ability to
stimulate cyclic-adenosine monophosphate (cAMP) production in response to GIP
stimulation as compared to the wild type GIP receptor (wtGIPR). The mutation of Thr-
342 to alanine resulted in an increased ability to stimulate cAMP production in response
to GIP stimulation in low expressing subclones. The wild type GIP receptor did not show
significant desensitization in response to preincubation with GIP, so it was not possible to
determine the result of mutations on receptor desensitization. GIPR/S248A was the only
mutant receptor to show a decrease in internalization rate and maximum internalization as
compared to the wtGIPR. Neither GIPR/S320A nor GIPR/T324A showed changes in
internalization rate or maximum internalization compared to the wtGIPR, while
GIPR/S339A and GIPR/T342A showed an increase in both these parameters. Therefore,
Ser-339 and Thr-342 may act as negative regulators of receptor internalization.
The above studies have added to the growing knowledge about GIP's signaling
pathways, its role in adipose tissue and its potential role in obesity and NIDDM.
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Extent |
8311743 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-07-06
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0089370
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2000-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.