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Biosynthesis of islet amyloid polypeptide. Elevated expression in mouse beta TC3 cells
S Nagamatsu1, M Nishi, D F Steiner
1Department of Biochemistry, University of Chicago, Illinois 60637.
The Journal of Biological Chemistry
|July 25, 1991
Summary
Islet amyloid polypeptide (IAPP) is synthesized, processed, and secreted by mouse beta TC3 cells. Despite high mRNA levels, IAPP translation is inefficient, with faster processing than proinsulin.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Biology
Background:
- Islet amyloid polypeptide (IAPP) is a key hormone produced by pancreatic beta cells.
- Understanding IAPP biosynthesis and secretion is crucial for studying pancreatic islet function.
Purpose of the Study:
- To investigate Islet amyloid polypeptide (IAPP) messenger RNA levels, biosynthesis, processing, and secretion in cultured mouse beta TC3 insulinoma cells.
- To compare IAPP processing with that of insulin.
Main Methods:
- Northern blot analysis to quantify IAPP mRNA levels.
- Metabolic labeling with [3H]leucine to study protein synthesis.
- Pulse-chase experiments to determine processing rates.
- Analysis of secretion pathways.
Main Results:
- IAPP mRNA was detected at significant levels in beta TC3 cells, approximately 60% of insulin mRNA.
- Pro-IAPP (Mr 7400) and mature IAPP (Mr 3900) were synthesized.
- Pro-IAPP processing to IAPP occurred rapidly (t1/2 ~25 min), faster than proinsulin to insulin conversion (t1/2 ~70 min).
- Both IAPP and insulin precursors were secreted via a constitutive pathway.
- Dexamethasone and forskolin did not affect IAPP mRNA or biosynthesis.
Conclusions:
- The IAPP gene is actively expressed in beta TC3 cells, resulting in IAPP biosynthesis, proteolytic processing, and secretion.
- IAPP mRNA is translated less efficiently than insulin mRNA.
- IAPP undergoes rapid post-translational processing and secretion, similar to insulin.