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Tissue-specific deletion of the retinoblastoma protein in the pancreatic beta-cell has limited effects on beta-cell
Rupangi C Vasavada1, Irene Cozar-Castellano, Darinka Sipula
1Division of Endocrinology, University of Pittsburgh School of Medicine, 200 Lothrop St., Pittsburgh, PA 15213, USA. vasavada@dom.pitt.edu
Abstract:
Animal studies show that G(1/S) regulatory molecules (D-cyclins, cdk-4, p18, p21, p27) are critical for normal regulation of beta-cell proliferation, mass, and function. The retinoblastoma protein, pRb, is positioned at the very end of a cascade of these regulatory proteins and is considered the final checkpoint molecule that maintains beta-cell cycle arrest. Logically, removal of pRb from the beta-cell should result in unrestrained beta-cell replication, increased beta-cell mass, and insulin-mediated hypoglycemia. Because global loss of both pRb alleles is embryonic lethal, this hypothesis has not been tested in beta-cells. We developed two types of conditional knockout (CKO) mice in which both alleles of the pRb gene were inactivated specifically in beta-cells. Surprisingly, although the pRb gene was efficiently recombined in beta-cells of both CKO models, changes in beta-cell mass, beta-cell replication rates, insulin concentrations, and blood glucose levels were limited or absent. Other pRb family members, p107 and p130, were not substantially upregulated. In contrast to dogma, the pRb protein is not essential to maintain cell cycle arrest in the pancreatic beta-cell. This may reflect fundamental inaccuracies in models of beta-cell cycle control or complementation for pRb by undefined proteins.
Insights
The retinoblastoma protein (pRb) is not essential for pancreatic beta-cell cycle arrest, contrary to established models. Conditional knockout mice showed no significant changes in beta-cell mass or function after pRb removal.
Area of Science:
- Cell Biology
- Endocrinology
- Molecular Biology
Background:
- G(1/S) regulatory molecules are critical for beta-cell proliferation, mass, and function.
- The retinoblastoma protein (pRb) is considered the final checkpoint molecule maintaining beta-cell cycle arrest.
- Global pRb loss is embryonic lethal, preventing in-vivo testing of its role in beta-cells.
Purpose of the Study:
- To investigate the role of the retinoblastoma protein (pRb) in maintaining pancreatic beta-cell cycle arrest.
- To determine if pRb inactivation leads to unrestrained beta-cell replication and increased beta-cell mass.
Main Methods:
- Developed two conditional knockout (CKO) mouse models.
- Inactivated both alleles of the pRb gene specifically in beta-cells.
- Analyzed beta-cell mass, replication rates, insulin concentrations, and blood glucose levels.
Main Results:
- Efficient pRb gene recombination was confirmed in beta-cells of CKO models.
- Limited or absent changes were observed in beta-cell mass, replication rates, insulin, and blood glucose.
- Upregulation of pRb family members p107 and p130 was not substantial.
Conclusions:
- The retinoblastoma protein (pRb) is not essential for maintaining cell cycle arrest in pancreatic beta-cells.
- Findings challenge established models of beta-cell cycle control.
- Potential complementation by undefined proteins or inaccuracies in current models may explain the results.
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