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Regulation and function of Skp2 in mediating p27 degradation during adipocyte hyperplasia

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Corinth Andrews Auld (Creator)
Institution
The University of North Carolina at Greensboro (UNCG )
Web Site: http://library.uncg.edu/
Advisor
Ron F. Morrison

Abstract: "Adipocyte hyperplasia is characterized by an increase in adipocyte cell number and contributes significantly to the development of obesity. A key question regarding adipocyte hyperplasia concerns the mechanism by which preadipocytes undergo the transition from quiescence to proliferation. Here we examined molecular processes regulating the cell cycle inhibitor, p27, during adipocyte hyperplasia. Previous literature has shown p27 regulation by transcriptional and post-transcriptional mechanisms in various cell types. In neoplastic cells, the F-box protein, Skp2, can target phosphorylated p27 protein for degradation via the 26S proteasome. Studies presented here indicated cell cycle-dependent decreases in p27 occurring during early stages of adipocyte differentiation due to accelerated protein turnover. Further analyses revealed Cdk2-dependent phosphorylation and polyubiquitylation of p27 during S phase progression. Additionally, the 26S proteasome was found to be essential for the decrease in p27 and subsequent cell cycle progression. It was determined that Skp2 is transiently expressed during preadipocyte replication and is, at least, partly responsible for increased degradation of p27. These are novel data suggesting the presence and role of Skp2 during adipocyte hyperplasia. It was demonstrated that expression of Skp2 is unique to preadipocyte proliferation and cannot be reproduced in fully differentiated, terminally growth-arrested adipocytes. Findings revealed transient and dramatic increases in Skp2 mRNA, preceding protein expression, due to promoter activation and not increased mRNA stability. These processes were significantly mediated by growth factor receptor signaling pathways involving PI3K and MAPK. Furthermore, inhibition of mTOR, a downstream target of PI3K important for translational control, produced a significant reduction in Skp2 protein but not mRNA. Collectively, these data suggest Skp2 expression during adipocyte hyperplasia is regulated at multiple levels and dependent on kinase-driven signaling pathways. The sesquiterpene lactone, helenalin, is a naturally occurring compound known to possess anti-proliferative and anti-inflammatory properties. This may be of benefit to the treatment of obesity as cells within adipose tissue have the capacity to proliferate and secrete pro-inflammatory cytokines. Studies presented here showed that helenalin prevents the increase in Skp2 mRNA by completely blocking Skp2 promoter activity. Further studies revealed helenalin's ability to prevent Akt phosphorylation, a key signaling molecule downstream of PI3K activation. Taken together, these findings provide valuable insight into molecular mechanisms regulating adipocyte hyperplasia. "--Abstract from author supplied metadata.

Additional Information

Publication
Dissertation
Language: English
Date: 2006
Keywords
Adipocyte hyperplasia, obesity, preadipocytes, molecular processes, cell cycle inhibitor, p27 regulation, neoplastic cells, preadipocyte proliferation
Subjects
Fat cells--Growth--Molecular aspects
Adipose tissues
Hyperplasia
Obesity--Research