Date of Award

2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biological Sciences

First Advisor

Andrew Swan

Keywords

Cell Cycle, Cyclin A, Polyploidy, Protein Interaction, Skp2

Rights

CC BY-NC-ND 4.0

Abstract

S phase kinase associated protein 2 (Skp2) is an E3 ubiquitin ligase, an established oncogene and an important G1-S regulator. The critical and well-studied target of Skp2 is the Cyclin dependent kinase inhibitor p27/Dap. Overexpression of Skp2 has been observed in a wide variety of cancer types and in most of these cancer types, downregulation of p27/Dap has also been observed. However, loss of Skp2 in mammals and in Drosophila also results in polyploidy in mitotic tissues. Polyploidy resulting from overexpression of Skp2 has been widely studied but there is still no clear understanding on how loss of Skp2 results in polyploidy. We found that loss of Skp2 results in premature degradation of Cyclin A and other mitotic cyclins - possibly by the premature activation of APC-CDH1/Fzr resulting in mitotic failure. The cells then enter a G-like state and start endoreplicating, causing polyploidy. Our results show that the N-terminus of Skp2 interacts directly with Cyclin A and is required for rescuing polyploidy in Skp2 null mitotic cells. We also showed that polyploidy resulting from overexpression of p27/Dap is different from polyploidy resulting from loss of Skp2. Our results show that the polyploid Skp2 null cells which enter mitosis, delay in prometaphase/metaphase of the cell cycle with the activation of the Spindle Assembly Checkpoint (SAC). These cells frequently undergo double stranded DNA damage and activates apoptosis and autophagy mediated cell death. Our results argue that it is not polyploidy but the entry of polyploid cells into mitosis that activates the checkpoints that cause apoptosis for genome stability.

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