Date of Award


Publication Type

Doctoral Thesis

Degree Name



Biological Sciences


Pure sciences, Biological sciences, DNA damage, Cellular senescence, Cyclin-dependent kinases, DNA damage response, Checkpoint activation


Lisa A. Porter




Cyclin-dependent kinases (Cdks) control progression through the cell cycle. Proper regulation of the events associated with each phase is critical for the cellular response to alterations induced by intrinsic and extrinsic signals. The requirement for tight, and highly-ordered control of the cell cycle is evident given the multi-mechanistic regulation of Cdks. Positive stimulation of Cdks is regulated by several means, such as activation by phosphorylation and interaction with cyclin proteins expressed only at specific times during the cell cycle. Negative regulation of Cdk activity involves interaction with Cdk inhibitors and modification at inhibitory residues. It is noteworthy, however, that regulation of Cdk activity is not exclusively accomplished through these mechanisms. The speedy/RINGO family of proteins is uniquely capable of activating Cdk1 and 2 in the absence of positive regulation. The originally identified human homolog of this family, Spy1, is expressed in a variety of human tissues and found at elevated levels in several human cancers. Abnormally elevated Spy1 bypasses checkpoint activation and suppresses apoptosis. A detailed understanding of how Spy1 is regulated is required to determine the contribution of Spy1 in normal cell cycle progression and cellular homeostasis. Here, I demonstrate that Spy1 is selectively regulated as part of the cellular response following DNA damage. Degradation of Spy1 depends on Chk2 and p53, and appears to occur via the ubiquitin/proteasome system. Moreover, Spy1 expression bypasses UV-induced premature senescence as well as replicative-induced senescence in the presence of p53 and p21. Knock-down of endogenous Spy1 accelerates the onset of senescence, suggesting a functional role for Spy1 in the regulation of cellular senescence. Furthermore, our work reports a unique mechanism of Cdk activation, in which Spy1 interacts with the Cdk2/p21 complex, promotes degradation of p21, and subsequently activates Cdk2. This level of cell cycle regulation may be explained as a "back-up" mechanism for cells to tolerate the alterations induced by various stimuli.