Novel Interactors of X-linked Inhibitor of Apoptosis Protein: Expression and Effects on Tumor Cell Death

Programmed cell death, or apoptosis, has during the last few years received a lot of attention because of its involvement in a large number of pathological conditions. Since death is always irreversible, it is important for cells to fully control the initiation and execution of this process. One of many apoptosis-regulatory proteins is XIAP, which blocks the action of caspases, a family of proteases that are important during apoptosis. However, apoptosis inhibitors have to be tightly controlled since too little cell death can lead to the development of tumors and other diseases. This thesis is the result of an aspiration to fully understand the function and regulation of XIAP.By using the yeast-2-hybrid system, we identified two novel binding partners of XIAP. The first, GPS2, was found to bind XIAP and inhibit its ability to block caspase-activity. In addition, GPS2 induced caspase-mediated cell death in two different tumour cell lines and XIAP inhibited this effect.The second binding partner, Nulp1, preferentially bound XIAP in the presence of the apoptosis-inducer staurosporine. Nulp1 induced or sensitized cell lines to cell death when overexpressed, but this was not blocked by caspase-inhibitors or XIAP, suggesting a different reason for binding than apoptosis regulation. With the aim to understand the Nulp1-XIAP interaction, we continued to study Nulp1 in vivo and in vitro. We studied three different splice variants of Nulp1 and found that they were regulated by poly-ubiquitination and nuclear shuttling. Also, Nulp1 was expressed in embryonic mice, especially in the cortical plate, hippocampal neurons and cerebellar granular neurons. Expression of Nulp1 decreased with age but was still present in cerebellar deep nuclei and Purkinje cells of adult mice.To summarize, we have identified GPS2 as an apoptosis-inducing factor and an inhibitor of XIAP in vitro, and Nulp1 as a XIAP-interacting protein during staurosporine-induced apoptosis.

Contents: Novel Interactors of X-linked Inhibitor of Apoptosis Protein: Expression and Effects on Tumor Cell Death

Introduction
1. An overview of cell death
1.1 Apoptosis
1.2 Necrosis
2. A closer look at apoptosis
2.1 Caspases
2.2 Apoptotic pathways
2.3 Major regulators of apoptosis
2.4 Translational regulation during cell stress
3. The Inhibitor of Apoptosis Protein family
4. X-linked Inhibitor of Apoptosis Protein
4.1 XIAP-inhibitors
4.2 XIAP as an ubiquitin ligase
4.3 XIAP in cell signalling
5. Physiological importance of XIAP
6. Using XIAP as a therapeutic tool
7. Novel interactors of XIAP
Aims
Materials and methods
Results and Discussion…