The role of gamma-tubulin in cell division and cancer

Although cancer is a heterogeneous disease, a common theme in tumor cells is the deregulation of cell division, which renders tumor progression independent of otherwise necessary internal or external growth cues. This may provide cancer cells with the ability to divide indefinitely, posing a threat to the rest of the organism. As the tumor grows, nearby healthy tissues are harmed, which eventually leads to system failure and death of the patient. Consequently, to deprive tumor cells of growth factor independent division is crucial for the treatment of cancer.
The aim of my thesis project was to examine novel roles of the protein gamma-tubulin during cell division, as defects in this process may lead to genomic instability that favor proliferation and tumor progression. We have described that gamma-tubulin contains a nuclear localization signal (NLS), and that SADB-mediated phosphorylation of an amino acid in close vicinity to this NLS leads to accumulation of gamma-tubulin in the nuclear compartment. Once nuclear, gamma-tubulin regulates cell cycle progression by moderating the activities of the E2F transcription factors. Another modulator of E2F transcriptional activity is the tumor suppressor protein Retinoblastoma (RB). Notably, gamma-tubulin and RB regulate each other’s expression, which results in increased gamma-tubulin protein levels in tumor cells lacking functional RB. Furthermore, gamma-tubulin depletion in these cells result in their death.
We have observed that the nuclear and cytosolic pools of gamma-tubulin are connected through the nuclear envelope by gamma-tubulin protein bridges that form a cellular meshwork. This meshwork is important during formation of chromatin-containing nuclei, as disruption of the gamma-tubulin DNA binding domain leads to formation of nuclear-like structures devoid of chromatin. In addition, we found that the meshwork of gamma-tubulin is implicated in the DNA damage repair response upon DNA double strand breaks.
Together, the findings presented in this thesis demonstrate that gamma-tubulin is in several ways important for cell cycle progression. Initially, during G1-to-S phase transition, apart from promoting centrosome duplication, gamma-tubulin moderates E2F transcriptional activities. During the formation of daughter cells, the protein is involved in both segregation of chromosomes between the cells, and in the formation of their nuclear envelopes. Moreover, inhibition of gamma-tubulin activity result in cell death in RB-negative cells, and consequently the protein can be considered as a new chemotherapeutic target for cancer treatment. Finally, the cytosolic and nuclear gamma-tubulin pools form a cellular meshwork throughout the cell, which affects DNA damage repair response.