Cell Cycle and Cancer - Research Units - Cell Cycle and Cancer - Research themes
Regulation of G1-S phase cell cycle progression
Significant evidence indicates that the chromatin-remodelling SWI/SNF complexes play an important role in regulating cell proliferation by regulating transcription and potentially, DNA replication. Cyclin E/CDK2 interacts genetically with core subunits of the SWI/SNF complex termed SWI2 and SWI3 suggesting that their phosphorylation regulates cell cycle progression. Consistent with this hypothesis, orthophosphate labelling experiments demonstrate that SWI3 is phosphorylated on CDK sites in cells. Further studies now involve characterising SWI2 and SWI3 phosphorylation sites to determine if these proteins are phosphorylated by cyclin E/CDK2 during G1-S phase cell cycle progression. In addition, studies will be performed to determine if the wild-type and phosphorylation site mutants of SWI2 and SWI3 have differential effects on regulating cell division and their chromatin remodelling activity or DNA replication.
These studies will be complemented by genetic studies in Drosophila melanogaster to evaluate the role of SWI2 and SWI3 in a whole organism by evaluating phenotypes associated with cell proliferation such as rough eye, altered wing notching and wing vein truncation. Transgenic flies have already been generated and interesting novel data obtained. SWI/SNF subunits are mutated in human cancer and function as tumour suppressors. This research will determine if the SWI/SNF tumour suppressor complex is a critical downstream target of cyclin E/CDK2. This work will also increase our understanding of how cyclin E regulates cell cycle transitions and how it can behave as an oncogene when overexpressed in human cancer.
Regulation of proliferation and metastasis by cyclin/CDK-mediated phosphorylation of the RBP1 tumour
A critical substrate of CDKs is the retinoblastoma tumour suppressor gene product, pRb, which inhibits G1-S phase cell cycle progression by inhibiting the activity of E2F transcription factors. pRb inhibits E2F through recruitment of histone deacetylases (HDACs) via a retinoblastoma binding protein (RBP1). Studies in our laboratory show that RBP1 is phosphorylated by CDKs leading to its dissociation from pRb, to affect its tumour suppressor function. In addition to binding to pRb, RBP1 binds to the breast cancer metastasis suppressor, Brms1, which is a transcriptional regulator. Brms1 function is important for suppressing the spread of breast cancer cells from the original tumour site to distant organs.
Our work suggests that CDK-mediated phosphorylation of RBP1 affects the metastasis suppressor function of Brms1. Further studies will involve understanding how CDK-mediated phosphorylation of RBP1 affects Brms1 metastasis suppressor function. These studies for the first time suggest that deregulated CDK activity can lead to increased proliferation and metastasis in breast cancer. Understanding the mechanisms of increased proliferation and metastasis of breast cancer cells is important for developing new therapeutic approaches to treat this disease.
Regulation of cell cycle control by the ubiqutin-conjugating and ubiquitin ligase enzymes
The ubiquitination pathway involves the covalent binding of the ubiquitin polypeptide to substrate proteins resulting in their recognition and proteolytic degradation by the proteasome. This pathway is involved in all aspects of eukaryotic biology and accounts for 80% of cellular protein turnover. The recent approval of the proteasome inhibitor bortezomib for the treatment of advanced multiple myeloma indicates that targeting the ubquitin/proteasome pathway offers new avenues for cancer therapy. Since the proteasome is non-specific and degrades most polyubiquitylated proteins, the development of drugs against specific lesions in the ubiquitin/proteasome pathway is important for increasing specificity and thus attaining better therapeutic outcomes for more effective cancer treatment. Ubiquitin-conjugating enzymes (UBCs) and ubiquitin-ligases (E3s) are pivotal enzymes in the ubiquitination pathway. Recent studies demonstrating increased expression of UBC3 and UBCh10 in human tumours, implicate a role for these molecules in human cancer development. Therefore, increased understanding of the molecular mechanisms of UBC and E3 action is pivotal towards evaluating the potential of these molecules as therapeutic drug targets.
Our laboratory has unveiled important regions in UBCs and E3s which are critical for the catalytic activity and cell cycle functions of these enzymes. The aim of this research is to completely characterise the importance of these regions for UBC and E3 function at a molecular level and in cell cycle progression. These sites may represent new drug targets to modulate UBCs and E3s for cancer treatment.