Dissecting the role of cathepsin V in breast cancer

Abstract

Breast cancer is the most common cancers diagnosed worldwide. Numerous advances in treatments to enhance efficacy and diminish side effects have resulted in increasing survival rates, however many challenges still remain, particularly in relation to disease recurrence, treatment resistance and metastasis. Therefore, more comprehensive understanding of the mechanisms that regulate breast cancer development will accelerate the development of new targeted therapeutic drugs.

Cathepsin V expression has been linked to many cancers including breast cancer. The expression of cathepsin V in breast cancer patients was found to be associated with distant metastasis; however, the regulatory function of cathepsin V has not been examined yet. This thesis is focused on elucidating the role of cathepsin V in breast cancer particularly within the luminal-A ER+ subtype.

A preliminary study in the Burden lab using KM plotter analysis revealed that high cathepsin V expression is associated with lower survival in breast cancer patients. However, more in depth analysis of the individual subtypes of breast cancer provided further information. Patients with basal-like or HER2-enriched breast cancer had a better prognosis if they exhibited high cathepsin V expression, whereas ER+ patients displayed an opposing effect, with high cathepsin V expression being associated with a poor prognosis. This suggested that cathepsin V may have a role in relation to ERα biology.

Through the development of genetic ablation models using cathepsin V shRNA in cell lines representative of different breast cancer subtypes, it was found that depletion of cathepsin V in luminal-A significantly decreased the rate of cell growth, while the opposite effect was observed in other subtypes. Furthermore, cathepsin V was illustrated to have a role in regulating ERα expression and its activities. Importantly, FoxA1 and GATA3, the crucial pioneer factors of ERα transcription, were also altered in cathepsin V depleted cells; therefore, it is plausible that cathepsin V may regulate ERα expression by mediating the expression of FoxA1 and GATA3. While the molecular mechanism by which cathepsin V regulates FoxA1 expression remains to be determined, the results have shown that depletion of cathepsin V in luminal-A breast cancer cells increases the expression of GATA3, which could be reduced by inhibiting PI3K signalling, suggesting that cathepsin V mediates GATA3 expression through PI3K pathway. Additionally, the depletion of cathepsin V increases the phosphorylation of Akt, subsequently increases the pSer9 GSK-3β and ultimately reduces GSK-3β expression, resulting in the reduction of GATA3 degradation. These results suggest that cathepsin V enhance GATA3 stability via the PI3K/Akt-GSK3-β signalling pathway.

Work presented herein has also identified that cathepsin V is present in the nucleus of luminal-A breast cancer cells. Nuclear localisation of cathepsin V appears to be cell-cycle dependent, with elevated levels detected during S and G2/M phases. Subsequent experiments suggest that cathepsin V contains a nuclear localisation signal, which is utilised for trafficking into the nucleus by interacting with the nuclear transport proteins, importins. Additionally, nuclear cathepsin V has a role in the regulation of histone H3 and H4 protein expression by controlling the histone chaperone sNASP, which is an essential protein for histone H3-H4 reservoir formation. The reduction of histone H3 and H4 affects progression through the cell cycle, as evidenced by the increasing cell population at G2 phase in cathepsin V depleted cells. 

In summary, this thesis examines the role of cathepsin V in breast cancer and has identified several novel molecular mechanisms by which this protease facilitates tumour development. These findings suggest that cathepsin V could represent a potential new therapeutic drug target for breast cancer, particularly within the luminal-A subtype. While further research is still needed, the work presented herein suggests that cathepsin V has a fascinating role within tumour biology.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Roberta Burden (Supervisor), Christopher Scott (Supervisor) & Niamh Buckley (Supervisor)