Theses (PhD in Medical and Health Sciences)

Cancer stem cells (CSCs) are a subpopulation of cancer cells with enhanced capabilities of self-renewal, differentiation and tumour initiation, and confer resistance to conventional chemotherapy. Therefore, it is important to elucidate the signalling and regulatory mechanisms in breast CSCs, which will be of useful for the development of effective CSC-targeted therapy. Sphingosine kinases (SPHKs) are conserved lipid enzymes that catalyse the formation of sphingosine-1-phosphate (S1P) through ATP-dependent phosphorylation of sphingosine. Although compelling evidence has indicated the oncogenic roles of SPHK1 in breast cancer, the functions of SPHK1 and its regulatory mechanisms in breast CSCs remain poorly understood. Accordingly, the study presented herein sought to investigate the functions of SPHK1 and its regulatory mechanism in human breast CSCs, as well as whether these would be different in non-stem breast cancer cells. This study first described the hyperactivation of SPHK1/S1P axis in breast CSCs as compared to non-stem breast cancer cells. By utilising mammosphere culture and RNA-interference (RNAi) approaches, this study identified the functional roles of SPHK1 in mediating the survival and proliferation of both non-stem breast cancer cells and breast CSCs derived from HCC38 and MDA-MB-468, of which knockdown of SPHK1 significantly inhibited cell proliferation and induced apoptosis in all tested non-stem breast cancer cells and breast CSCs, while ectopic expression of SPHK1 promoted the survival and mammosphere forming efficiency of breast CSCs. Subsequently, this study discovered interferon (IFN)/STAT1 signalling as key targets of SPHK1 through global proteomic analysis and validated a novel mechanism by which SPHK1 promotes the survival of both non-stem breast cancer cells and breast CSCs via STAT1 suppression. Depletion of SPHK1 was found to induce apoptosis in both non-stem breast cancer cells and breast CSCs in a STAT1-dependent mechanism, along with activation of type I and II IFN signalling. Furthermore, this study reported that SPHK1 inhibitors including FTY720 and PF-543 synergised doxorubicin sensitivity in targeting both non-stem breast cancer cells and breast CSCs. In conclusion, this study identified SPHK1 as a key regulator of cell survival and proliferation in both non-stem breast cancer cells and breast CSCs, which warrants further development for theragnostic purposes in breast cancer treatment.