Cross-regulation of prosaposin and sphingosine kinase signalling in prostate cancer

Abstract

Sphingosine kinase 1 (SPHK1) is an oncogenic enzyme that is upregulated in a wide range of human tumours and is associated with cancer progression. The product of SPHK1's activity, sphingosine-1-phosphate (S1P) enhances metastatic potential by promoting cancer cell migration and invasion. Our group has previously reported that in vitro SPHK1 inhibition potentiates the effects of docetaxel, a key treatment of prostate cancer (PCa) , implicating a potential therapeutic role of SPHK1. The work presented in this thesis investigated the mechanism for docetaxel-induced apoptosis in PC-3 cells and demonstrated the involvement of a SPHK1-dependent and -independent pathway. The dose-dependent inhibition of SPHK1 by docetaxel led to an initial loss of enzyme activity followed by a decrease in SPHK1 expression. Further, in prostate cancer cell models, SPHK1 inhibition had a significant chemosensitising potential leading to a 4-fold reduction in the effective dose of docetaxel. To link SPHK1 upregulation in prostate cancer cells to potential targets, this thesis investigated the nature of the interaction between SPHK1 and prosaposin signalling pathways. Prosaposin is a neurotrophic secreted protein involved in cancer progression and chemoresistance. Recent reports suggest that prosaposin activates SPHK1 in cancer cells. The work presented in this thesis demonstrates that prosaposin knockout mice exhibited a significant reduction in SPHK1 activity in the prostate and seminal vesicles. In prostate cancer cell lines, SPHK1 activity correlated with the amount of secreted, but not intracellular prosaposin. This suggests a role of the prosaposin/G-protein coupled receptor (GPCR) signalling. Blocking prosaposin in prostate cancer cells induced a signi cant decrease in SPHK1 activity and expression. Conversely, exogenous prosaptide TX14A, derived from the trophic sequence of saposin C, enhanced SPHK1 activity and expression. In turn, SPHK1 activation is essential for prosaposin secretion, but not for its expression. Increased SPHK1 expression or exogenous S1P triggered prosaposin secretion, while inhibition of SPHK1 using speci c siRNA or pharmacological inhibitor drastically decreased prosaposin secretion. Both prosaposin and SPHK1 signalling pathways were shown to be involved in the production of cytokines and angiogenic factors such as plasminogen activator inhibitor-1 (PAI-1), macrophage migration inhibitory factor (MIF) and pentraxin-3 (PTX3). These findings were extended to the clinical situation, and circulating prosaposin levels were determined in prostate cancer patients and compared with healthy controls. Increased prosaposin serum levels correlated with tumour stage. The work in this thesis has shown for the rst time the cross-regulation of prosaposin and SPHK1 in prostate cancer. The mechanism and physiological relevance of this cross-regulation in prostate cancer cells has been investigated and found to have significant therapeutic and biomarker potential in advanced prostate cancer.