Intercellular signalling in the prostate cancer tumour microenvironment: implications for disease progression

Advanced prostate cancer (PCa) is incurable and presents a significant clinical challenge. Once metastasised, crosstalk between PCa cells and other cells within the tumour microenvironment (TME) is critical for tumour growth. Hence, disrupting tumour-stroma interactions has been highlighted as a therapeutic strategy to combat disease progression. However, the intercellular signalling mechanisms underpinning PCa cell growth within the TME are still incompletely understood. Enhanced signalling via the bone morphogenetic protein-6 (BMP-6) and mammalian target of rapamycin complex 1 (mTORC1) pathways is frequently observed in advanced PCa tumours. Whilst the role of BMP-6 in regulating PCa cell growth remains an area of intense debate, the growth-promoting effects of mTORC1 is well-established. Nonetheless, targeting mTORC1 has had limited clinical success in PCa, hence further understanding of the upstream and downstream mechanisms by which mTORC1 regulates cell growth is crucial. Therefore, this thesis focuses on understanding the ways in which these two major growth factor signalling pathways regulate tumour growth and the implications for devising future therapeutic strategies.
Here, I characterise the effect of BMP-6 on the growth of several PCa cell lines and identify a role for the androgen receptor (AR) and retinoblastoma (Rb) in mediating the growthsuppressive effects of BMP-6. I show that treatment with BMP-6 inhibits AR activity, Rb phosphorylation and cell growth in the androgen-responsive LNCaP cell line, but not in the androgen-insensitive derivative cell line C4-2B. Furthermore, I propose that alterations in this pathway enables cells to escape the growth-inhibitory effects of BMP-6 in advanced PCa and provide support for this model using existing clinical data. I also examine the effects of altered mTORC1 signalling on the secretion of extracellular vesicles (EVs) - membrane-bound intercellular messengers that carry multiple molecular cargoes and can reprogram target cell behaviour. I show that decreased mTORC1 activity alters the type of EVs released from two different PCa cell lines in a manner reminiscent of an event recently characterised by our lab in colorectal cancer cells, termed an ‘exosome switch’, and that these EVs alter the growth of recipient PCa cells in vitro. Furthermore, I reveal that androgen-deprivation represents another form of microenvironmental stress that regulates a similar event in PCa cells and identify increased secretion of AR-positive EVs as a novel characteristic of the switching response. Furthermore, I provide evidence to suggest that these ‘switched’ EVs include a heterogeneous population of vesicles consisting of at least two different EV subtypes. Finally, I show that upstream of mTORC1, the amino acid transporter PAT4 acts, in part, to mediate the stimulatory effects of androgens on mTORC1 activity and PCa cell growth, and I propose a further role for PAT4 in regulating the exosome switching event in PCa.
In summary, my research reveals novel ways in which signalling via the BMP-6 and mTORC1 pathways regulate PCa cell growth and highlights potential clinical targets and therapeutic avenues for the treatment of patients with advanced PCa.

Advisor: Goberdhan, D