Sphingolipids and Prostate Cancer: complex molecules for a complex disease

Prostate cancer (PCa) is the second most incident cancer in men worldwide, and despite vast amounts of research it remains one of the major epidemiological burdens in oncology. The prostate gland is regulated by male androgens that activate the Androgen Receptor (AR), a transcription factor and a master regulator of several different signaling pathways, contributing towards normal development of the organ. In PCa, these hormones exert an important role in the onset and progression of the disease, and therefore have been the main therapeutic target for PCa patients. Anti-androgens are a common therapy in prostate cancer (PCa), targeting Androgen Receptor (AR) signaling or hormone production; however, these therapies fail due to selection of highly aggressive castration-resistant PCa cells (CRPCa). Therefore, novel and more efficacious therapeutics and better molecular understanding of this disease are extremely important to help PCa patients. In this Thesis, I explored the potential of ceramide-centric therapeutics in PCa and the mechanisms underlying the intersection between AR and sphingolipid metabolism. Ceramides are sphingolipids, a class of bioactive lipids that play structural and signaling roles in cells and have been shown to possess anti-tumor properties by triggering cell death of tumor cells. Because ceramides are hydrophobic, delivery is problematic. Enter, Ceramide nanoliposomes (CNL); these nanoparticles have now demonstrated great promise in multiple different tumor models, and more importantly in a completed multi-institutional Phase 1 NCI trial (NCT02834611). While ceramides have been speculated as highly selective apoptotic lipids, CNL is the only delivery modality to have completed Phase 1 testing, demonstrating enhanced pharmacokinetic (PK) parameters and minimal toxicities in patients.
Here, we demonstrate that elevating endogenous ceramide levels with administration of exogenous Ceramide Nanoliposomes (CNL) was efficacious in AR-negative prostate cancer cell lines with limited efficacy in AR-positive cells. This effect is mediated through reduced de novo sphingolipid synthesis in AR-positive cells. We show that anti-androgens elevate de novo generation of sphingolipids via SPTSSB, a rate-limiting mediator of sphingolipid generation. Moreover, pharmacological inhibition of AR increases the efficacy of CNL in AR-positive cells through de novo synthesis, while SPTSSB knockdown limited CNL’s efficacy in AR-negative cells. Alluding to clinical relevance, SPTSSB is up-regulated in advanced PCa patients after anti-androgens treatment. We also show that AR stimulation leads to altered expression of several sphingolipid metabolic enzymes that can be explored in future studies. Moreover, we determine that the anti-tumor effect of C6-ceramide in PCa cells is due to a specific chiral conformation of its lipid structure, suggesting that the bioactive lipid itself as well as its chiral-specific metabolism is crucial for the treatment efficacy.
These findings emphasize the relevance of AR regulation upon sphingolipid metabolism, support the potential of CNL as a PCa therapeutic, and open new avenues to be pursued concerning the importance of the intersection between sphingolipid metabolism, androgens, and AR in PCa.