An estimated 1 in 7 men will develop prostate cancer (PCa) with many progressing to advanced castrate-resistant disease. Unlike other tissue types, normal prostate cell growth and development is heavily dependent on the androgen receptor (AR) signaling pathway. While the introduction of novel AR antagonists for clinical treatment has improved outcomes, most castration-resistant prostate cancer (CRPC) patients ultimately develop resistance to these therapies. A need exists to better understand the mechanisms that control the transition of prostate cells from a hormone-dependent to castrate-resistant state.
Androgens strongly influence the metabolic state of PCa cells to favor sustained cellular growth. We hypothesize there are effectors working in conjunction with AR to coordinate alterations to androgen-dependent metabolism that are linchpins in the orchestration of the transition to CRPC. Leading candidates are members of phosphoinositol (PI) pathways, which have a high frequency of alteration in PCa (i.e phosphoinositide 3-kinase (PI3K)). Herein we explore a family of poorly understood lipid kinases called the type II phosphatidylinositol-5-phosphate 4-kinases (PI5P4Ks) and predict them to be critical regulators of cancer cell survival. PI5P4Ks are druggable targets that act by phosphorylating the lipid phosphatidylinositol-5-phosphate (PI 5-P) at the 4 position of the inositol ring to generate phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2; PIP2). We implicate the three PI5P4K isoforms (PI5P4Kα, PI5P4Kβ, and PI5P4Kγ) encoded by the genes PIP4K2A, B, and C, to be important regulators of cancer metabolism that play a role in the maintenance of prostate biology and oncogenesis.
Analysis of transcript data revealed expression of PIP4K2A, B, and C in primary PCa patient samples, which was correlated with an AR activation gene signature and hotspot tumor suppressor deletion. As well, isoform expression was assessed for differential expression in relation to an integrated neuroendocrine prostate cancer mRNA score (TCGA; n=333). PI5P4Kα and PI5P4Kβ protein was detected in primary and advanced prostate cancer using optimized antibodies of patient tissue TMAs (n= 72). Using in vitro LNCaP cell models, siRNA knockdown systems were tested to evaluate the molecular consequence of targeting PIP4K2A and PIP4K2B in androgen-dependent systems. Stable knockdown using fluorescently labeled lentiviral shRNA constructs significantly reduced proliferation of shPIP4K2 treated cells. As well, we have produced a prostate-specific PI5P4K knockout mouse model by expressing probasin-driven Cre in a homozygous 129/SvEv Pip4k2aflx/flx murine strain. Finally, implementation of a discovery-based metabolomic platform (Metabolon HD4) was used to profile the overall shift in metabolite species that results from downregulating the expression of PIP4K2A in androgen-dependent cell models.
In summary, we have developed novel insights into the role of a family of noncanonical PI kinases in prostate biology. There are a growing number of PI3K/AKT inhibitors being tested in combination with androgen deprivation therapy in clinical trials, but there is still almost nothing known about the potential crosstalk of the greater PI kinase network. These data convincingly implicate a fundamental role for PI5P4Ks in PCa androgen signaling and metabolism, as well as lay the foundation of phenotypic understanding of what PI5P4K is responsible for in the prostate.
Citation Format: Joanna Triscott, Matteo Benelli, Verena Sailer, Davide Prandi, Brooke Emerling, Francesca Demichelis, Lewis Cantley, Mark A. Rubin. Towards understanding noncanonical phosphatidylinositol kinases in the maintenance of prostate metabolism [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A078.