Androgen Receptor Signaling and Metabolic and Cellular Plasticity During Progression to Castration Resistant Prostate Cancer

Abstract

Metabolic reprogramming is associated with re/activation and antagonism of androgen receptor (AR) signaling that drives prostate cancer (PCa) progression to castration resistance, respectively. In particular, AR signaling influences the fates of citrate that uniquely characterizes normal and malignant prostatic metabolism (i.e., mitochondrial export and extracellular secretion in normal prostate, mitochondrial retention and oxidation to support oxidative phenotype of primary PCa, and extra-mitochondrial interconversion into acetyl-CoA for fatty acid synthesis and epigenetics in the advanced PCa). The emergence of castration-resistant PCa (CRPC) involves reactivation of AR signaling, which is then further targeted by androgen synthesis inhibitors (abiraterone) and AR-ligand inhibitors (enzalutamide, apalutamide, and daroglutamide). However, based on AR dependency, two distinct metabolic and cellular adaptations contribute to development of resistance to these agents and progression to aggressive and lethal disease, with the tumor ultimately becoming highly glycolytic and with imaging by a tracer of tumor energetics, ¹⁸F-fluorodoxyglucose (¹⁸F-FDG). Another major resistance mechanism involves a lineage alteration into AR-indifferent carcinoma such a neuroendocrine which is diagnostically characterized by robust ¹⁸F-FDG uptake and loss of AR signaling. PCa is also characterized by metabolic alterations such as fatty acid and polyamine metabolism depending on AR signaling. In some cases, AR targeting induces rather than suppresses these alterations in cellular metabolism and energetics, which can be explored as therapeutic targets in lethal CRPC.

Keywords: aerobic glycolysis; androgen receptor; castration-resistant prostate cancer; fatty acid metabolism; metabolic reprogramming 18F-FDG; mitochondria; neuroendocrine.

Figure 1

Metabolic reprogramming is involved in malignant transformation of prostatic cells. (A) Normal prostate epithelial cells express zinc transporter ZIP1 facilitating intracellular accumulation of zinc ion, which contributes to inhibition of m-aconitase (ACO2) at mitochondria. This inhibition results in truncation of tricarboxylic acid cycle (TCA) cycle and release of citrate to the extracellular space. Citrate production is supported by increasing the substrate pools for citrate synthase, acetyl-CoA (Ac-CoA) and oxaloacetic acid (OAA) at the mitochondria. OAA is supplied as the result of action of mitochondrial aspartate aminotransferase (GOT2) on L-aspartate. The level of mitochondrial acetyl-CoA is associated with increased expression of pyruvate dehydrogenase E1 component subunit alpha (PDHE1α) of pyruvate dehydrogenase complex (PDH). From bioenergetic point of view, normal prostatic cell is supported by aerobic glycolysis. (B) Marked decrease in zinc levels due to depletion of ZIP1 represents an essential early event in the development of PCa malignancy, which relieves m-aconitase to establish a complete TCA cycle. These metabolic alterations are functionally related to low citrate level and the general low avidity of ¹⁸F-FDG in primary PCa. Fatty acids (FA) are incorporated through CD36, followed by CPT1-mediated entry into mitochondria to serve as the substrate for fatty acid oxidation (FAO). L-Glutamine also serves as the precursor of TCA cycle intermediates after conversion into L-glutamate. ATP-citrate lyase (ACLY) cleaves citrate to produce acetyl-CoA to serve as the substrate for fatty acid synthase (FASN). (C) Further malignant transformation promotes glycolysis (through increased expression of glycolytic enzymes). While lipogenic trait is enhanced, multiple combinations of/all energy source pathways are theoretically available at this stage. Therefore, it is important to determine which metabolic pathway dominates for survival of given tumors for the future metabolism-based precision therapy.

Figure 2

Androgen receptor (AR) status defines four distinct groups of prostate cancer (PCa). Four distinct groups of PCa display the resistance mechanism to anti-AR therapy. AR signaling supports survival and growth of PCa and suppresses transdifferentiation into neuroendocrine. (1, 2). Loss of AR signaling derepresses expression of NE gene signatures required for NE phenotypes (3). Double-negative PCa bypasses AR requirement without NE phenotype (4).

Figure 3

Immunohistochemical images for the expression of androgen receptor (AR), prostate-specific antigen (PSA), and neuroendocrine PCa (NEPC) markers. (A) Expression of full-length AR (AR-FL) and PSA in castration-sensitive (CS) PCa and AR-FL and AR-V7 in CRPC. Note uniform nuclear staining of AR-V7. CS and CRPC correspond to group (1) and (2) in Figure 2 , respectively. (B) Expression of specific markers for each CRPC type. CRPC adenocarcinoma are positive for AR and its transcriptional target PSA but negative for NEPC markers chromogranin A (CHGA) and synaptophysin (SYP). NEPC is positive for NEPC markers and negative for AR and PSA. DNPC is negative for AR, PSA, and NEPC markers (image courtesy of Dr. Colm Morrissey at University of Washington). Representative images for the data in Figure 2 (2: Adeno-CRPC, 3: NEPC, 4: DNPC). Scale bar=20 µm.

Figure 4

Molecular basis of actions of positron emission tomography (PET) radiotracers in prostate cancer (PCa). Acetate is converted to acetyl-CoA (Ac-CoA) which serves as a substrate for FASN to produce fatty acids (FA). After enzymatic modification by choline kinase (CK), ¹¹C-choline is incorporated into cell membrane as the form of phosphatidylcholine. After incorporation into cell, ¹⁸F-FDG undergoes phosphorylation by hexokinase (HK) and accumulates as the form of ¹⁸F-FDG-6-P. Mitochondrial membrane potential (ΔΨ_m) drives accumulation of 4-[¹⁸F]fluorobenzyl triphenylphosphonium (¹⁸F-FBnTP) at mitochondria. ¹⁸F signal is indicative of respiration-competent functional mitochondria. Binding of ligand dihydrotestosterone (DHT) activates full-length AR as a transcriptional factor to upregulate target genes such as PSMA. Accordingly, the presence of full-length AR can be monitored by ¹⁸F-FDHT. It is noteworthy that constitutively active AR variant fails to bind to ¹⁸F-FDHT. Accordingly, ¹⁸F-FDHT negativity does not necessarily mean tumors are negative for any form of AR. ⁶⁸Ga-labeled antagonistic ligand for PSMA can be used to monitor tumors with active AR signaling.