Abstract
Olfactory receptor OR51E2, also known as a Prostate Specific G-Protein Receptor, is highly expressed in prostate cancer but its function is not well understood. Through in silico and in vitro analyses, we identified 24 agonists and 1 antagonist for this receptor. We detected that agonist 19-hydroxyandrostenedione, a product of the aromatase reaction, is endogenously produced upon receptor activation. We characterized the effects of receptor activation on metabolism using a prostate cancer cell line and demonstrated decreased intracellular anabolic signals and cell viability, induction of cell cycle arrest, and increased expression of neuronal markers. Furthermore, upregulation of neuron-specific enolase by agonist treatment was abolished in OR51E2-KO cells. The results of our study suggest that OR51E2 activation results in neuroendocrine trans-differentiation. These findings reveal a new role for OR51E2 and establish this G-protein coupled receptor as a novel therapeutic target in the treatment of prostate cancer.
Significance Prostate cancer is the second most common cancer in men. Most deaths from prostate cancer are due to the progression of localized disease into metastatic, castration-resistant prostate cancer characterized by increased number of neuroendocrine-like cells. These neuroendocrine-like cells are non-proliferating, terminally differentiated cells. Olfactory receptor OR51E2, also known as a Prostate Specific G-Protein Receptor, is highly expressed in prostate cancer, and its expression correlates with disease progression. Here, we identify and validate novel endogenous ligands for this receptor. We show that activation of OR51E2 by newly-discovered prostate cancer-relevant agonists facilitates cellular transformation, resulting in neuroendocrine trans-differentiation, a characteristic phenotype of castrate resistant prostate cancer. Our results establish this G-protein coupled receptor as a novel and therapeutic target for castration-resistant prostate cancer.
Highlights
Discovery of novel agonists for olfactory receptor OR51E2/PSGR highly relevant to prostate cancer pathology
Activation of OR51E2 receptor by agonist N-acetyl-N-formyl-5-methoxykynurenamine (AFMK) results in release of 19-hydroxyandrostenedione (19-OH AD) from the prostate cancer cells indicating its endogenous production
Activation of OR51E2 receptor by 19-OH AD, AFMK, and propionic acid decreases anabolic and proliferative signals
Activation of OR51E2 receptor by 19-OH AD and AFMK increases markers specific for neuroendocrine trans-differentiation (NEtD)
Ablation of the OR51E2 gene in prostate cancer cells treated with agonist 19-OH AD significantly reduces neuron-specific enolase
INTRODUCTION
G-protein coupled receptors (GPCRs) have emerged as important factors in tumor growth and metastasis 1–3. Several GPCRs, such as the 5HT1c serotonin receptor 4, the M1, M3, and M5 muscarinic receptors 5, and the a1B-ADR adrenergic receptor 6, can function as oncogenes when persistently activated. These GPCRs, which are normally expressed in fully differentiated, post-mitotic neuronal cells, are able to induce cellular oncogenic transformation when introduced to an ectopic environment of proliferating cells and activated by agonist 7,8. In addition to oncogenes and tumor-suppressor genes essential for cancer initiation and progression, autocrine and/or paracrine secretion of GPCR-activating molecules and their downstream signaling events affect tumor growth, survival, and metastasis 1,9–14.
Olfactory receptors (ORs) are the largest family of GPCRs present in the olfactory epithelium but are also found in various ectopic or non-olfactory locations such as prostate, heart, placenta, embryo, erythroid cells, spleen, kidney, gut, tongue, and carotid body 15–17. Some ectopic ORs also play roles in chemotaxis18, muscle regeneration19, blood pressure regulation20, and hypoxia response21. OR51E2, or Prostate Specific G-protein Receptor (PSGR), is one of the most highly conserved and broadly expressed ectopic ORs 22–24. OR51E2 is present in healthy prostate tissue and shows significantly increased expression in prostate intraepithelial neoplasia (PIN), prostate adenocarcinoma (PC), and castration-resistant prostate cancer (CRPC) 25–32.
What is the role of OR51E2 in prostate cancer?
The majority of prostate tumors start as androgen-dependent adenocarcinomas. As localized cancer progresses to a metastatic state, the number of neuroendocrine (NE)-like cells increases, contributing to the development of a highly aggressive form of castrate-resistant prostate cancer (CRPC) known as neuroendocrine prostate cancer (NEPC)33,34. Many clinical studies have demonstrated a correlation between neuroendocrine trans-differentiation (NEtD) and PC progression with poor prognosis 35. Tumor-derived NE-like cells are localized in tumor foci and are non-proliferating, terminally differentiated cells rich in serotonin and positive for NE markers, including neuron-specific enolase (NSE) and chromogranin A (CGA) 35.
The molecular mechanism underlying development of a neuroendocrine phenotype in PC is not fully understood. In the androgen-dependent PC cell line LNCaP, serum deprivation and agents that increase cAMP also increase expression of NEtD markers and genes indicative of neuronal phenotype 36.
In the olfactory system, ORs signal via the canonical cAMP pathway37, and several reports have indicated cAMP-mediated signaling for ectopic ORs38–40. Furthermore, high expression of the OR downstream targets adenylate cyclase 3 and Gaolf was recently identified in prostate tissue, supporting the role of a cAMP-mediated pathway in ectopic OR activation22,41. We hypothesized that agonist-mediated activation of OR51E2 increases cAMP and facilitates cellular transformation, resulting in NEtD. Thus, ectopic expression of this GPCR in proliferating cells and ligand-dependent activation could enable this receptor to function as an oncogene.
Recently, it has been demonstrated that overexpression of OR51E2/PSGR in a PSGR-Pten (Δ/Δ) mouse model accelerates PC development and progression 42. Furthermore, β-ionone, an agonist for OR51E2, decreased proliferation and increased invasiveness of human PC cells 43–45.
In this paper, we aimed to identify biologically relevant OR51E2 ligands using a combination of in silico investigations and experimental validation, and we also set out to study the effects of these ligands on androgen-dependent LNCaP cells. Currently identified OR51E2 agonists include the short-chain fatty acids acetate and propionate 20,46, steroid derivatives, β-ionone 43, and lactate 21. However, it is not known whether activation of OE51E2 by endogenous ligands is involved in PC pathogenesis.
Here, we virtually screened >2,500 metabolites, experimentally validated 55 of these candidates in vitro, and ultimately identified 24 new agonists and 1 antagonist for the human OR51E2 receptor. Among the agonists, we identified 19-hydroxyandrostenedione (19-OH AD)—which is synthesized by aromatase, an enzyme highly expressed in NEPC and CRPC—and N-acetyl-N-formyl-5-methoxykynurenamine (AFMK), a tryptophan and melatonin metabolite. We detected endogenous production of 19-OH AD in the LNCaP cells stimulated with AFMK agonist. The identity of the newly discovered agonists, as well as significant differences in metabolomics signatures in agonist-stimulated PC cells indicative of non-proliferation, prompted us to further investigate their effects on cell viability, cell cycle status, and several NE markers to examine if OR51E2 receptor activation drives NEtD.
RESULTS
In Vitro Validation of Ligands Predicted in Silico
A structural model of OR51E2 was made in silico using Modeller (see Materials and Methods for details). To experimentally validate our virtual library screening (VLS) predictions, we used an in vitro heterologous expression system 47 in which Hana 3A cells transfected with the OR51E2 receptor were stimulated with the top candidate ligands. Responses were subsequently measured with a cAMP-mediated luciferase reporter gene assay. First, a small library of 33 compounds identified as PC-associated metabolites was selected from the Human Metabolome Database HMDB 48. These compounds were identified from previous reports 49–51. Additional compounds previously identified as OR51E2 agonists were also included: 1,4,6-androstatriene-17-β-ol-3-one, 1,4,6-androstatriene-3,17-dione, 6-dehydrotestosterone, and β-ionone43. Thus, a total of 37 compounds were used for the initial, small-scale VLS (Table S4). Two scoring functions, Score and mfScore, were used to predict the best binders. The top 9 compounds from each score list (italicized in Table S4) were then tested in vitro, and the following metabolites were identified as novel agonists for OR51E2 (bold and italics): bradykinin, kojibiose, glycylglycine, L-histidinol N-acetylglutamic acid, and D-alanyl-D-alanine. We also confirmed the previously reported agonist 1,4,6-androstatriene-3,17-dione (Figure S2)43.
Next, a larger library of 2,511 human metabolites from the HMDB was selected for virtual screening and docking into the receptor pocket (Figures 1A and 1B). Here, the top potential ligands (in italics, Tables S1 and S2) were tested in vitro using a biologically relevant concentration range reported in the HMDB, and concentration-response curves were subsequently produced. In total, 55 compounds were tested (9 and 46, from the smaller and larger screens, respectively), and 24 agonists (Figures 1C, and S2 and S3) and 1 antagonist for OR51E2 (Figure 1C) were identified. In each experiment, OR51E2-expressing Hana3A cells were also stimulated with a known agonist, 1 mM propionic acid (PA), so we were able to compare the efficacy of each metabolite relative to PA. Furthermore, potency values (EC50) were determined (Table 1). Glycylglycine was the most efficient agonist, while L-histidinol was the most potent. Diverse metabolites were discovered as novel agonists for OR51E2. Concentration-response curves for metabolites from the large VLS screen that did not activate the receptor are presented in Figure S4.
Some of the newly discovered agonists are: 19-hydroxyandrostenedione (19-OH AD), a hypertensive steroid (vasopressor) secreted by the adrenal gland 52–54, an intermediate in estrogen synthesis from testosterone 55 also found in porcine testes 56 and rat ovarian granulosa cells 57; acetyl-N-formyl-5-methoxykynurenamine (AFMK), a melatonin and kynurenamine metabolite previously reported to be abundant in aggressive PC 58; estradiol 59; adenosine-2,3-cyclic phosphate, a positional isomer of the second messenger 3’,5’-cAMP 60; 8-hydroxyguanine, a marker of DNA damage; α-ketoglutaric acid, an intermediate in the citric acid cycle; urea; glycine; and palmitic acid.
Because in situ estrogen production is an important factor in prostate carcinogenesis, and since the expression of aromatase, the enzyme that synthetizes estrogens from androgens, is increased 30-fold in PC, we decided to further examine the presence and production of 19-OH AD in LNCaP cells61,62.
Endogenous 19-OH AD production upon OR51E2 activation with AFMK
We have developed a liquid chromatography/mass spectrometry (LC-MS/MS) assay for the measurements of 19-OH AD in the cell media. LNCaP cells were stimulated with newly discovered agonist, 250 μM AFMK, for 3 days, and the 19-OH AD was measured in the cell medium (Figure 2). We used CD-phenol free RPMI1640 medium (Figure 2A) and regular RPMI1640 medium (Figure 2B) to estimate production of 19-OH AD. Three times more 19-OH AD was detected when cells were stimulated in CD-phenol free medium (0.83 vs. 0.27ng/mL). No 19-OH AD was detected in unstimulated cells (dotted lines in Figures 2A and 2B).
Metabolomic Signatures of LNCaP Cells Treated with Selected OR51E2 Agonists
In addition to the newly discovered agonists 19-OH AD and AFMK, we also selected the previously identified OR51E2 agonist propionic acid (PA) for metabolomics analysis 46. Cells were incubated with 100 nM 19-OH AD, 250 μM AFMK, and 1 mM PA for 72 h, and non-targeted metabolomics analysis was performed to identify metabolites that differed significantly in the cell lysate and conditioned medium (CM). Agonist treatment resulted in pronounced intra- and extra-cellular changes in metabolomic signatures, as seen in heat maps (Figures S5 and S6, respectively). Differentially expressed features/metabolites identified in each group using the t-test (P < 0.05) and fold change (2-fold or greater) are presented in Figures S5-S10 and Tables S5-S10.
The top 15 differentially expressed extracellular metabolites are presented in Figures 3A, 3B, and 3C. All 3 agonists also produced robust intracellular decreases in amino acids, especially serine and threonine, and also 2 glycolytic intermediates: glucose-6-P and fructose-6-P (Figure 3D, 3E and 3F). Furthermore, agonist treatment resulted in decreases in both methionine and glycine levels. In addition to decreased lactic acid, significant decreases were noted in fumaric, malic, and succinic acids, all intermediates of the TCA. Interestingly, we detected an increased level of phosphoenolpyruvate (PEP) in all agonist-treated samples (fold change analysis, Figures S7B, S8B and S9B). We also observed decreased levels of myoinositol, inosine, adenosine, asparagine, aspartate, and guanosine, which have been previously reported as being depleted in metastatic PC tissue 49, indicating that activation of OR51E2 by agonists in LNCaP cells produces metabolic signatures similar to those observed in human metastatic tissues. Agonist treatment also reduced levels of intracellular urea and spermine/spermidine, in accordance with recently reported data showing reduced spermine during malignant transformation 63. Accordingly, we also detected decreased intracellular levels of ornithine, a precursor of polyamines.
Fold change analysis of the CM revealed significantly increased glutamine (25-, 11-, and 51-fold, for 19-OH AD, AFMK, and PA, respectively), indicating that agonist-treated cells do not have an increased demand for glutamine as highly proliferating cancer cells usually do (Figures S10B, S11B, and S12B). Thus, these results argue for a non- or a low-proliferative phenotype. Decreased levels of docosanoic and decanoic acid and increased levels of asparagine were also prominent in the CM in all 3 treatments (Figure 4G). The pathways most affected, as identified by MetaboAnalyst, were the serine, threonine, and glycine; alanine, aspartate, and glutamine; ketogenesis; arginine and proline; and beta-alanine metabolic pathways (Figure 4F).
Time-dependent Modulation of Cellular Proliferation
Our metabolomics results indicated reduced capacity for anabolic reactions in LNCaP cells following receptor activation, which prompted us to further analyze the effects of OR51E2 agonists on cellular proliferation. Moreover, because melatonin reduces cell proliferation, we examined whether AFMK, a melatonin metabolite, also reduces or inhibits cell proliferation 64. LNCaP cells were stimulated with various concentrations of 19-OH AD and AFMK for 4 days and analyzed every 24 hours using an ATP viability assay. At day 4, both agonists significantly decreased the number of viable cells when compared to the control, non-stimulated cells (Figure 5A). This effect was dependent on cell seeding density; at higher plating densities (40% to 50%), the effect was not observed.
Cell Cycle Arrest
Next, we determined if the decrease in cell viability during agonist stimulation is attributable to increased apoptosis or cell-cycle arrest. Cells were treated with various concentrations of agonists for 3 and 7 days. Both agonists increased the fraction of cells in the G0/G1 phase and decreased the number of apoptotic cells (Figure 5B). Our results are in agreement with previous reports that the majority of cells with a neuroendocrine-like phenotype show signs of resistance to apoptosis 65.
Neuroendocrine Markers
To assess the effect of selected OR51E2 agonists on NEtD, we analyzed transcript levels after 3 and 12 days of stimulation of the following neuroendocrine, epithelial, and receptor genes: NSE; α-methylacyl-CoA racemase (AMACR); keratins 5, 8, and 18; voltage-gated Ca channel a1H (Cav3.2); androgen receptor (AR); and OR51E2 receptor. NSE was used to specifically identify NEtD status, and although LNCaP (being a cell line established from lymph-node metastatasis) cells already express low levels of NSE, treatment with OR51E2 agonists significantly increase levels of the NSE transcript (Figures 5C, 5D, and 5E). These results correlate well with the increased intracellular level of PEP detected in the metabolomic analysis (Figures S7B, S8B and S9B), since the glycolytic enzyme enolase catalyzes PEP synthesis. a-AMACR is an enzyme essential for isomerization of branched-chained fatty acids and is present at very low levels in healthy prostate and increased in PC and NE-like cells 66. Since NE-like tumor cells express AMACR, we investigated whether activation of OR51E2 also increased AMACR levels. Indeed, AMACR levels were significantly increased after 3 and 12 days of agonist stimulation (Figures 5C, 5D, and 5E).
Furthermore, 19-OH AD decreased the AR transcript after 12 days, and although AFMK also showed a similar trend, it did not reach statistical significance (Figure 5D). Normal basal prostate epithelial cells are positive for K5, and expression of K5 is also associated with the epithelial-to-mesenchymal transition during tumor progression and metastasis 67. K5 and K8 were not detected in the agonist-stimulated cells (data not shown). A significant decrease in K18, a luminal secretory marker, was found after 12 days of stimulation with 100 nM 19-OH AD (Figure 5D). Although Ca+2 entry through the voltage-gated calcium channel α1H (Cav3.2) was previously reported to be involved in NEtD of LNCaP cells when cultured in steroid-free conditions, we did not detect changes in its transcript levels (data not shown) 68.
OR51E2 Knock-out Confirms NE-like Phenotype Upon Receptor Activation
To confirm receptor involvement in the agonist-mediated increase of NSE, OR51E2 was deleted using a CRISPR-Cas9 method (Figure 5F). We designed 3 gRNAs to target Cas9 to the OR51E2-gene and generated a lentiviral sgRNA-Cas9 vector to deliver gRNA into cells. The efficiency of each gRNA was measured, and we observed that by using sgRNA #1, OR51E2 was abrogated in 80% of cells. These OR51E2-knockout cells were exposed to 1 μM 19-OH AD for 3 days and analyzed for the presence of specific markers (Figures 5G and 5H). A statistically significant decrease of NSE in OR51E2-knockout cells in comparison with control cells was observed (from 0.579 ± 0.043 in control to 0.377 ± 0.04, P < 0.05, n = 4 biological replicates, Figure 5G), confirming that increased NSE during stimulation with agonists is at least partially a receptor-mediated phenomenon (Figures 5C and 5D).
DISCUSSION
As the number of ectopic olfactory receptors associated with diverse pathological states continues to increase 15,17, the implications and significance of these receptors will be greatly enhanced by receptor “deorphanization” (i.e., defining the ligands). Previously, we successfully identified novel ligands for mouse OR using a similar in silico approach with VLS 69. Here, we present a highly successful approach of combining in silico and in vitro analyses to identify novel biologically relevant ligands for the human ectopic OR OR51E2. This method can be used to elucidate ligand specificities of other ectopic ORs. Once identified, these new ligands can help define the role and function of ORs in cancer and other diseases.
Among the newly discovered metabolites identified as OR51E2 agonists, several were previously reported to be associated with PC, including bradykinin, kojibiose, glycylglycine, N-acetylglutamic acid, and D-alanyl-D-alanine 49. Thus, our results indicate that these metabolites are likely endogenous agonists. New agonists with known biological roles were also discovered: epitestosterone, known to be a major metabolite of androstenedione and testosterone 70; and androstanedione (also known as 5α-androstane-3,17-dione), an intermediate in steroid synthesis71.
In addition to these agonists, we also identified a previously under-reported metabolite of the complex steroid biosynthetic network, 19-hydroxyandrost-4-ene-3,17-dione (19-OH AD) 55,72,73. 19-OH AD is produced by aromatase P450 (CYP19A1), which catalyzes the irreversible aromatization of the androgens androst-4-ene-3,17-dione and testosterone and their consequent conversion to estrogens (http://www.brenda-enzymes.org/enzyme.php?ecno=1.14.14.14). We detected this testosterone metabolite in agonist-stimulated prostate cancer cells. These results demonstrate that 19-OH AD is actively produced by cancer cells when the OR51E2 receptor is activated. Thus, we demonstrate that 19-OH AD is an endogenous agonist produced by activation of OR51E2 in prostate cancer cells.
Aromatase is increased 30-fold in metastatic PC 62, and aromatase-knockout mice have a reduced incidence of PC following exposure to testosterone and estrogen, indicating that aromatase metabolites, mainly 19-OH AD and estradiol, are likely involved in prostate carcinogenesis. Results from our study demonstrate that 19-OH AD is a potent OR51E2 agonist (EC50= 1.5−10 M) and support the notion that increased in situ estrogen production via 19-OH AD is an important factor in PC 61.
AFMK is a metabolite of melatonin 74. Previous studies demonstrated that melatonin reduces proliferation of LNCaP cells, leading to NEtD, and the phenotype was not reversed by melatonin receptor antagonists, suggesting that additional receptors may be mediating this process 64,75. Our results demonstrate that OR51E2 is a receptor for AFMK, a melatonin metabolite, and although its EC50 is in the μM range, much higher than reported blood concentrations (<65 pM) 76, the local tissue concentration may reach μM and mM levels as has been recently shown for keratinocytes 74. An additional source of AFMK might be a tryptophan metabolic pathway 77. Tumors produce high levels of tryptophan and kynurenic acid metabolites 78. Significant amplification of tryptophan-2,3-dioxygenase (EC 1.13.11.11) TDO2, which catalyzes the oxidation of L-tryptophan to N-formyl-L-kynurenamine, was observed in NEPC and PC 79,80. Thus, in more advanced stages of PC, AFMK production may be increased via this tryptophan metabolic pathway. Recently, this pathway was shown to regulate the immunosuppressive microenvironment of various tumors81.
We also identified bradykinin as an agonist for OR51E2. In general, kinins are released during the inflammatory reaction and they are involved in angiogenesis and tumorigenesis 82,83. Our results indicate that the activation of OR51E2 by bradykinin can also happen in the early stages of PC, when the inflammatory milieu is predominant. Prostatic secretions of PC patients have elevated levels of human kallikrein 2 84, which produces bradykinin and thus stimulates proliferation of androgen-independent PC cells in later stages of PC 85.
The OR51E2 antagonist 13-cis RA is an endogenous component of human serum, and many of its actions can be explained by isomerization to all-trans RA and 9-cis RA, which both act via retinoid receptors. However, since 13-cis RA does not have potent gene regulatory activity, additional pathways via membrane receptors have been proposed to explain its pharmacological and anti-inflammatory actions 86. Our results demonstrate that 13-cis RA acts via the OR51E2 receptor when expressed heterologously.
OR51E2 receptor activation by 19-OH AD, AFMK, and PA induced pronounced metabolic reprograming of LNCaP cells, with the most significant changes being decreased intracellular serine and threonine levels. Because metabolism of these amino acids includes one-carbon metabolism, which provides cofactors for biosynthetic reactions in highly proliferating cells, intracellular depletion may indicate a general decrease in anabolic reactions 87. Furthermore, an intracellular decrease in aspartate, which is normally required for protein, purine, and pyrimidine synthesis, and an increase in the CM indicate that agonist-activated LNCaP cells are not preparing for proliferation. We also detected decreased intermediates of glycolysis (glucose-6-phosphate and fructose-6-phosphate) in activated cells. Agonist treatment decreased intracellular lactate, suggesting a slower rate of glycolysis. An intriguing result was the increased intracellular level of phosphoenolpyruvate (PEP). We also found increased NSE transcription for the glycolytic enzyme enolase, which catalyzes the formation of PEP, indicating predominance of the PEP-forming reaction.
NSE is not only a marker of neuronal differentiation and maturation characteristic of neurons and neuroendocrine cells 88, but it also has an important role in synaptogenesis 89 and is reported to be stable in the high-chloride environment characteristic of neurons, in which it reaches a concentration of 2% to 3% of the soluble protein 90. Taken together, these results indicate that receptor activation results in a neuronal-like phenotype of LNCaP cells.
Cystine was increased in the medium after 19-OH AD and AFMK treatment (Tables S12 and S13). In healthy cells, cystine is transported into cells and reduced to cysteine, which can then be utilized for synthesis of gluthatione, a protective antioxidant. As a consequence of rapid cell growth during tumorigenesis, the production of reactive oxygen species increases, providing a proliferative signal for glutamine to enter the cell and, after deamidation, condense with cysteine to form a precursor of glutathione. However, in our experiments, the medium, but not the cells, showed increased levels of glutamine and cysteine, indicating a reduction in protective oxidative and proliferative signals in agonist-stimulated cells. The alanine/aspartate/glutamine pathway is the most affected biochemical pathway during NEtD of LNCaP cells induced by steroid-reduced medium, which corroborates our results 91 (Figure 4F). In cancer-associated fibroblasts, asparagine and aspartate are involved in glutamine synthesis92, and our experiments showed decreased intracellular levels of these amino acids, suggesting increased use for intracellular synthesis of glutamine. These results might also indicate a decreased cellular influx of asparagine, since it is abundant in CM. Flux studies will be necessary to determine the exact relationship between glutamine synthesis and transport in PC cells upon receptor activation.
To explain the role of OR51E2 in PC, we propose the following model: agonist stimulation generates new cells through asymmetric division and gradually increases the subpopulation of terminally differentiated cells expressing neuroendocrine markers (see graphical abstract).
NE-like cells from PC are characterized by increased expression of NSE and AMACR and decreased expression of K18 and AR 34,93,94. Increased expression of NSE and AMARC and decreased expression of AR and K18 following 19-OH AD and AFMK treatment demonstrate that these OR51E2 agonists induce a neuroendocrine phenotype. We confirmed that this effect is receptor-mediated, as treatment of OR51E2-knockout LNCaP cells significantly reduced the NSE and AMACR transcript levels.
Cell proliferation and differentiation have an inverse relationship, and terminal differentiation coincides with proliferation arrest and exit from the division cycle 95. Our results demonstrate that agonist treatment during the first 3 days induces cell proliferation at a rate similar to control cells, but after 4 days the viability of these cells, as measured by ATP content, was significantly reduced. Our results also demonstrate that receptor activation results in a new subpopulation of cells that undergoes G0/G1 cell cycle arrest and has decreased DNA synthesis, which is concordant with the results from our metabolomics analysis. Cellular senescence is an irreversible growth arrest, and senescent cells actively suppress apoptosis 96. We found that agonist treatment decreases the fraction of apoptotic cells, indicating that growth arrest likely induces cellular senescence. Future studies are needed to confirm the irreversibility of this process.
Furthermore, recent whole-exome sequencing of NEPC and CRPC showed an overlap in genomic alterations, and in both demonstrated increased amplification of the OR51E2 gene, supporting our hypothesis that this receptor contributes to the NE-phenotype of PC 79,80.
Prostatic adenocarcinomas typically contain foci of non-proliferating NE-like cells that increase in number as cancer progresses 97. Although these cells are non-mitotic, proliferating carcinoma cells have been found in their proximity, suggesting that the non-proliferating NE-like cells likely provide paracrine stimuli for growth of the surrounding carcinoma cells 98,99. Our results demonstrate that activation of OR51E2 by newly-discovered PC-relevant agonists induces and/or facilitates cellular transformation, resulting in NEtD, a characteristic phenotype of CRCP. This indicates that activation of OR51E2 in PC might contribute to development of non-proliferating foci. Our data demonstrate that activation of OR51E2 results in NEtD and establish this GPCR as a novel and therapeutic target for NEPC and CRPC.
Author Contributions
Conceptualization, T.A.; Methodology, T.A., J.B., M.J.M., I.S. and S.Y.K.; Investigation, T.A., J.B., M.J.M., S.Y.K., I. S. and S. L.; Writing, T.A.; Writing, Review, and Editing, T.A., J.B., M.J.M., S.L., and H.M.; Resources, H.M.; Supervision, T.A. and H.M.
Acknowledgements
We would also like to thank Professors J. Heitman, J. Huang, M. P. Dewhirst and J. A. Chi, and Drs. C. de March and T. Zhou for their critical review on the manuscript. This study was supported by the NIDCD to H. Matsunami. The authors declare no conflict of interest.