Cytokine dysregulation
Dysregulation of cytokines secreted by cancer cells, including chemokines, growth factors, pro-angiogenic factors, anti-angiogenic factors, adipokines, soluble receptors and extracellular proteases, and their associated stroma are associated with drug resistance in many diseases (Jones, Huang et al., 2016). PCa cells express autocrine interleukin-6 (IL-6), which is a frequently studied pro-inflammatory cytokine with respect to Enz resistance. IL-6 activates the Janus kinase (JAK)/STAT3 pathway, which is associated with prostate tumour growth and progression. The IL6-STAT3 axis was previously validated as a mediator of resistance to Enz, and the STAT3 inhibitor AG490 reverses ENZ resistance in CRPC (Culig and Puhr, 2018 , Canesin, Krzyzanowska et al., 2020 , Liu, Zhu et al., 2014). In addition, Enz increased high mobility group box 1 (HMGB1) levels promote tumour-associated macrophage (TAM) recruitment and polarization. Activated TAMs secrete IL-6 to enhance Enz-induced NED and directly facilitate HMGB1 transcription via STAT3. Hence, blocking the feedback loop between neuroendocrine differentiation factors and TAMs by targeting IL-6-STAT3 with tocilizumab and HMGB1 silencing may be a potential method for overcoming Ent resistance (Wang, Peng et al., 2018). Leukaemia inhibitory factor (LIF) and oncostatin M (OSM) are IL-6-like cytokines that are increased in the prostate epithelial cells of cancer patients. LIF activates STAT3 to promote CRPC and NED, while OSM induces morphological changes, the EMT, and migration of PCa cells via the JAK/STAT3 pathway. Similarly, IL-8 and IL-11 promote PCa cell progression through STAT3 signalling (Canesin, Krzyzanowska et al., 2020).
IL-23 is a driver of CRPC that is generated by myeloid-derived suppressor cells (MDSCs). MDSC infiltration and IL-23 were increased in blood and tumour samples from CRPC patients. IL-23 directly acted on the pSTAT3-RORγ axis to drive AR and its splicing variants and downstream target gene transcription. Combined, anti-IL-23 and Enz can effectively inhibit AR activity and induce tumour cell apoptosis. Thus, blocking MDSC recruitment to the tumour or inhibiting IL-23 can be potential therapeutic strategies for Enz-resistant CRPC patients (Calcinotto, Spataro et al., 2018).
The C-X-C motif chemokine ligand 12 (CXCL12) and its receptor CXCR4 are overexpressed in prostate cancer cells and tissues and are associated with tumour progression. The intracellular CXCL12γ isoform was found to be expressed in CTCs of m-CRPC patients and PCa cells with neuroendocrine phenotypes. The overexpression of CXCL12γ induced CSC and neuroendocrine phenotypesvia CXCR4-mediated protein kinase C/NFκB signalling, leading to tumour progression and drug resistance (Jung, Cackowski et al., 2018). The seven-transmembrane G protein–coupled chemokine receptor CXCR7 was the most upregulated gene in Enz-resistant CRPC and was directly regulated by AR. Elevated CXCR7 interacted with β-arrestin 2, which recruited mitogen-activated protein kinase (MAPK) proteins to phosphorylate extracellular regulated protein kinase (ERK) 1/2. MAPK/ERK inhibitors have been shown to be effective in suppressing Enz-resistant CRPC. Moreover, macrophage migration inhibitory factor (MIF), a ligand for CXCR7, induced the expression of cell cycle genes by activating the AKT pathway. The MIF/CXCR7/AKT signalling pathway facilitated CRPC growth and metastasis independent of the CXCL12/CXCR4 axis. In addition, interleukin-8 (IL-8) secretion and vascular endothelial growth factor (VEGF) were upregulated by CXCR7, leading to tumour angiogenesis. CXCR7 also has been reported to interact with the epidermal growth receptor (EGFR) and stimulate increased levels of phospho-EGFR (p-EGFR). The effect of the combination of CXCR7 inhibitors and Enz was manifested by significant growth inhibition of PCa cell line xenografts and PDX models of CRPC (Li, Fong et al., 2019 , Rafiei, Gui et al., 2019 , Luo, Azad et al., 2018). Similarly, other cytokines, such as fibroblast growth factor, granulocyte-macrophage colony-stimulating factor, and IL-10, were found to be elevated in the circulation of mCRPC patients resistant to Enz (Pal, Moreira et al., 2019).
MicroRNAs and Enz resistance
MicroRNAs (miRNAs) are short endogenous RNA molecules consisting of 19-25 nucleotides that regulate gene expression after transcription. It has been reported that miRNAs function as oncogenes or tumour suppressors in cancer. In particular, miRNAs have been shown to be involved in the regulation of cellular responses to drugs (Si, Shen et al., 2019). In CRPC, miR-346, miR-361-3p, and miR-197 have been proven to augment AR activity via a novel mechanism by which a 6.9-kb AR is directly associated with the 3′UTR to stabilize the transcripts, which are increased after long-term Enz treatment (Fletcher, Sulpice et al., 2019). MicroRNA-194 (miR-194) functions as a novel post-transcriptional regulator of trans-differentiation in prostate cancer and is negatively correlated with signal-axis AR. Notably, miR-194 trans-differentiated PCa cells into a neuroendocrine-like cell by targeting FOXA1, which may contribute to Enz resistance (Fernandes, Toubia et al., 2019).
The loss of miR-101 or miR-27a during the progression of PCa contributed to the upregulation of orphan nuclear receptor COUP-TFII expression, which in turn upregulated the expression of the forkhead domain transcriptional factor FOXM1 and structural protein of kinetochore CENPF, two of the most important oncogenes in PCa. The expression of miR-101 or miR-27a was drastically decreased in Enz-resistant clones, while overexpression of miR-101 or miR-27a increased the efficacy of Enz treatment (Lin, Kao et al., 2016). The expression of miR‐30c‐1‐3p/miR‐103a‐2‐5p was significantly downregulated in PCa cells. MiR‐30c‐1‐3p/miR‐103a‐2‐5p overexpression inhibited the expression of AR‐V7, which was directly bound to the AR‐V7 3′‐UTR, suppressing CRPC cell proliferation. Importantly, the effects of miR‐30c‐1‐3p/miR‐103a‐2‐5p overexpression was significantly reversed by AR‐V7 overexpression upon Enz treatment (Chen, Yao et al., 2019). Targeting miR‐30c‐1‐3p/miR‐103a‐2‐5p may be a selective method of Enz-resistant therapy.
MiR-644a is a broad-spectrum cellular pathway regulator that downregulates the expression of multiple tumour microenvironment drivers, including AR coregulators, c-MYC, BCL-XL, and BCL-2. Importantly, miR-644a contributes to the downregulation of EMT markers and the elevation of E-cadherin. In addition, the Warburg effect is suppressed by miR-644a via collective inhibition of c-Myc, AKT, IGF1R, and GAPDH expression. In particular, experiments have shown that miR-644a sensitized tumours to Enz therapy. Thus, miR-644a can function as an auxiliary agent in the treatment of Enz resistance (Ebron, Shankar et al., 2019).
Genepolymorphisms
In recent years, a number of studies have focused on the association between single nucleotide polymorphisms (SNPs) in androgen-related genes and prognoses. Several SNPs also had impacts on CRPC progression and overall patient survivalc (Aragon, Cendon et al., 2019). Despite their roles in tumour development, there was a link between SNPs and Enz resistance. Y-box binding protein-1 (YB-1) is a critical factor in CRPC progression because it regulates the expression of AR-V7. Inhibiting YB-1 enhanced the effect of Enz in CRPC. The (rs12030724 polymorphism in the YB-1 gene affected the expression of YB-1 in PCa tissues (YB-1 positive rate was 14.3% in TT, 40.0% in AT, and 52.9% in AA, P = 0.04) and the probability of metastatic prostate cancer progression (AT/TT vs AA, hazard ratio = 0.49, 95% confidence interval = 0.32 to 0.77, P = 0.001) (Shiota, Fujimoto et al., 2016). In another experiment, researchers investigated genetic polymorphisms in several antioxidant enzymes that were involved in regulating various stimulus-induced oxidative stress states to modulate AR and CRPC progression. Only the GSTM3 rs7483 SNP was associated with significant progression risk (AG/GG versus AA; HR; [95% CI], 0.45 [0.25–0.79], P = 0.0047). The levels of intracellular reactive oxygen species and GSTM3 expression were upregulated in both castration-resistant and Enz-resistant cells (Shiota, Fujimoto et al., 2017 , Fujimoto, Shiota et al., 2017).
According to an ARMS-PCR analysis of the androgen receptor-coding gene in 50 patients with drug-resistant PCa and 50 patients with non-drug-resistant PCa in the human population of the Isfahan province, Enz resistance was significantly correlated with the rs137852595 SNP (P-value = 0.005) and rs137852574 SNP (P-value = 0.039). In patients with these SNPs, the binding of the drug to the active site of AR was inaccurate, leading to a reduction in the binding energy of the drugs with the receptors (Alizadeh, Sazegar et al., 2018 , Kaviani, Sazgar et al., 2018).