Figure 2 | Glucocorticoid receptor overexpression. The
expression of GR is upregulated after inhibition of AR. AMFR mediate the
loss of 11β-HSD2 to maintain cortisol concentrations, which leads to Enz
resistance. The loss of TLE3 is another reason of GR upregulation. In
addition to the signal target inhibitors, GR/AR dual antagonists Z19
suppresses the transcriptional activity of both AR and GR. GR,
glucocorticoid receptor; GC, glucocorticoid; p23, the small, acidic
co-chaperone; FKBP52, a large peptidyl-proyl cis–trans isomerase
immunophilin protein; AMFR, ubiquitin E3-ligase autocrine mobility
factor receptor; 11β-HSD2, 11β-hydroxysteroid dehydrogenase-2; TLE3,
transducin-like enhancer of split 3; PI3K/AKT, phosphatidylinositol
3-kinase/ protein kinase B; KLF9, Kruppel-like factor 9; PSA, prostate
specific antigen.
Activation of Wnt
signalling
Accumulating
evidence suggested that Wnt signalling plays complex roles in CRPC and
has therapeutic effects of AR-targeting agents. The Wnt family
consists
of 19 cysteines and secreted lipoglycoproteins, which regulate stem cell
self-renewal and cell proliferation, migration, and differentiation
during embryonic and organ development. Wnt proteins bind to multiple
transmembrane frizzled (FZD) and diverse coreceptors, such as
low-density lipoprotein receptor
(LRP)4, LRP5, and LRP6, inactive
tyrosine-protein kinase transmembrane receptor
(ROR)1 and ROR2, and
tyrosine-protein kinase (RYK), to
activate
canonical
(β-catenin-dependent) and
noncanonical (β-catenin-independent)
signalling pathways (Murillo-Garzon and Kypta,
2017).
The Wnt/β-catenin pathway is the
best-studied canonical Wnt signalling pathway. It has been reported to
affect cell proliferation and differentiation and the
epithelial-to-mesenchymal transition (EMT) transition in prostate
cancer. The Wnt ligands form complexes with FZD receptors and
coreceptors, including LRP5 or
LRP6. The complex is inhibited after receptor activation, leading to a
blockade of the phosphorylation of
β-catenin
by both casein kinase Iα and glycogen synthase kinase-3β (GSK-3β).
β-catenin is a multifunctional protein that regulates cell adhesion and
gene activation. A characteristic of canonical Wnt signalling is the
stabilization and nuclear localization of β-catenin. Genomic and RNA-seq
analyses demonstrate that Wnt
/β-catenin signalling is activated in Enz-resistant CRPC, partially due
to a reduction in β-TrCP-mediated ubiquitination. Moreover, the
connection between AR and β-catenin is stronger in CRPC tissues than it
is in normal tissues. The upregulation of stem-like genes upon the
activation of the Wnt/β-catenin pathway contributes to Enz resistance by
reactivating AR signalling (Zhang, Cheng et al., 2018). β-catenin is
interacts with AR and is an AR coactivator, as indicated by β-catenin
armadillo repeats directly interacting with the LBD of AR.
Sex-determining region Y(Sry)-related high-mobility group (HMG) box 9
(SOX9), a member of the SOX family of HMG DNA-binding domain-containing
transcription factors, regulated β-catenin/T cell factor activity and
may be upregulated downstream of Wnt/β-catenin signalling. It was
induced by Wnt signalling and positively regulated multiple genes of Wnt
pathways. Moreover, AR was one of the SOX9-regulated proteins in PCa
cells. The
SOX9-AR-Wnt/β-catenin
signalling axis in PCa led to the synergistic aberrant expression of the
target genes involved in cell viability, multiplication, and
differentiation, resulting in the early emergence of invasive and
aggressive CRPC. Therefore, targeting the SOX9-AR-Wnt/β-catenin
signalling axis may provide a potential strategy for Enz-resistant CRPC
treatment (Khurana and Sikka, 2019). In addition, PRKAR2B is also a
critical oncogenic gene in CRPC, which belongs to the tetrameric enzyme
PKA. PRKAR2B is upregulated in CRPC cells, inducing the EMT and
activating the Wnt/β-catenin pathway. Further research indicated that
miR-200b-3p, miR-200c-3p, and X-box-binding protein 1 were upstream
regulators of PRKAR2B in prostate cancer. Notably, miR-200b-3p and
miR-200c-3p were strongly downregulated in CRPC cells and negatively
correlated with the expression of PRKAR2B. Given the multiple key
factors regulating PRKAR2B, they are additional candidates for
therapeutic targets for treatment (Sha, Han et al., 2018 , Xia, Han et
al., 2020). The small-molecule Wnt/β-catenin inhibitor CWP232291
inhibited β-catenin and AR, inducing endoplasmic reticulum stress and
upregulating the pro-apoptotic transcription factor CHOP. It may also be
a candidate for CRPC treatment when chemotherapy fails (Pak, Park et
al., 2019).
Noncanonical Wnt signalling is divided into planar cell polarity (PCP)
and Wnt-Ca2+ pathways. Upon Wnt binding to FZD
receptors and tyrosine-kinase-like coreceptors, noncanonical Wnt
signalling is activated, leading to the recruitment and activation of
Dishevelled (DVL). The PCP pathway has two parallel branches related to
small GTPases, including Rho and Rac. Rho activates Rho-associated
kinase (ROCK), and Rac is linked to c-Jun N-terminal kinase (JNK).
Cytoskeletal and transcriptional changes associated with these small
GTPases regulate cell adhesion and migration. The
Wnt-Ca2+ pathway stimulates Ca2+release from the endoplasmic reticulum after being activated, thereby
promoting the activation of G-proteins, protein kinase C (PKC),and
Ca2+/calmodulin-dependent
kinase type II (CaMKII)
(Murillo-Garzon and Kypta, 2017). RNA-seq of single prostate circulating
tumour cells (CTCs) indicated the activation of the noncanonical Wnt
signalling pathway induced by anti-androgens. Notably, the noncanonical
Wnt ligand Wnt5A enhanced the survival of AR-positive LNCaP cells in the
presence of Enz (Miyamoto, Zheng et al., 2015). Subsequent research
validated that the finding that the expression of Wnt5A, RhoA, and ROCK
was increased in Enz-resistant PCa cells. Moreover, the combination of
the ROCK inhibitor
Y27632 and Enz synergistically
inhibited 22RV1‐derived xenograft tumour growth. Additionally, ROCK
deletion combined with Enz inhibited the invasion and migration of
Enz-resistant PCa cells by suppressing the EMT (Chen, Liu et al., 2020).
In addition to the activation of Wnt signalling pathways, Wnt-activating
mutations may affect Enz therapy in CRPC. Since Wnt-activating mutations
are present in 10-20% of advanced prostate cancers, a recent study
found that metastatic CRPC (mCRPC) patients with Wnt pathway mutations
showed poor therapeutic effects after abiraterone or Enz treatment. Even
after adjusting for multivariate factors and considering the alterations
in other vital tumour suppressor genes, the negative prognostic impact
of Wnt pathway mutations persisted. Targeting Wnt alterations may
provide another idea for Enz-resistant treatment (Isaacsson Velho, Fu et
al., 2020). Lombard et al. took this novel step and explored the role of
Wnt-secretion-mediating proteins (Lombard, Liu et al., 2019). RNA-seq
and gene set enrichment analysis were utilized to validate the
expression profile changes in Enz-resistant C4-2B-MDVR cells compared to
that of the parental C4-2B cells.
Wntless
(WLS) was needed for the secretion
of all Wnts and mediated the transport of Wnts. WLS was found to be
overexpressed in the resistant cells, regulating Wnt signalling and
cellular viability. Furthermore, WLS regulated AR and AR variant
expression and downstream signalling. These findings support the idea
that targeting Wnt secretion may be a potential strategy for treating
Enz-resistant CRPC