Advances in landscape and related therapeutic targets of the prostate tumor microenvironment

Abstract

The distinct tumor microenvironment (TME) of prostate cancer (PCa), which promotes tumor proliferation and progression, consists of various stromal cells, immune cells, and a dense extracellular matrix (ECM). The understanding of the prostate TME extends to tertiary lymphoid structures (TLSs) and metastasis niches to provide a more concise comprehension of tumor metastasis. These constituents collectively structure the hallmarks of the pro-tumor TME, including immunosuppressive, acidic, and hypoxic niches, neuronal innervation, and metabolic rewiring. In combination with the knowledge of the tumor microenvironment and the advancement of emerging therapeutic technologies, several therapeutic strategies have been developed, and some of them have been tested in clinical trials. This review elaborates on PCa TME components, summarizes various TME-targeted therapies, and provides insights into PCa carcinogenesis, progression, and therapeutic strategies.

Introduction

Prostate cancer (PCa), the second most common cancer worldwide and the fifth leading cause of cancer-related mortality in males, has been a major health issue for decades [1]. During the past 10 years, 25%‒30% of PCa patients who have undergone radical prostatectomy progress to the advanced disease stage. Advanced PCa has an unfavorable relapse rate and progresses to androgen-independent or castration-resistant prostate cancer (CRPC), which has a dismal prognosis [2] because it is incurable by current standard therapies. Therefore, new therapeutic strategies and prognostic markers are urgently needed to develop ideal PCa treatments with the help of therapeutic modality evolution; moreover, such discoveries will lead to further studies on tumors as well as the tumor microenvironment (TME).

The PCa TME is intricate and includes stromal, immune, endothelial, and neural crest cells, among others, all of which release substances such as chemokines, cytokines, extracellular matrices, and matrix-degrading enzymes. Together, they sculpt a sophisticated and pathogenic entity, presenting multiple hallmarks that enhance tumor progression and invasion [3].

This review mainly elaborates on the critical components of the PCa TME and how TME-infiltrated cells function in PCa carcinogenesis, metastasis, immunosuppression, and drug resistance. We also summarize the therapeutic approaches and drugs currently in clinical trials based on different tumor microenvironment compositions and related targets. This study provides a systematic understanding of how the TME functions in PCa initiation, metastasis, and drug resistance and provides potential new therapeutic targets for PCa.

Prostate Tumor Microenvironment Composition

The TME exhibits a highly interlinked and sophisticated signal network and provides a fertile niche for tumor survival, evolution, and metastasis [4]. It also supports the hallmark capabilities and characteristics of cancer cells [5]. Understanding PCa tumorigenesis and maximizing the efficacy of all therapies require methodological research on the behavior of various cell types, extracellular matrix (ECM), and various biological factors included in the PCa microenvironment ( Figure 1).

Figure 1 .

Constituents of prostate tumor microenvironment

PCa TME consists a dense ECM, soluble factors and proteins, and various stromal cell and immune cells that express numerous therapeutic targets.

Cancer-associated fibroblasts (CAFs)

Fibroblasts and smooth muscle cells account for the majority of the stromal cell population in PCa and metastatic castration-resistant prostate cancer (mCRPC) [6]. The tumor/stroma ratio and expression of stromal markers are crucial for prognosis and tumor development [7]. CAFs are the most prevalent stromal cells expressing multiple markers, such as fibroblast activating protein (FAP), platelet-derived growth factor receptor-β (PDGFRB), fibroblast-specific protein-1 (FSP-1), and α-smooth actin (α-SMA). These markers enable CAF function in tumor development, invasion, CRPC progression, and bone metastases [8]. CAFs release vascular endothelial growth factor (VEGF), interleukin (IL)-6, IL-8, fibroblast growth factors (FGFs), transforming growth factor-β (TGFβ), matrix metalloproteinases (MMPs), hepatocyte growth factor (HGF), and growth differentiation factor-15 (GDF-15) [9], stimulating angiogenesis, luminal cell proliferation, and tumor invasion following the loss of the basal cell layer and eventually accelerate ECM component deposition and tenascin C (TNC) secretion [10]. CAFs remodel the ECM and increase tumor invasion by initiating epithelial-to-mesenchymal transition (EMT) through the production of factors such as MMPs and extracellular vehicles (EVs) [11], which are exosomes that carry microRNA-409 (miR-409) [12] and CXC-chemokine ligand (CXCL) 12 [13]. Increased IL-6 secretion by CAFs is considered to be a mechanism underlying the androgen-independent progression of PCa via the PI3K-AKT, STAT3, and MAPK/ERK pathways [14]. Additionally, CAFs increase cancer cell motility via Eph-ephrin signaling [10].

Unlike in normal fibroblasts, the upregulation of CXCL12, FAP, FN1, and POSTN [10], which code for stromal proteins, is considered a CAF indicator. TGFβ-dependent LRRC15 ⁺ CAFs dictate the tumor-fibroblast setpoint to promote tumor growth, directly suppress CD8 ⁺ T-cell function, and limit responsiveness to checkpoint blockade [15]. Zhang et al. [16] reported that neuregulin 1 (NRG1) released by CAFs promotes resistance to androgen deprivation therapy (ADT) in tumor cells by activating the NRG1/HER3 axis. Shen et al. [17] found that CAFs can activate YAP1/TEAD1 signaling and increase the secretion of nerve growth factors (NGFs), thus promoting PCa perineural invasion (PNI). Overall, CAF transformation into normal fibroblasts and blockage of cancer cells are the main CAF-based targeted therapeutic approaches.

Immune cells

Miscellaneous immune and inflammatory cells are found in the TME, the infiltration of which is closely related to tumor progression, together establishing the immune microenvironment hallmark. As a ″cold″ tumor [18] with low immune cell infiltration, PCa shows little response to standard immunotherapies [19]. A previous study [20] investigated why immunotherapies do not work well against PCa after examining 10 different types of immune cells that infiltrate PCa. It was concluded that the pro-PCa immunosuppressive population of cells, including M2 macrophages, activated mast cells, and neutrophils, is upregulated in the active stage, whereas the remaining immune cells, such as cytotoxic T cells, remain in the quiescent stage.

Tumor-associated macrophages (TAMs)

The prostate TME is preferentially enriched with myeloid cells, of which TAMs account for the majority [17]. Both blood- and embryonic-derived macrophage populations are contained in the prostate; however, whether macrophage lineages from different origins exhibit different functions in PCa progression has not been clarified [21].

TAMs can be recruited by chemokines and cytokines surrounding the tumor and constantly gather on the PCa surface to conduct their main functions, such as recruitment, polarization, maintenance, and infiltration [22]. Colony-stimulating factor-1 (CSF-1)/CSF-1 receptor (CSF-1R) signaling promotes TAM recruitment [23] and stimulates M1 polarization to the M2 phenotype [24]. M2-like polarization can also be triggered by the mutant p53 gene, which appears in nearly 20% of PCa cases [25]. Polymerase I and transcript release factor (PTRF)/cavin-1 expression is reduced in PCa by upregulating inflammatory signals and attracting TAMs, which establish a pro-tumor TME [26]. TAM infiltration is related to depletion of the PTEN gene and overexpression of CXCL8 [21]. The urokinase-type plasminogen activator/urokinase-type plasminogen activator receptor (uPA/uPAR) axis has also been reported to be related to TAM infiltration [27].

The vast majority of biological factors secreted by TAMs participate in PCa progression. Derived CXCL1, complement component 5a (C5a), and C-C motif chemokine ligand-2 (CCL2) promote prostate intraepithelial neoplasia (PIN) formation [28]. CCL2 modulates TAM levels and TME phenotypes [29]. CCL-4 primarily promotes PCa by downregulating P53/ PTEN expression in RWPE-1 cells [30]. Excessive IL-6 levels increase STAT3 phosphorylation [31]. IL-8 modulates the STAT3/MALAT1 axis to promote tumor growth [32]. Secreted vascular endothelial growth factor (VEGF) promotes angiogenesis, tumor progression, tumor recurrence, and unfavorable prognosis [ 33– 35] . Increased expression of miR-95 in TAMs-derived exosomes induces tumor growth, proliferation, and EMT by binding to the downstream target gene JUNB [36]. The presence of M2-like TAMs is also correlated with a high Gleason score [37], high prostate-specific antigen (PSA) level, poor prognosis, and high mortality risk [38]. Meanwhile, M2-like TAMs produce compensatory growth factors, such as IL-8, which activates the MAPK/ERK signaling pathway [39], and IL-6, which acts as a stimulant of the TGFβ/SMAD2 axis and p38/MAPK [40], to promote neuroendocrine differentiation (NED), which brings about drug tolerance in late-stage PCa [41].

M2-like TAMs enhance the metastatic potential of PCa [42] by secreting numerous modulators [ 43– 47] . TAMs contribute to bone metastasis by expressing CaM kinase (CaMK) kinase 2 (CaMKK2) and releasing cytokines that help the bone microenvironment promote PCa development [48]. Additionally, M2-like TAMs invoke NF-κB and STAT2 signals, induce CXCL5 secretion, and foster bone metastasis [49]. TAMs attach to vessel-invasive circulating tumors and transform into circulating cancer-associated macrophage-like cells (CAMLs), thereby facilitating the dissemination of tumor cells to distant organs [50]. A recent study [51] revealed that a lipid-loaded TAMs subset, macrophage receptor with collagenous structure (MARCO)-expressing TAMs, is activated by IL-1β from PCa cells and facilitates metastasis by releasing CCL6.

According to the results of a cohort study [52], TAM is essential for the success and efficacy of ADT. Semaphorin 3A (Sema3A) was upregulated and caused monocyte recruitment and M2-like polarization via NRP1 receptor activation in patients receiving ADT for a longer period [53]. M2-like TAMs are also closely associated with poor prognosis after PCa immunotherapies [54]. Protein phosphatase-1 (PP1), a TAM-secretory factor, is responsible for triggering and maintaining M2 polarization, increasing the expression of p-glycoprotein, and promoting chemotherapeutic resistance in PCa [55].

T cells

Cytotoxic T lymphocytes (CTLs), or CD8 ⁺ T cells, are involved in tumor clearance when recruited by various cytokines, such as CCL5, CXCL9 [56], and CXC-chemokine receptor (CXCR6) [57]. Although CD8 ⁺ T cells are responsive to PCa-specific antigens, they still exhibit low infiltration and immune checkpoint activation. Tumor cells express decoy receptors [58] or negative checkpoints, such as PD-1, CTLA-4, LAG3 [59], and TIM3 [60], which induce T-cell anergy. Kaur et al. [61] evaluated 312 primary prostate tumors and concluded that increased T-cell infiltration density is closely related to ERG positivity (median 309 vs CD3 ⁺ T cells/mm ²; P=0.0004) and PTEN loss (median 317 vs CD3 ⁺ T cells/mm ²; P=0.001). Thus, higher FOXP3 ⁺ T-cell density was associated with an increased propensity for metastasis (HR=12.89, P=0.02), and higher CD8 ⁺ T-cell density led to unfavorable clinical progress [62]. Immune cell infiltration is strongly related to antitumor immune responses, maintenance, and prognostic predictions in solid tumors [63]. However, low T-cell infiltration and T-cell anergy in most PCa TMEs maintain an immune-privileged site [64], which reduces the effectiveness of immunotherapy. The unfavorably weak cytotoxicity is related to numerous TME components. TAMs either exclude CD8 ⁺ T cells from the tumor mass via granulin-induced fibrosis or impair T-cell activities by inhibiting its receptor CD3ζ chain [65]. Elevated arginase-1 (Arg1) expression in TAMs induces metabolic starvation in effector T-cells by depleting l-arginine and reducing the efficiency of the CTL cycle. Expression of PD-1 [66] on TAMs leads to phagocytosis of the anti-PD-1 receptor in T cells [67] and renders immunotherapy ineffective, which leads to poor T-cell activity and infiltration.

CD4 ⁺ T cells are also critical for an immunotherapeutic response. CD4 ⁺ T cells can differentiate into either T helper (Th) or Treg cell subsets in response to the cytokine milieu [68]. Th cells include Th1, Th2, and Th17 cells that participate in cellular and humoral immunity by releasing diverse cytokines [64], such as HIF-1α, IL-6, IL-21, and IL-23 [69]. HIF-1α promotes the differentiation of Th17 cells by inducing ROR-γt transcription and inhibiting the differentiation of Treg cells in an active process that degrades the Foxp3 protein. IL-6 stimulates naive CD4 ⁺ Tells to differentiate into Th17 cells via phosphorylation, which induces the upregulation of Th17-specific genes ( ROR-γt, IL-17, and IL-23). By activating STAT3 and increasing ROR-γt expression, IL-21 promotes the differentiation of Th17 cells. IL-23 maintains Th17-cell differentiation by boosting the production of Th17-specific cytokines, such as ROR-γt.

B cells

Compared with the decreased T-cell infiltration percentage in PCa, the B-cell infiltration percentage significantly increases [20] and has an unfavorable influence on antitumor responses [70]. By secreting lymphotoxins, B cells recruited by CXCL13 in residual cancer clones drive malignant cell proliferation through NF-κB, IKKA-STAT3, or BMI1 signaling [71]. By expressing the inhibitory ligand PD-L1 and secreting IL-10, B cells can suppress the immune system and withstand chemotherapy [72]. Additionally, TGFβ-dependent inhibition of CTL activation by IgA class-switched IL10 ⁺ PD-L1 ⁺ plasma cells can impede antitumor efficacy [72].

Tregs

Normally, Tregs regulate the expansion and activation of T and B cells to suppress inappropriate immune reactions and balance immune homeostasis. Increased expression of Axl, MerTK, and Tyro3 receptor kinases in M2 macrophages stimulates lymphocyte influx and subsequent differentiation into regulatory T cells (Tregs) [73]. In the PCa TME, a high-level accumulation of Tregs contributes to suppressing effective tumor immunity [ 74, 75] in different ways [76]: i) Degradation of ATP by expressing CD39 and CD73, which release cAMP and transform AMP into adenosine; ii) Release of effector T-cell inhibitory cytokines, such as IL-10, IL-35, and TGFβ [77]; iii) Reduction of T-cell access to APCs and IL-2 by binding with CD86, LAG-3, and Nrp-1 on DCs; and iv) Induction of CTL apoptosis by releasing perforin/granzyme B or by establishing FasL/F binding. Generally, Tregs primarily contribute to the suppression of antitumor immune responses via contact- and cytokine-dependent communication with T cells, APCs, and innate immune cells.

NK cells

NK cells exert essential functions in immunosurveillance similar to innate immune cells [68]. Normally, NKs release lytic granules to eliminate tumors with downregulated expression of MHC class I or upregulated ligands which together activate NK cell receptors [78], and an association between low NK cell infiltration and poor cancer prognosis has been proven [79]. They are also capable of promoting T-cell infiltration and eliciting inflammatory responses through cytokine and chemokine secretion [80]. However, these antitumor processes are suppressed because of the low levels of IL-12 secretion by TAMs [81] and the expression of TGFβ from MDSCs in the PCa TME [82]. Increased levels of M2-like TAMs with poor antigen-presenting capacity may generate cytotoxicity desensitization of natural killer (NK)-mediated PCa cells, reduce immune surveillance, enhance immunological suppression, and benefit PCa survival. Additionally, circulating NKs are inadequate in PCa patients [83], which leads to a reduction in degranulation capabilities and interferon-γ (IFN-γ) release, thereby promoting tumor growth. IL-15 promotes NK cell activation and proliferation, and IL-15 receptor agonists in combination with immunotherapy have been considered a novel hinge that can enhance NK cell-based immunotherapy [84].

Tumor-associated neutrophils (TANs)

Neutrophils can be polarized into antitumor subtypes (N1) and pro-tumor subtypes (N2) under modulation by various cytokines, such as granulocyte colony-stimulating factor (G-CSF), TGFβ, and IL-1β [85], and this dual function presumably reflects their remarkable adaptability in response to environmental signals [86]. N1 TANs upregulate CD8 ⁺ T-cell infiltration and are energized by releasing IL-12, CCL3, and CXCL9. N2 TANs promote TME remodeling and tumor invasion by secreting proteases, such as MMP-9 [87], neutrophil elastase [88], and cathepsin G [89]. They also promote T-cell anergy by secreting ARG1 and iNOS [90]. CD66ce ⁺ neutrophils are relatively increased in PCa compared to benign tissues and are therefore considered to promote PCa progression [85]. Moreover, a high neutrophil-to-lymphocyte ratio (NLR) can act as a predictor of a poor prognosis in PCa [91].

MDSCs

With the development of immuno-oncology, novel concepts have been proposed to provide a better understanding of the distinct immune environment in cancer. MDSCs are a highly heterogeneous population of immune cells originating from the bone marrow that accumulate in the TME via a CXCR2-dependent approach [92]. MDSCs with polymorphonuclear (PMN-MDSC) or monocytic (M-MDSC) morphology are the two distinct MDSC subpopulations [93] that exhibit immunosuppressive activities, including inhibition of T-cell activation, dendritic cell maturation, induction of anergy of NKs, and promotion of de novo expansion of Tregs in PCa [94] by direct cell-cell contact or the release of soluble cytokines and short-lived factors, such as IL-10, CXCR4 [95], TGFβ, prostaglandin E 2 (PGE 2), L-Arg, and indoleamine 2,3-dioxygenase (IDO) [96]. PMN-MDSCs are constantly referred to as N2 TANs because they express the same cell surface phenotypes (CD11b ⁺Ly6G ⁺Ly6C ^– cells in mice and CD66b ⁺CD14 ^–CD11b ⁺CD15 ⁺ cells in humans), whereas M-MDSCs express Gr-1 ⁺CD11b ⁺Ly6C ⁺Ly6G ^– in mice and HLA-DR ^-CD11b ⁺CD33 ⁺CD14 ⁺CD15 ^– in humans [93]. Influenced by tumor-released chemokines, MDSCs can differentiate into pro-tumor cells, dampen antitumor immunity, and create a fertile tumor premetastatic niche. In addition to the establishment of an immunosuppressive TME, MDSCs facilitate tumor ADT resistance and CRPC progression by secreting IL-23 [97], and M-MDSCs are closely related to a higher disease burden and poor clinical outcomes in PCa by encouraging TAM polarization from M1 to M2 [98]. Additionally, IL-1 [99] and VEGF [100] released by M2-TAMs lead to MDSCs proliferation and immunosuppressive features in the TME. Activated by various cytokines, such as IL-6, IL-10, GM-CSF, and VEGF, in the PCa TME, the STAT3 signaling pathway [101] plays a critical role in generating MDSCs and inducing T-cell suppression. In general, MDSCs accumulation is associated with tumor progression and poor prognosis [102].

Epithelial cells

The classification of prostate epithelial cells has expanded to basal epithelial, luminal epithelial, neuroendocrine, hillock, and club cells [103]. Song et al. [104] identified PCa-enriched club cell states that are believed to be androgen-responsive through a single-cell analysis and further compared prostate epithelial organoids with prostate tissues in terms of their molecular and cellular features. They found that prostate epithelial organoids harbor tumor-associated epithelial cell states that give rise to common TME responses [105] and are associated with prostate carcinogenesis [104]. Once carcinogenesis is initiated, epithelial cells produce TGFβ and kallikrein 4 (KLK4), which convert the initial stromal fibroblasts into CAFs. Epithelial cells are also critical in shaping cellular plasticity by becoming mesenchymal and migratory, or EMT, which contributes to tumor invasion, metastasis and therapy resistance [106].

ECM and biological molecules

The tumor ECM is a stiff environment composed of fibrin, proteoglycans, cytokines, growth factors, hormones, and physical and chemical parameters. Together, the ECM establishes complex crosstalk with the TME and is essential for basic cell support, development, migration, communication, and metabolism.

Tumor development and colonization are overwhelmingly supported by the ECM [107]. Mast cells secrete MMP-9 and MMP-2, which in turn stimulate the release of VEGF and fibroblast growth factor (bFGF) [108]. The increase in MMP-9 and MMP-2 can be explained by the effect of Dickkopf-3 (DKK3), a TGFβ inhibitor, in both prostate epithelial and stromal cells [109]. MMPs can also be secreted by cancer cells, CAFs, and epithelial cells [110]. The expression of glycoprotein tenascin-C also increases in the PCa ECM and is notably related to poor prognosis [111]. Expression of RGD (Arg-Gly-Asp) domains on ECM proteins enables them to bind integrin αvβ3 on PCa cells, recruit FMS-like adaptor kinase (FAK), and promote further signaling pathways that contribute to cell adhesion and motility in the bone microenvironment [112]. The degradation of ECM and resulting evasion of adhesion are the first steps in PCa migration and invasion of the blood arteries, lymph nodes, and bone [113].

Exosomes, nanosized EVs with thicknesses between 30 and 150 nm, are released from cells after multivesicular bodies (MVBs) fuse with the plasma membrane. These vesicles consist of a lipid bilayer membrane enclosing a cargo of biomolecules, including proteins, lipids, RNA, and DNA. Exosomes mediate intercellular communication by transferring their cargo to recipient cells to modulate target cellular functions, and reports have suggested that they contribute to PCa progression, premetastatic niche establishment, and organ-specific metastases [114]. Chen et al. [115] carried out a single-cell analysis that discovered that cancer-derived EVs prime T cells to express KLK3, which migrate to lymph nodes and attract PCa cell metastasis.

Numerous chemokines and cytokines are involved in cell recruitment, inhibition, and cellular communication [116]. CSF-1, granulocyte colony-stimulating factor (G-CSF), and CCL-2 secreted from tumor or stromal cells are associated with the recruitment and formation of tumor-associated immune cells. IL6 and CXC chemokines are capable of recruiting and differentiating polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), dendritic suppressor cells, and M2-like TAMs [117]. TGFβ/TGFR plays a pivotal role in PCa progression. Increased TGF hinders the differentiation of Th1 cells to generate PCa bone metastases resistant to ICI treatment [118]. TGFβR1 can be activated by the binding of TGFβ to TGFβR2 [119]. Consequently, SMAD2/SMAD3 is phosphorylated and translocated into the nucleus and stimulates target gene expression after complexation with SMAD4, which eventually results in PCa aggressiveness, metastasis, and poor prognosis [120]. Galunisertib (LY2157299; NCT02452008), a TGFβ receptor I antagonist, significantly inhibited tumor growth and progression in a TRAMP-C1 cell line-derived subcutaneous tumor model [13]. Furthermore, M7824 (MSB0011359C), a novel bifunctional fusion protein targeting both PD-L1 and TGF-β, is currently being evaluated in PCa patients [NCT04633252].

Tertiary lymphoid structures (TLSs)

TLSs are structured aggregates of cellular composition, including B cells, T cells, DCs, follicular dendritic cells (FDCs), follicular reticular cells (FRCs), and high endothelial venules (HEVs), in nonlymphoid tissues that result from chronic inflammation such as cancer [ 121, 122] . The intricate process of producing TLSs involves interactions with immune cells. CXCL13 and IL-7 secreted by lymphocytes or stromal cells recruit T lymphoid tissue inducer cells (LTis), which are replaced by Th17 cells, B cells, or M1 TAMs during the initiation of TLS [123]. Lymphotoxin-α1β2 (LTα1β2) expressed on LTis is then activated by binding to the LTβ receptor (LTβR) on stromal cells that secrete chemokines and attract lymphocytes from adjacent HEVs. These cells, together with CD4 ⁺ Th-polarized T follicular helper (TFH) cells, FDCs, and mature DCs, build up the TLS structure [124].

TLSs have been reported to function in anti-tumor immunity [122], improve immunotherapy in some solid tumors [125], and determine the level of tumor-infiltrating lymphocytes [126]. Regarding PCa, García-Hernández et al. [127] investigated 17 patients and observed TLSs in the tumor invasion margin. Moreover, Tbet ⁺ T cells and CD8 ⁺ T cells are abundant in TLSs from patients with spontaneous PCa remission. However, unlike pancreatic [128], stomach [129], and lung cancer [130], for example, the prognostic value of TLS in PCa has not yet been reported. Even so, TLS-inducing agents can turn immune-cold tumors into immunogenic tumors because unencapsulated TLSs expose immune cells to tumors [122], which can be a novel therapeutic approach [124].

Bone marrow microenvironment

Bone metastasis is a destructive and incurable outcome that occurs in over 90% of PCa patients [1]. Pro-tumor cells in the TME secrete tumor-promoting factors, reshape the ECM, and induce angiogenesis, which has been previously stated. These alterations in the tumor TME facilitate dormant cancer cell reactivation, proliferation, and escape [131]. During these sessions, disseminated cancer cells (DCCs) lead to the differentiation of osteoclastic precursors and promote osteolytic metastasis [132]. After bone metastasis takes place, a novel and distinct tumor microenvironment is shaped in the bone marrow and is believed to be critical for a better understanding of PCa progression. A fine image for biomarkers and cell types of bone metastatic response to systemic therapy is still limited. Kfoury et al. [133] performed single-cell analysis of the bone marrow prostate. They observed emerging TAMs, tumor inflammatory monocytes (TIMs) and dysfunctional T cells and increased expression of CCL20, an inducer of T-cell exhaustion and immunosuppression, on TAMs and TIMs, which indicates that a CCL20–CCR6 inhibitor could be a potential therapy to relieve bone metastatic prostate cancer.

Overall, stromal cells, immune cells, and cancer cells interact mutually, together with abundant soluble factors and proteins; they construct an immunosuppressive network and account for a series of TME hallmarks [134].

Hallmarks of the PCa TME

Hypoxic niche

Under the rapid growth of tumor cells, the insufficiency in the vasculature for hemoperfusion [135] leads to a hypoxic environment. This situation induces hypoxia-inducible factor-1α (HIF-1α), which signals and stimulates vascular endothelial cells to upregulate the transcription of VEGF and ANGPT1 in the tumor, which activates tumor angiogenesis [136]. HIF-1α increases inducible nitric oxide synthase (iNOS), Arg-1, and “don’t eat me” protein expression, which facilitates the escape of tumors from immune cells. In PCa, hypoxia is associated with an increased mutational load of somatic variations and alterations in PTEN oncogenes [136].

Acidic niche

The lactate metabolism and hypoxia stated previously result in the formation of a unique acidic niche in the tumor microenvironment. Tumor-derived lactic acid fuels regulatory T cells and induces M2 polarization in macrophages [137]. Lactate, as a sophisticated modulator of immune cell function, can coexist with the co-product of glycolysis. Feng et al. [138] discovered that subcutaneous injection of lactate into MC38 mice is responsible for increased TCF-1 expression on CD8 ⁺ T cells, which offers encouraging suggestions for enhancing the therapeutic effects of immunotherapies.

Metabolism microenvironment

Alterations in metabolism in the TME occur after the rapid growth of tumors and then reprogram the TME to induce hypoxia, oxidative stress, and acidification [139]. The metabolism of tumor-associated immune cells differs, with Tregs and M2-like macrophages depending on oxidative phosphorylation produced by fatty acid oxidation (FAO), whereas activated neutrophils, M1-like macrophages, and iNOS-expressing dendritic cells (DCs) mostly rely on glycolysis. M2-like TAMs are addicted to glucose, which leads to their tumor-promoting and drug-resistant characteristics [140]. Tumor metabolites, including lactate, fatty acids [141], PGE2, tryptophan, cholesterol, arginine [142], and glutamine, are known to affect tumor immunity [143]. CAFs play a supportive role in PCa oxidation by releasing lactate through aerobic glycolysis and forming cellular bridges to convert mitochondria into tumor cells, which then sustain their promotion [144]. L-Arg is a non-essential amino acid whose reduction generates T-cell dysfunction by reducing the half-life of the CD3-chain T-cell antigen receptor [142]. Epidemiological evidence indicates that an increased PCa risk is related to the high consumption of high-fat food and dairy products [145].

Innervated niche

Recent studies have shown that innervation via the sympathetic (adrenergic) and parasympathetic (cholinergic) nervous systems [146] stimulates tumor development and invasion [147]. Therefore, nerves and individual axons infiltrate the blastema and are necessary for tissue growth and remodel supervision [148]. Hence, an innervated microenvironment is created during this process. Three theories of neurogenesis in the PCa TME have been proposed: neuronal neurogenesis, stem cell neurogenesis, and the central nervous system [149]. Nerves within and around tumors regulate tumor growth and participate in the invasion of neighboring tissues [150], also known as PNI, which indicates poor prognosis in PCa [151]. In the PCa TME, adrenergic nerves are present within the stroma, while PCa cells are intermixed or surrounded by the complex stroma containing axonal branches of the autonomic nervous system secreting specific neurotransmitters (NT) [149]. NGFs and NTs released by Schwann cells and axons stimulate tumor growth by regulating inflammation, axonogenesis, and angiogenesis [152]. PCa cells secrete NGFs, brain-derived nerve factors, pleiotrophin, or neuregulin, which facilitate cancer innervation [153]. However, the relationship between nerve density and poor PCa prognosis and the mechanisms by which cellular components in innervated niches induce tumor invasion have not been fully elucidated. In summary, although the innervated niche of PCa is not fully understood, it has promising potential as a therapeutic target.

Immune environment

To determine how immune cells are related to the TME, Liu et al. [154] designed an algorithm called PhenoAligner that categorizes different immune cell phenotypes into N-type (niche-associated type) and M-type (malignancy-associated type) to reveal how tumor characteristics and host organs affect immune cell phenotypes. Owing to the absence of tumor-associated antigens, insufficient T-cell infiltration, and abundant immunological regulatory cells, PCa is currently classified as a “cold” tumor [61] with an immunosuppressive microenvironment. Thus, TME components are considered critical in maintaining the immunosuppressive niche that helps cancer cells avoid immune destruction at the cellular level. CAFs play a crucial role in dampening immune responses by recruiting immunosuppressive cells via cytokine/chemokine release [155]. Immunosuppressive cells (Tregs, MDSCs, and TAMs) and biological factors released together cause immunosuppression in PCa. The secretion of IFN-γ, TGFβ, TNF-α, IL-6, IL-17, IL-15, IL-27, or complement factor C5a or activation of intracellular ERK/MEK, AKT-mTOR, NF-κB, WNT, and JAK/STAT pathways in cells promote PD-L1 upregulation in PCa [156]. A novel study [157] identified high EP4 (PTGER4) expression in epithelial cells and various immune cells in the PCa TME, which is believed to be related to MDSC activity and T-cell anergy. Sahu et al. [158] recently reported that a better evaluation of the dynamic vascular events (tumor angiogenesis, leukocyte trafficking) within the tumor immune microenvironment (TIME) is reliable for indicating topical immunotherapies in solid tumors.

Mechanical environment

The mechanical environment is composed of intracellular and extracellular components, intercellular signaling, and stromal cells, which provide structural and functional support for tumor growth and metastasis [159]. Pathological conditions promote aberrant ECM alterations, and initial tumor-related abnormalities are observed in the TME [160]. Excessive deposition of collagen and hyaluronic acid, increased lysyl oxidase (LOX) activation or expression level, and radial alignment of collagen fibres enhance ECM stiffness, which is considered to be related to PCa progression, higher Gleason score [161], and castration resistance [162]. Both tumor cells and CAFs contribute to increased ECM stiffness and alterations in signaling pathways by overproducing ECM components [163]. ECM stiffness can stimulate EGFR, which activates the downstream PI3K/AKT pathway and promotes AR translocation to the nucleus. Stiffness-activated FGFR activates downstream signaling, such as MAPK, to drive prostate cancer progression. Alternatively, ECM stiffening gives rise to WNT signaling pathway activation [162] and leads to aberrant tumor vasculature [164].

Generally, these hallmarks establish connections to activate PCa initiation, promotion, and metastasis, thereby leading to variable prognosis. Decreased M1/M2 ratios, reduced Th1 infiltration, and functionally inactivated T cells lead to poor prognosis in PCa [165]. Increased glucose uptake sustains hexosamine biosynthesis pathway-dependent O-GlcNAcylation to promote cancer spread and chemoresistance, whereas M2-like TAMs have the highest individual capacity to take up intertumoral glucose. Cathepsin B expression is positively correlated with OGT expression in human TAMs, both of which predict chemotherapy response and prognosis in cancer patients [166]. Tregs demonstrate good adaptivity to high levels of lactate in the TME due to the upregulated expression of CD36 [167].

Emerging Therapeutic Strategies Targeting the PCa TME

ADT

Traditional therapies are tumor-specific and are applied depending on the severity of cancer [168]. Localized PCa patients receive deferred treatment or active local therapy, with or without ADT. Metastatic PCa undergoes standard treatment: ADT, the backbone of metastatic PCa [169], with gonadotropin-releasing hormone (GnRH) antagonists/agonists, followed by treatment with docetaxel plus prednisolone and continued ADT after disease progression. ADT is a conventional and fundamental treatment for individuals with PCa; nevertheless, within two years, nearly all patients receiving ADT will inevitably progress to advanced CRPC. The incidence of mCRPC after ADT has been associated with an immunosuppressive TME [170]. Castration treatment enhances IL-8/CXCR2 axis expression in prostate epithelial cells and attracts defective neutrophils, namely, PMN-MDSCs, which inhibit efficient nutritional intake and antigen recognition and lead to reduced proliferation and function of CD 8 T cells by secreting IL-23 and producing reactive nitrogen [171]. A current clinical trial is targeting this new finding [NCT03689699]. Therefore, significant and extensive attempts have been made to develop efficient treatment procedures.

Although end-stage PCa has a high mortality rate, a relatively long window is available for experimental therapies, especially when PCa has a low tumor mutational burden (TMB) [20], which generates immunogenic potential.

Immunotherapies

Sipuleucel-T, the first therapeutic cancer vaccine that was approved by the FDA in 2010, is an autologous, active cellular immunotherapy. The phase III Immunotherapy for Prostate Adeno Carcinoma Treatment (IMPACT) [172] study revealed a significant increase of 4.1 months in the treatment group′s median overall survival (OS) compared to the placebo group. However, there were no statistically significant variations in the median progression times between the two patient groups. Another mainstream immunotherapy for PCa is the humanized monoclonal anti-PD-1 antibody pembrolizumab, which received accelerated approval from the United States Food and Drug Administration (FDA) in 2017. Marcus et al. [173] collected data on 149 patients and reported an objective response rate of 39.6% (95% CI: 31.7-47.9), with 78% of responses lasting 6 months or more.

However, immunotherapies have not achieved optimal and durable therapeutic effectiveness in PCa treatment. This is associated with poor cytotoxic cell infiltration, recruitment of inhibitory immune cell populations, and production of pro-tumorigenic cytokines [174], which characterize PCa as a cold tumor. In particular, the PCa TME establishes chronic inflammatory and immunosuppressive networks in advanced metastatic PCa and causes difficulties in activating immune-targeting treatments [175]. Recent phase II clinical trials [NCT02601014 and NCT02787005] have revealed that ICIs are only effective for a minority of patients, and the disease control rate does not exceed 20% in PCa. Additionally, the overall response rate of PCa patients to anti-PD1/PD-L1 antibodies is 10%‒20% [176].

The effectiveness of cancer immunotherapy is intimately associated with the properties of the TME. In recent years, PCa therapies that modify the TME have gradually been developed since treatments that target the TME rather than tumor cells preserve genetic stability [177] and reduce drug resistance ( Table 1).

Table 1 Current clinical trials targeting PCa TME components

Targeted TME components	Target	Drug	Mechanism of action	Status	Trial identifier
T cell	PD-1*	Pembrolizumab+ Olaparib/ Lenvatinib	Anti PD-1 mAb+ PARP inhibitor /TKI	Phase 1/2	NCT02861573
		Pembrolizumab+ Carboplatin+ Etoposide	Anti PD-1 mAb+ Chemotherapy	Phase 1/2	NCT02861573
		Pembrolizumab+ Navarixin	Anti PD-1 mAb+ CXCR1/CXCR2 antagonist	Phase 2	NCT03473925
		Pembrolizumab+ Enzalutamide	Anti PD-1 mAb+ AR inhibitor	Phase 1/2 Phase 3 Phase 3	NCT02861573 NCT04934722 NCT03834493
		Pembrolizumab+ Docetaxel	Anti PD-1 mAb+ Chemotherapy	Phase 3	NCT04907227
		Cetrelimab+ Apalutamide	Anti PD-1 mAb+ AR inhibitor	Phase 1	NCT03551782
	PD-L1*	Durvalumab+ Tremelimumab+ metronomic vinorelbine	Anti PD-L1 mAb+ Anti CTLA-4 mAb+ antiangiogenic therapy	Phase 1/2	NCT03518606
		N-803+/– M7824	IL-15+/– TGFβ/PD-L1 bispecific Ab	Phase 2	NCT05445882
	PD-1, CTLA-4	Ipilimumab+ Nivolumab	Anti CTLA-4 mAb+ anti PD-1 mAb	Phase 2	NCT03570619
	LAG-3*, PD-L1	FS118	LAG-3/PD-L1 bispecific Ab	Phase 1/2	NCT03440437
	LAG-3, CTLA-4, PD-1	XmAb®22841+Pembrolizumab	LAG-3/CTLA-4 bispecific Ab+ Pembrolizumab	Phase 1	NCT03849469
	PD-1, CD39*, A2AR*, A2BR	SRF617+ Etrumadenent+ Zimberelimab	CD39 inhibitor+ A2aR and A2bR antagonist+ anti PD-1 mAb	Phase 2	NCT05177770
T cell	PD-1, A2AR, A2BR	Etrumadenent+ Zimberelimab+ Enzalutamide	A2aR and A2bR antagonist+ anti PD-1 mAb+ AR inhibitor	Phase 1/2	NCT04381832
		Etrumadenent+ Zimberelimab	A2aR and A2bR antagonist+ anti PD-1 mAb	Phase 1	NCT03629756
	A2AR	AZD4635+ Durvalumab Oleclumab/	A2AR antagonist+ anti PD-L1 mAb/ anti CD73 mAb	Phase 1	NCT04089553
	A2AR, PD-1	NIR178+ PDR001	A2AR antagonist+ anti PD-1 mAb	Phase 2	NCT03207867
	A2AR, PD-L1	Ciforadenant+/– Atezolizumab	Anti PD-L1 mAb+/– A2AR antagonist	Phase 1	NCT02655822
	CD73, A2AR, PD-1	CPI-006+ Ciforadenant+ Pembrolizumab	Anti CD73 mAb+/– A2AR antagonist+ anti PD-1 mAb	Phase 1	NCT03454451
	BTLA*	JS004	Anti BTLA mAb	Phase 1	NCT04278859
	B7-H3	Enoblituzumab	Anti B7-H3 mAb	Phase 2	NCT02923180
	T-cell ACT	KLK2 CAR-T cells+ bridging therapy	CAR-T cells	Phase 1	NCT05022849
		PSMA CAR-T	CAR-T cells	Phase 1	NCT05354375
		P-PSMA-101 CAR-T cells	CAR-T cells	Phase 1	NCT04249947
		PSCA-Targeted CAR-T cells	CAR-T cells	Phase 1/2	NCT02744287
		CART-PSMA-TGFβRDN cells	CAR-T cells	Phase 1	NCT03089203
		CARTEPC	CAR-T cells	Phase 1/2	NCT03013712
		CTM-N2D	CAR-T cells	Phase 1	NCT04107142
		TmPSMA-02 CAR-T cells	CAR-T cells	Phase 1/2	NCT05489991
		4SCAR-PSMA T cells	CAR-T cells	Phase 1/2	NCT04429451
		NeoTCR-P1	TCR-T cells	Phase1	NCT03970382
DC	4-1BB*	Anti-PSCA-CAR-4-1BB/TCRzeta-CD19t-expressing T-lymphocytes	CAR-T cells+ Agonistic anti-4-1BB Ab	Phase 1	NCT03873805
		CB307	Trispecific humabody on CD137, PSMA, HAS	Phase 1	NCT04839991
	TLR3	Rintatolimod+ Aspirin+ Radical Prostatectomy+ IFN alpha-2B	TLR3 agonist+ Aspirin+ Surgery+ IFN alpha-2B	Phase 2	NCT03899987
		PolyICLC+ Durvalumab+ Tremelimumab	TLR3 agonist+ anti CTLA-4 mAb+anti PD-L1 mAb	Phase 1/2	NCT02643303
	DC vaccination	DC1 vaccine	Vaccination	Phase 2	NCT00970203
		T-cell receptor alternate reading frame protein (TARP) dendritic cell vaccine	Vaccination	Phase 1	NCT00972309
		Myeloid dendritic cells (mDC) vaccinations	Vaccination	Phase 2	NCT02692976
		Plasmacytoid dendritic cells (pDC) vaccinations	Vaccination	Phase 2	NCT02692976
		DC/PC3	Vaccination	Phase 1/2	NCT00345293
		Prodencel; an autologous dendritic cell therapeutic tumor vaccine	Vaccination	Phase 1	NCT05533203
		mRNA transfected DC	Vaccination	Phase 2	NCT01446731
B cell	CD40*	Ad-sig-hMUC-1/ecdCD40L vector vaccine	DC vaccine	Unknown	NCT02140996
CAF	FGFR	TKI258	Tyrosine kinase inhibitor (TKI)	Phase 2	NCT01741116
		TT-00420+ Nab-Paclitaxel	Multikinase inhibitor (MKI)+ Chemotherapy	Phase 1/2	NCT04742959
	GFRAL	NGM120	Anti GFRAL Ab	Phase 1/2	NCT04068896
	VEGFR*	Cabozantinib+ Atezolizumab	Tyrosine kinase inhibitor (TKI)+ anti PD-L1 mAb	Phase 1	NCT03170960
		PTK787	VEGFR inhibitor	Phase 2	NCT00134355
		Pazopanib	Tyrosine kinase inhibitor (TKI)+ anti PD-L1 mAb	Phase 2	NCT01832259
		Axitinib	VEGFR inhibitor	Phase 2	NCT01385059
		ESK981	VEGFR inhibitor	Phase 2	NCT03456804
		Bevacizumab+ Temsirolimus	Anti VEGF mAb+ mTOR inhibitor	Phase 1/2	NCT01083368
		Tivozanib+ Atezolizumab	TKI+ anti PD-L1 mAb	Phase 1/2	NCT05000294
	EphB2*	sEphB4-HAS+ Pembrolizumab+ Computed tomography	EphrinB2 inhibitor+anti PD-1 mAb+ Computed tomography	Phase 2	NCT02717156
TAM	CSF-1R	PLX3397+ Antiandrogen therapy Radiation therapy	CSF1R inhibitor+ Antiandrogen therapy+ Radiation therapy	Phase 1	NCT02472275
NK	NKG2D	NKG2DL-Targeting CTM-N2D	CAR-T Cells	Phase1	NCT04107142
	NK-cell ACT	anti-PSMA CAR NK cells	CAR-NK Cells	Early Phase 1	NCT03692663
MDSC	CXCR2*	AZD5069+ Enzalutamide	CXCR2 antagonist+ AR inhibitor	Phase1/2	NCT03177187
Epithelial cell	EGFR	Panitumumab	Anti EGFR mAb	Phase 1	NCT00054574
		Erlotinib	EGFR inhibitor	Phase 2	NCT00148772
		Afatinib Oral Tablet+Zenocutuzumab+Enzalutamide	Anti EGFR/HER2 agent+bispecific anti HER2/HER3 Ab+AR inhibitor	Phase 2	NCT05588609
		AZD0530	EGFR inhibitor	Phase 2	NCT00558272
	ErbB2*	Lapatinib ditosylate+ laboratory biomarker analysis Correlative studies	ErbB2 inhibitor	Phase 2	NCT00103194
		EGFRvIII peptide based vaccine	Vaccination	Phase 1	NCT00023634
Nerve cell	Cynapse	Botox-injection	Botulinum Toxin A	Withdrawn	NCT01520441
	β-adrenergic Receptor	VT-122	β-adrenergic blockers	Terminated	NCT01857817
		Propranolol hydrochloride+ Laboratory biomarker analysis correlative studies	β-adrenergic blockers	Phase 2	NCT03152786
		Carvedilol	β-adrenergic blockers	Phase 2	NCT02944201
Soluble Factors and Proteins	ARG1	PEG-BCT-100	rhArg 1	Phase 1	NCT02285101
	mTOR	Everolimus	PI3K-AKT-mTOR signaling inhibitor	Phase 3	NCT03580239
		RAD001	mTOR1 inhibitor	Phase 2	NCT00657982
	TGFβ	LY2157299+ Enzalutamide	TGFβR inhibitor+ AR inhibitor	Phase 2	NCT02452008
	IL-8	BMS-986253+ Nivolumab+ Degarelix	Anti IL-8 mAb+ anti PD-1 mAb+ GnRHR antagonist	Phase 1/2	NCT03689699
	IL-6	CNTO 328+ Docetaxel	IL-6 inhibitor+ Chemotherapy	Phase 1	NCT00401765

ACT, adoptive cell therapy; TKI, tyrosine kinase inhibitors; AR, androgen receptor; mAb, monoclonal antibody; * marked targets are expressed on multiple cells which have been illustrated in Figure 1.

Although most of the early immune checkpoint blockade (ICB) therapies for PCa have not illustrated satisfying therapeutic effects and have ceased due to no significant OS changes [NCT01057810 and NCT03016312], current clinical trials that combine ICB with chemotherapies [NCT02861573 and NCT04907227], AR inhibitors [CT02861573 and NCT03551782] and antiangiogenic therapy [NCT03518606] are either recruiting or active. BAY1082439, a PI3Kα/β/δ inhibitor capable of restricting immunosuppression, is undergoing a phase 1 clinical trial for solid tumors [NCT01728311]. Recently, Qi et al. [178] proposed that poor-T-cell-infiltrated PCa can benefit from the combination of PD-1 antibody and intermittent BAY1082439 treatment, which induces intratumoral CD8 ⁺ T cell clonal expansion and prolonged T cell-inflamed phenotype even after drug withdrawal. Zippelius et al. [179] described the potential of PD1-TIM3 and PD1-LAG3 human bispecific antibodies. In recent years, bispecific antibodies have been applied in several ongoing clinical trials for PCa, including NCT03849469 (LAG3-CTLA4 bispecific antibody), NCT03440437 (LAG3-PDL1 bispecific antibody), and NCT05445882 (TGFβ/PD-L1 bispecific antibody). Activated NKs express immune checkpoints [ 180, 181] as well, and therefore, immune checkpoint blockade immunotherapies, including anti-PD1 and anti-CTLA4 antibodies, can reactivate NKs to conduct antitumor activities [182]. Additionally, neutralizing TGF improves CD8 ⁺ T cell and NK-mediated antitumor immune responses and raises beneficial chemokine distributions that stimulate the recruitment and activation of antitumor neutrophils [183]. Only one TGFβ inhibitor (LY2157299) is registered for PCa and is designed in combination with enzalutamide [NCT02452008]. Other T-cell targets are also designed in clinical trials, such as CD39 inhibitor [NCT05177770], A2AR antagonist [NCT04089553], B7-H3 antibody [NCT02923180], and CD73 antibody [NCT03454451].

TME-targeted therapies

TAM-targeted therapeutic strategies include preventing TAM recruitment, depleting TAMs, reprogramming TAMs, exploiting antitumor functions, and infusing engineered macrophages [21]. Experimental models have shown that CSF1R inhibitors (CSF1Ri) help reduce TAM numbers [184] and alter pro-tumor phenotypes. While most CSF1R antibody trials are registered for solid tumors, only one trial of CSF1R combined with antiandrogen and radiation therapies and as a laboratory biomarker is designed for PCa and is currently in phase 1 [NCT02472275]. Firdaus et al. [185] proposed that the nitric oxide (NO)-CSF1Ri combination has the potential to act as a therapeutic agent, promote CSF1Ri efficiency and restore control over TME. The CD47-mediating signal “don’t eat me” is a potential target for invigorating TAM antitumor activities [186], but no clinical trial targeting CD47 is registered as a clinical trial at present. Numerous therapeutic targets on TAMs have been listed in a latest review [54], such as ARG1, B7-S1R, TIM-3, and Trem1. However, these targets have not been registered in clinical trials for prostate cancer specifically, except for one clinical trial designed for solid tumors that target Trem1 [NCT04504942].

The normalization and reprogramming of CAFs by targeting GPR77 [187], vitamin D receptor (VDR) [188], and platelet-derived growth factor receptor (PDGFR) [189] have shown therapeutic potential. CXCR2, the key moderator in neutrophil mobilization, is the most promising target for neutrophil inhibition [190]. CXCR2 inhibitors show antitumor ability in lung cancer [191] and PCa [192], sensitizing hepatocellular carcinoma (HCC) to ICB therapy [193] and enhancing the chemotherapy effect [191]. A novel study [194] supports the combination of ICB and a CXCR2 antagonist, and one clinical trial is now registered [NCT03473925] for PCa conditions. CAF with GPR30 knockdown (CAF-shGPR30) [195] disrupts TAM polarization to M2 and secretion of IL-6, which benefits tumor invasiveness.

MDSCs, together with Tregs and immunosuppressive Th cells, establish a pro-tumor TME that restrains T-cell entrance [72] and reduces immunotherapy effectiveness. Therefore, therapeutic strategies targeting these cells have been developed and tested. Current strategies proposed to target MDSCs include (i) depletion of MDSCs, (ii) impairment of MDSC function through inhibition of immunosuppressive mediators, (iii) disruption of MDSC recruitment and tumor trafficking, and (iv) stimulation of MDSC maturation by promoting their differentiation [94]. Re-educating MDSCs is an attractive therapeutic approach to restrain immunosuppression, and animal models have shown that MDSCs can be trained to take on an antitumorigenic phenotype in response to immune system activation that mimics bacterial infection. This is accompanied by an increase in TH1 cytokines, a decrease in T-cell-suppressive factors, and the differentiation of MDSCs into M1-like macrophages [196]. Books et al. [197] showed that depleting the level of cationic amino acid transporter 2 (CAT2), an L-Arg transporter on MDSCs, can prevent immunosuppression and tumor growth induced by MDSCs. Consequently, T-cell infiltration is considered to be inversely related to MDSC frequency and can be reactivated after MDSC inhibition. Peng et al. [157] found that a novel EP4 antagonist, YY001, can inhibit MDSC function and recast T-cell infiltration. Moreover, the combination of YY001 with PD-1 immunotherapy resulted in a robust antitumor immune response. Although Treg-targeted therapies have not been extensively studied, promoting Treg cell consumption is critical for delaying ADT resistance [121]. Anti-CD25 antibodies selectively depleting Tregs are effective in enhancing the effectiveness [198] of cancer vaccines and improving immunotherapy responses in cancer [199].

Therapies targeting different TME hallmarks have also shown therapeutic potential. ECM-targeted therapies mainly target HIF-1α [200], HAS/hyaluronan/CD44 [NCT00834704] [201], fibronectin [NCT01058538] and MMPs [NCT03486730 and NCT00033215]. However, these clinical trials are designed for solid tumors in general, not specifically for PCa. The pH Low Insertion Peptide (pHLIP) [202] assists STING agonists (STINGa) in targeting acidic cells in the TME, such as CAFs, TAMs, MDSCs, and DCs, which leads to a reduction in TME acidity levels, promotes pro-tumor cells, T-cell recruitment, infiltration, and tumor eradication, and prevents T-cell cytotoxicity from being impaired.

Adoptive cell therapies (ACTs)

A better understanding of the TME in PCa and achievements in other solid tumor treatments will benefit the development of more effective therapies. ACT, especially chimeric antigen receptor T-cell immunotherapy (CAR-T), has demonstrated significant therapeutic promise in the treatment of hematologic malignancies in the past decade [ 203– 206] , which offers innovative guidance for the management of solid tumors. However, a small therapeutic benefit is gained in solid tumors due to the immunosuppressive TME that leads to unsatisfactory immunotherapeutic outcomes. To date, no CAR-T therapies have received approval for solid tumors. A high level of TGFβ is believed to inhibit T-cell-mediated immunity. Hence, Tang et al. [207] found that CAR-T cells present a better tumor-eliminating ability after knocking down endogenous TGFβ receptor II ( TGFBR2) by CRISPR/Cas9. Another strategy for fortifying CAR-T efficacy is armoring CAR-T cells with CAR that targets highly specific PCa tumor-associated antigens, such as PSCA, PSMA, STEAP1, DLL3, and CEA [208]. The occurrence of CART-PSMA-TGFβRDN cells [NCT03089203], a PSMA-targeting TGFβ-insensitive armored CAR T, is regarded as a milestone of PCa immunotherapy, bringing innovative prospects for PCa treatment [209]. In recent years, the engineering of innate immune cells has appeared to be promising for solid tumor treatment. Unlike T cells, NKs are capable of directly recognizing target cells regardless of MHC, which largely reduces the incidence of graft-versus-host disease (GVHD) caused by CAR-T cells [210]. The resources of NK cells for CAR-NK cell manufacturing are not restricted to autologous NK cells. The NK92 cell line, umbilical cord blood, and induced pluripotent stem cells (iPSCs) are practical NK resources and have been used in clinical trials [NCT03692663, NCT03415100, NCT03056339]. CAR-NK cells also have advantages over CAR-T cells in decreasing CRS and neurotoxicity, more diverse tumor-eliminating mechanisms, and limited lifespan [211]. CAR-M, a primary human macrophage modified with CAR, is designed to reprogram the TME by recruiting NKs and T cells, presenting antigens to tumors, and transforming M2-like TAMs into M1-like TAMs [212]. CAR-iMACs [213] are CD19-targeted CAR macrophages induced by pluripotent stem cells (iPSCs) that can polarize to M1-like macrophage target recognition and induce tumor killing. Based on macrophages derived from iPSCs, CAR-iMACs provide the possibility of allogeneic treatment without triggering graft-versus-host disease or host rejection [214]. Other CAR-M designs, such as CAR-M (MerTK) [215] and CAR-147 [216], will not be further discussed in this review.

In conclusion, restoring control over the TME in PCa seems practical for better control and elimination of tumors. Distinct tumor types illustrate diverse preferences and reactions to different therapies due to the discrepant distribution of targets on TME components, TME hallmarks, and tumor characteristics. Increasing resources and energy should be devoted to unidentified targets and potential therapeutic trials for PCa and to draw lessons from extensive solid tumor treatment strategies.

Prospectives

The recognition of the TME has been rapidly updated in recent years. Hanahan et al. [6] reviewed the hallmarks of cancer this year and indicated that senescent cells have been reported as an important cellular component in the TME; however, the role of such cells in the PCa TME has not been investigated. Marin et al. [217] provided promising evidence that senescent cells can efficiently activate DCs and antigen-specific CD8 ⁺ T cells by releasing alarmins, activating interferon signaling, enhancing MHC class 1 machinery, and expressing senescence-specific self-peptides. Therefore, future induction of tumor senescence can activate autologous CD8-dependent antitumor immune responses. Thus, research on the TME composition of lymph nodes and bone marrow metastasis of PCa is coming into sight, giving hope for new approaches against PCa progression.

PCa is considered as “cold” tumor capable of suppressing the immune response, especially in the late stage, and effective therapies that can be sustained for as long as possible are urgently needed. As a result of research on the PCa TME, novel predictors of efficacy and resistance to immunotherapy, potential candidates for antigen-directed therapies, immunosuppressive TME-targeted drugs, and a combination of multiple immunotherapies have emerged. PSMA has shown great potential in radioligand (177Lu-PSMA-617 [218]) and adoptive cellular therapies (TmPSMA-02 [210] and P-PSMA-101-CAR-T [NCT04249947]). The increasingly integrated recognition of the TME will lead to insights into tumor and organ relevance.

In this review, we summarize six hallmarks of the PCa TME and illustrate numerous therapeutic strategies targeting its diverse compartments. Currently, researchers have sought to discover innovative targeted drugs that eliminate tumors directly, sensitize various antitumor cells, and avert drug resistance. Immunotherapeutic drugs have become increasingly popular and have important prospects, especially when applied with either traditional or novel therapies [NCT03849469, NCT04148937 and NCT03910660]. Meanwhile, more integrated images of TME elements and targeted treatments are gradually emerging. Various cancer vaccines [NCT05533203 and NCT03946800], adoptive cell therapies [NCT03089203 and NCT03970382], and oncolytic viruses [NCT04097002] have been developed.

PCa remains a critical and serious health issue worldwide. Therefore, a thorough understanding of the early detection of abnormalities in peritumoral tissue and early editing and reprogramming of the PCa TME will be helpful for effectively inhibiting disease progression. Technology and big data analyses have played critical roles in pushing this course forward, and single-cell RNA sequencing (scRNA-seq) [219], fluorescence in situ hybridization (FISH) [220], human organ-on-chips (also known as organ chips) [221], and artificial intelligence [222] have been applied in recent studies.

In general, this review provides new insights into the PCa TME and promotes precision medicine for the benefit of the increasing PCa patient population.

COMPETING INTERESTS

The authors declare that they have no conflict of interest.

Funding Statement

This work was supported by the grants from the Science and Technology Commission of Shanghai Municipality (No. 18410750200 to S.R.), the National Natural Science Foundation of China (No. 82125025 to S.R.), the “Doctoral Innovation Fund” project of Changzheng Hospital (No. BSCX2022-18 to D.L.), the 2021 Naval Medical University Basic Science Research Fund (Youth Start-up Fund, No. 2021QN29 to Y.C), the 2022 National Natural Science Fund (Youth Fund, No. 82203138 to Y.C.), and the 2022 Pujiang Scholar Program Award (No. 22PJ1402700 to Y.H.).