The role of protein post-translational modifications in prostate cancer

Phosphorylation influences the molecular mechanism of prostate cancer

Phosphorylation is one of the most common PTMs (Singh et al., 2017). Tyrosine kinases such as focal adhesion kinase (FAK) (Liu et al., 2023), fibroblast growth factor receptor 1 (FGFR1) (Wang et al., 2018) and cyclin-dependent kinases are involved in protein phosphorylation or dephosphorylation pathways that promote the occurrence and development of cancer by inducing cancer cell proliferation, inhibiting cancer cell apoptosis, inducing cancer cell invasion and metastasis, inducing cancer cell angiogenesis, and inducing cancer stem cell proliferation, and so on (Liu et al., 2021a). Therefore, targeting phosphorylation pathway is one of the potential pathways for anticancer drug development. PI3K/Akt/mTOR and Ras/MAPK are two important signaling pathways implicated in PCa development (Fig. 1) (D’Abronzo & Ghosh, 2018). Eukaryotic translation initiation factor 4E (eIF4E) is phosphorylated by mitogen-activated protein kinase-interacting kinases 1 and 2 (Mnk1/2) in response to rapamycin (mTOR), and phosphorylation of eIF4E increases the rate of oncogene translation (such as c-Myc and Akt) and promotes drug resistance (such as everolimus and gemcitabine) in prostate cancer (D’Abronzo & Ghosh, 2018). Extracellular signal-regulated kinase 2 (ERK2) phosphorylates the leukemia suppressor receptor (LIFR) S1044 and subsequently activates the protein kinase B (AKT) signaling pathway to phosphorylate AKT S473, which induces the expression of a number of proliferation and transformation genes (Shao et al., 2019). COPS3, the third subunit of the COP9-CSN complex, is highly expressed in PCa tissues and promotes the epithelial-mesenchymal transformation (EMT) of PCa by increasing the phosphorylation level of P38 MAPK, thereby promoting the proliferation, migration and invasion of prostate cancer cells (Zhu et al., 2019). Tousled-like kinase mediates the phosphorylation of NEK1, an amitotic gene A-associated kinase 1, leading to DNA damage and promoting the development of CRPC (Khalil et al., 2022). ErbB-2 is phosphorylated by Src kinase and phosphorylated by AKT, which promotes PCa cell proliferation and migration through PI3K/AKT (Gao et al., 2016). In PCa, loss of nuclear FOXP3 is usually accompanied by low expression of TSC1, which induces c-Myc transcription and protein phosphorylation to synergistically increase c-MYC expression and activate mTOR signaling (Wu et al., 2019). Overexpression of wild-type (WT) EphA7 in PCa cells resulted in reduced tumor volume and increased tumor apoptosis in primary tumors compared to site-mutated phosphorylated EphA7 (Li et al., 2017).

Targeting phosphorylation sites for diagnosis and treatment of prostate cancer

Based on the above molecular mechanisms by which phosphorylation affects prostate cancer, targeting these targets and pathways can be developed for clinical applications in prostate cancer. Phosphorylation controls the androgen receptor (AR) and the PTEN/PI3K/AKT/mTOR axis (McAllister et al., 2020). Cyclin-dependent kinase (CDK) is a serine/threonine kinase that plays a key role in cell cycle progression and has been proposed as a promising target for cancer therapy (Wang, Bode & Zhang, 2023). CDK1 and AKT phosphorylate Ser81 and Ser213 of AR, respectively, while phenethyl caffeic acid (CAPE) reduces the protein levels and activity of CDK1 and AKT and inhibits the phosphorylation of Ser81 and Ser213 on AR, thereby regulating the stability of AR, implying the possibility of using CAPE as a treatment for advanced PCa (Kuo et al., 2019). The Proviral Integration site for Moloney murine leukemia virus (PIM) kinases are a family of oncogenic Ser/Thr kinases (Casillas et al., 2018). An unbiased proteomic screen identified Abl-interactor 2 (ABI2), an integral member of the wave regulatory complex (WRC), as a PIM1 substrate, Ser183 is phosphorylated by PIM1 to promote prostate cancer invasion, while PIM inhibitors can reduce prostate cancer metastasis (Jensen et al., 2023). Androgen deprivation therapy (ADT) is commonly used to treat advanced prostate cancer. Nonetheless, most patients progress to castration-resistant prostate cancer (CRPC) after being treated by ADT. PCa develops a key restrictive androgen biosynthetase of CRPC, 3β-hydroxysteroid dehydrogenase type 1 (3βHSD1) (Hettel & Sharifi, 2018). BMX is a member of TEC family nonreceptor tyrosine kinase. In the presence of dehydroepiandrosterone (DHEA), bone marrow kinase on the X chromosome (BMX) is activated and phosphorylates 3βHSD1 to form an active dimer that promotes the conversion of DHEA to dihydrotestosterone (DHT) (Hettel & Sharifi, 2018). DHT interacts with AR to promote the progression of PCa to CRPC. The BMX inhibitor zanubrutinib can effectively block this process, thereby inhibiting (Qiu, 2023). ErbB-2 can be used as a biomarker for invasive PCa when hyperphosphorylated (Chuang et al., 2010). Androgens induce an increase in p66Shc protein, a 66 kDa proto-oncogene of Src and collagen homolog (Shc), resulting in increased cell ROS levels, ROS oxidation and inactivation of cellular prostatic acid phosphatase (cPAcP), preventing cPAcP from dephosphorylating ErbB-2. This results in ErbB-2 activation and the CRPCa phenotype of PCa cells. Therefore, inhibition of ErbB-2 with antiandrogens significantly reduces the risk of tumor recurrence in PCa xenotransplantation in mouse (Miller, Ingersoll & Lin, 2019). Therefore, using ErbB-2 as the target and combining various methods to treat PCa is worthy of further study. LQB-118 is a pterocarpanquinone with proven antineoplastic activity, which regulates the proliferation, death and migration/invasion of PCa cells via a negative regulator of the Akt/GSK3 signaling pathway. As a result, LQB-118 may be used in the treatment of metastatic prostate cancer, either alone or in combination with another chemotherapeutic agent (Martino et al., 2023). Serine/threonine protein phosphatase 5 (PPP5C) is a member of the protein serine/threonine phosphatase family that regulates the phosphorylation of protein serine/threonine residues and activates or inactivates the corresponding substrates. PPP5C is highly expressed in PCa tissues. After the downregulation of PP5C expression, the phosphorylation of ERK1/2 and JNK is significantly increased, which leads to the arrest and apoptosis of tumor cells in G0/G1 phase, thus inhibiting cell proliferation in PCa cells. Therefore, PPP5C may become a new diagnostic biomarker and therapeutic target for PCa (Lv et al., 2018). For a summary of studies on the impact of protein phosphorylation on the diagnosis, treatment, and prognosis of PCa, please refer to the Table S2.

Glycosylation influences the molecular mechanism of prostate cancer

The modification of glycosylation is mainly catalyzed and regulated by various glycosyltransferases and glycosidases (Samaržija, 2021), which play an important role in the origin and development of malignant tumors. Most tumor markers used in clinical applications are glycoproteins (Butler & Huang, 2021). Studies on the influence of glycosylation modification on the occurrence and development of prostate cancer mainly include 2, 6-sialylation, core fucosylation, branched N-glycans, and LacdiNA (GalNAcβ1–4GlcNAc) glycosylation (Kałuza, Szczykutowicz & Ferens-Sieczkowska, 2021). The structures and types of these glycosylation modifications are illustrated in Fig. 2.

ST6 β-galactoside α-2,6-sialtransferase 1 (ST6GAL1) is an enzyme that catalyzes the addition of 2,6-linked sialic acids to terminal N-linked glycans. Its upregulation in prostate cancer has been found to promote tumor growth and metastasis (Garnham et al., 2019; Gratacós-Mulleras et al., 2020). Growth differentiation factor 15 (GDF15) is a member of the TGF-β superfamily with immunomodulatory functions, and its high expression is often associated with cancer progression. High expression of GDF15 is associated with poor survival in prostate cancer patients. GDF15 N70 is modified by N3H4F3 and N6H3F2, and the glycosylation of GDF15 is completely inhibited when cells are treated with the N-linked glycosylation inhibitor Tunicamycin (TM). GDF15 inhibits EGFR activity, and GDF15 N70 glycosylation can reduce the inhibition of EGFR, thereby leading to the development of castration resistance, which provides a good basis for the future development of selective inhibitors based on GDF15 glycosylation for the treatment of CRPC (Wang et al., 2022). Neural cell adhesion molecule L1 (L1CAM) was found to be a mediator of the pro-invasive effects of FUT8. L1CAM is a highly glycosylated protein known to regulate cell attachment, invasion, and migration in several cancers. α (1,6) Fucosylaminotransferase (FUT8) is an enzyme involved in N-glucosylfucosylation. Higher levels of L1CAM cleavage were observed in FUT8-silenced cells, whereas FUT8 overexpression reduced L1CAM cleavage. Thus, FUT8 drives cancer cell invasion and tumor metastasis, in part due to reduced cleavage of core-fucosylated L1CAM (Mao et al., 2023). Meanwhile, FUT8 mediates glycosylation of several proteins including EGFR, TGF-beta receptor (TGFBR), E-cadherin, PD1/PD-L1, and β1-integrin, and plays an important role in promoting the malignant phenotype of tumor cells (Mao et al., 2023).

Targeting glycosylation sites for diagnosis and treatment of prostate cancer

These studies suggest that further research into protein glycosylation may lead to the development of new biomarkers or drugs for the diagnosis and treatment of prostate cancer. The level of core fucosylated glycoproteins in the serum of prostate cancer patients is significantly increased, and core fucosylated prostate-specific antigen (PSA) shows potential as a diagnostic biomarker for distinguishing prostate cancer from other prostate diseases, such as benign prostatic hyperplasia (BPH) (Liao et al., 2021; Höti et al., 2020). Serum prostate-specific antigen (sPSA) could not distinguish between poorly differentiated, moderately differentiated, and highly differentiated PCa. The integration of N-glycosylation profiles and prostate volume changes into a single urinary glycosylation profile marker (UGM) capable of distinguishing between BPH, PCa and prostatitis has high potential as a PCa biomarker (Vermassen et al., 2015a; Vermassen et al., 2015b). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been employed to detect changes in O-glycans in prostate cancer cells, including increased O-glycan levels, decreased complex O-glycans, and increased sialylation of O-glycans (Bai et al., 2020). The N-acetylgalactosaminyltransferase 7 (GALNT7) belongs to a large family of 20 GALNT isoenzymes and is characterized for exclusively glycosylating proteins/peptides that already carry GalNAc moieties added by other GALNT-isoenzymes. GALNT7 can alter the O-glycosylation of membrane and secreted proteins. It has been found that the levels of GALNT7 in urine and blood of patients with CRPC are higher than normal, and its diagnostic value exceeds that of PSA alone (Scott et al., 2023). Treatment with Myc inhibitors (10074-G5 or 10058-F4) induces the IRE- α-XBP1s pathway to trigger fructose-6-phosphoamidotransferase-1 (GFAT1) and increased protein glycosylation. When Myc inhibitors are used in combination with GFAT-1 inhibitors (DON), there is a synergistic effect in inhibiting prostate cancer cell proliferation and migration, suggesting that targeting Myc and GFAT-1 is a novel approach that may represent a strategy for the treatment of prostate cancer (Zhang et al., 2023).

Ubiquitination influences the molecular mechanism of prostate cancer

Speckle-type POZ protein (SPOP), the most commonly mutated tumor suppressor gene in human primary prostate cancer (Gang et al., 2019), is an E3 ubiquitin ligase that inhibits tumor growth by breaking down cancer-promoting substrates. Wild-type (WT) SPOP induces the expression of caprin1,3-phosphoinositol-dependent protein kinase-1 (PDK1) (Shi et al., 2019). SPOP can hinder tumor growth by inhibiting the activity of AKT kinase (Jiang et al., 2021). Additionally, it can facilitate the degradation of heterogeneous nuclear ribonucleoprotein K (HnRNPK) by promoting its ubiquitination, thereby inhibiting the proliferation of prostate cancer cells (Wu et al., 2021). The SPOP/CUL3/RBX1 complex inhibits PCa progression through ubiquitination of cyclin E1 (Ju et al., 2019). However, mutant SPOP has different effects. It promotes the degradation of transcription factor 2 (ATF2), leading to the proliferation and migration of prostate cancer cells (Ma et al., 2018). Additionally, mutant SPOP can increase androgen production, androgen receptor (AR) activation, and the growth of PCa cells (Shi et al., 2021). The ubiquitin-conjugating enzyme E2S (UBE2S) regulates the stability of p16 and β-catenin through K11-linked ubiquitination, thereby promoting the migration and invasion of PCa cells (Peng et al., 2022). The ubiquitination process requires ATP and hexokinase (HK) is the first rate-limiting enzyme in glycolysis (Nogueira, Patra & Hay, 2018). Docetaxel can activate the expression of hypoxia-inducing factor 1 (HIF-1) and thereby increase the expression of SUMO-specific protease 1 (SENP1), which mediates HK2 desumoylation and promotes HK2 binding to mitochondria (Shangguan et al., 2021). Melatonin reduces the expression of SENP1, which mediates the desumoylation of HDAC1, a key factor in AR transcription activity (Hao et al., 2022). Furthermore, F-box and WD repeat domain containing 2 (FBXW2) can target EGFR for ubiquitination and degradation, thereby inhibiting the proliferation and metastasis of PCa cells (Zhou et al., 2022). USP16 and USP33, as the deubiquitinating enzymes of c-Myc, regulate the proliferation of PCa cells by deubiquitinating and stabilizing the expression of c-Myc. By down-regulating the expression of USP16 and USP33, the growth of PCa cells in vitro was significantly inhibited in vivo (Ge et al., 2021; Ding et al., 2021). Ovarian tumor deubiquitinase 6A (OTUD6A) is highly expressed in prostate cancer tissues, and OTUD6A stabilizes Brg1 and AR expression by removing FBXW7-mediated polyubiquitination of the K27 junction of Brg1 and the SPOP-mediated K11 junction of AR (Fu et al., 2022). BRCA1-associated protein 1 (BAP1) is a deubiquitinating enzyme that can inhibit prostate cancer progression by stabilizing the expression of PTEN and downregulating the PI3K-Akt pathway (Deng et al., 2021).

Targeting ubiquitination sites for diagnosis and treatment of prostate cancer

The use of protein ubiquitination and deubiquitination to affect protein stability can also achieve the goal of disease treatment. Nobiletin, a compound, has the ability to specifically promote the degradation of AR-V7 through the K48-linked ubiquitination form of the AR splice variant 7. It achieves this by preventing the interaction of the deubiquitinating enzymes USP14 and USP22 with AR-V7. In addition, Nobiletin also enhances the sensitivity of CRPC to enzalutamide, effectively inhibiting the growth of CRPC (Liu et al., 2021b). UBC9 mediates the SUMOylation of signal transducer and activator of transcription 4 (STAT4). Inhibiting UBC9 with 2-D08 can promote the activation of tumor-associated macrophage (TAM) and CD8 T cells, preventing the progression of PCa (Xiao et al., 2023). USP14 is one of the related proteins of kinesin family member 15 (KIF15) and acts as a deubiquitinating enzyme, preventing AR and AR-V7 degradation, thereby increasing prostate cancer resistance to enzalutamide (Gao et al., 2021). At the same time, lncRNA PCBP1 antisense RNA 1 (PCBP1-AS1) can also stabilize USP22-AR/AR-V7 complex formation, enhance AR and AR-V7 deubiquitination, and promote CRPC progression and resistance to enzalutamide (Zhang et al., 2021b). The ubiquitin-specific peptidase 1 (USP1) functions as a deubiquitinating enzyme, while SNS-032 as a kinase inhibitor, inducing apoptosis and downregulating USP1 expression, thereby inhibiting PCa proliferation (Liao et al., 2022). Moreover, overexpression of USP7, USP10 and USP12 in PCa cells is associated with poor prognosis in PCa patients and may be used as prognostic markers (Guo et al., 2023). Furthermore, one of the induced degradation techniques developed for target proteins is PROTAC (Proteolytic Targeting Chimera) technology by forming ternary complexes that link the target proteins with E3 ligases (Qi et al., 2021). AR degraders developed using PROTAC technology, such as ARD-61, ARV-110, ARD-2128, and ARD-266, have shown significant inhibitory effects on cancer cell proliferation and have the potential to overcome drug resistance (Yedla et al., 2023; Kargbo, 2022). Most prostate cancer patients treated with docetaxel develop resistance to docetaxel, and nuclear protein 1 (NUPR1), a stress-inducible transcription factor, confers docetaxel resistance to prostate cancer cells, suggesting that NUPR1 plays a role in docetaxel resistance (Schnepp et al., 2020). Additionally, some RNAs are involved in the ubiquitination process. For instance, circ_0006156 regulates S100A9 protein expression via the ubiquitination process, thereby inhibiting R transfer in PCa cells (Zhang et al., 2022a). Detailed studies on the impact of protein ubiquitination on prostate cancer diagnosis, treatment, and prognosis can be found in the Table S3.

Acetylation modification affects prostate cancer

Tumor protein D52 (TPD52) can be acetylated by lysine acetyltransferase 2B (KAT2B), creating antagonism with histone deacetylase 2 (HDAC2) and preventing the interaction between TPD52 and the HSPA8 member, resulting in tumor growth impairment. This represents a target for PCa treatment (Fan et al., 2021). Lysine acetyltransferase 2A (KAT2A) acetylates AR and induces its translocation from the cytoplasm to the nucleus, resulting in increased transcription activity of the AR target gene PSA, thereby increasing resistance to abiraterone (Lu et al., 2021). CBP/P300 mediates the acetylation of HOXB13, AR, JMJD1A, SKP2 and other proteins and promotes the emergence and development of PCa, making it the key factor of antiandrogen resistance of CRPC(Waddell, Huang & Liao, 2021; Nguyen et al., 2022; Xu et al., 2020; Rezaeian et al., 2023). CBP/P300-related factors can promote the degradation of β-catenin through acetylation and thus inhibit prostate cancer progression (Zhou et al., 2019). Furthermore, in CRPC, carnitine palmitoyl transferase 1 A (CPTIA), a rate-limiting step that catalyzes the conversion of acyl coenzyme A to acyl carnitine, provides acetyl groups to histones to promote tumor growth and anti-androgen resistance (e.g., enzalutamide) (Joshi et al., 2019). In patients with advanced PCa, the degree of acetylation of the H3 in their tissues is significantly increased (Puzyrenko et al. , 2022). Furthermore, N-α-acetyltransferase 10 (NAA10), as an acetyltransferase that acetylates both N-terminal amino acid and internal lysine residues of proteins, which has been found to promote the proliferation and migration of prostate cancer cells, as well as induce autophagy (Kim et al., 2020). On the other hand, acetyl-CoA acetyltransferase 1 (ACAT1), known as a protumor factor in prostate cancer, has been shown to promote the occurrence and development of the disease by inhibiting autophagy and eliminating reactive oxygen species (Guan et al., 2022). SIRT5 inhibits PI3K and mediates PI3K/AKT/NF-B signaling to suppress prostate cancer metastasis (Choi et al., 2022). Moreover, SIRT5 promotes the activity of the MAPK signaling pathway through ACAT1, enhancing the proliferative, migratory, and invasive abilities of prostate cancer cells (Guan et al., 2020).

Acetylation modification may be used as a therapeutic target in prostate cancer

Since acetylation modification affects the occurrence and development of prostate cancer, acetylation modification can also achieve the purpose of treating prostate cancer. Docetaxel, a semisynthetic taxane, has exhibited significant single-agent activity against prostatic tumors (Pienta, 2001). However, drug resistance and toxicity often occur during treatment (Aragon-Ching & Madan, 2018). Therefore, it is clear that acetylation and deacetylation of proteins affect the sensitivity of prostate cancer to drugs. Kruppel-like factor 5 (KLF5), a member of the KLF family, regulates the expression of a large number of novel target genes and is involved in a variety of cellular functions including desiccation, proliferation, apoptosis, autophagy and migration. Transforming growth factor-β (TGF-β) can induce a process known as acetylation of Kruppel-like factor 5 (KLF5) (Ac-KLF5), which promotes bone metastasis in PCa by activating the C-X-C chemokine receptor type 4 (CXCR4). The use of the CXCR4 inhibitor AMD3100 has been shown to increase tumor sensitivity to docetaxel and inhibit bone metastasis in PCa (Zhang et al., 2021a). Ac-KLF5 also plays a role in regulating prostate development (Zhang et al., 2020). Nitazoxanide which is an anti-parasitic drug with potent antiviral activity as an inhibitor can suppress Ac-KLF5-induced bone metastasis in PCa by regulating KLF5 function (Huang et al., 2023). SIRT3 is involved in regulating the acetylation level of various proteins (such as HSD17B4, ACO2, etc.) to affect protein stability, and it can serve as a diagnostic marker for predicting PCa progression (Huang et al., 2020). Notably, mitochondrial aconitase (ACO2), a nuclear encoded tricarboxylate cycling enzyme, has significantly increased activity. AR can regulate the expression of SIRT3 by binding to steroid receptor coactivator 2 (SRC-2). In the absence of SRC-2, the expression of SIRT3 is enhanced, and the acetylation of ACO2 is reduced. Increased expression of SRC-2 and decreased expression of SIRT3 serve as genetic markers for the accumulation of prostate cancer metastases (Sawant Dessai et al., 2021). Phosphoenolpyruvate carboxykinase subtype 2 (PCK2), which has complex functions in addition to regulating glucose metabolism, may promote tumorigenesis by reducing acetyl-CoA levels through a reduction in the TCA cycle. PCK2 is therefore a potential therapeutic target for aggressive prostate tumours (Zhao et al., 2017).