As prostate cancer progresses, the combination of PTEN and TP53 gene deletions and rearrangements of the ERG gene serves as a crucial driving factor (35–37) Among these alterations, the incidence of ERG rearrangement is the highest (38). The transcription factor known as ERG, which is produced by the ERG gene, was initially identified in colon cancer cells in 1987 and is recognized as a new member of the E-26 transformation-specific (ETS) oncogene family (39, 40). The ETS family of genes has been shown to play crucial roles as regulatory elements in the transcription process and are directly implicated in the mechanisms of cell proliferation, differentiation, and angiogenesis (41, 42). TMPRSS2 is a transmembrane protease found in normal prostate epithelium and is also present in semen (43). Both ERG and TMPRSS2 are located on chromosome 21, approximately 3 Mb apart (44, 45). Studies have found that in about 50% of prostate cancer cases, intronic deletions between the TMPRSS2 and ERG genes lead to the fusion of the TMPRSS2 promoter with ERG, a fusion that exists in over 90% of ERG-overexpressing prostate cancers (46). A recent observational study has also reported similar findings (47). TMPRSS2 is one of the target genes of the AR (48), and when androgens stimulate TMPRSS2-ERG fusion-positive prostate cancer cells, a significant increase in ERG expression can be observed (49). H3K9/14 acetylation is a chromatin mark that is highly localized to the 5’ regions of transcriptionally active human genes (50). Research by Nguyen et al. indicates that ERG can promote the expression of YAP in prostate cancer cells by influencing the H3K9/14 acetylation of the YAP promoter. Furthermore, the study revealed that silencing YAP effectively inhibited the trends of cell growth and tumor transformation induced by ERG overexpression in prostate epithelial cells. This indicates that YAP serves as an essential intermediate factor in the ERG-mediated transformation of prostate epithelial cells and the invasion of tumor cells (51). Recent studies have clearly established that the TMPRSS2-ERG fusion plays a significant role among the primary drivers of malignant transformation in normal prostate tissue (52). It is proposed that the TMPRSS2-ERG fusion event activates the proto-oncogenic properties of ERG within normal prostate epithelium (53). Moreover, the aberrant expression of ERG resulting from the TMPRSS2-ERG fusion is closely associated with increased cell proliferation, neovascularization, and invasive behavior in prostate cancer (54, 55). Based on the results of Nguyen et al., this phenomenon is likely mediated by YAP. In summary, AR modulates YAP expression through TMPRSS2-ERG, which has significant implications for the progression of prostate cancer.

In addition to promoting the transcription of YAP, AR can also enhance its protein levels by influencing the translation process of YAP protein. Research conducted by Salem et al. observed that when androgens stimulated LNCaP cells, the protein level of YAP increased in correlation with the intensity and duration of stimulation, while no significant changes were noted in YAP mRNA levels (56). Furthermore, the study found that AR activation-induced increase in YAP activity was inhibited by the translation inhibitor cycloheximide (CHX) but was not affected by the proteasome inhibitor (MG132), indicating that AR activation influenced YAP’s translation process rather than protein degradation. Consequently, Salem et al. propose that AR promotes the aberrant activation of YAP by modulating the translation process of YAP protein, thereby influencing cell proliferation and metastasis. Although the precise mechanisms by which AR regulates protein translation in prostate cancer remain to be fully elucidated, existing studies suggest that AR may indirectly regulate the mTOR signaling pathway to participate in the protein translation process (57). Kallikrein-related peptidase 4 (KLK4) is one of the target genes of AR, which can enhance mTOR activity by inhibiting REDD1 (58). mTOR is a serine/threonine kinase that is categorized into two distinct complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) (59). Notably, mTORC1 can phosphorylate ribosomal protein S6 kinase and eukaryotic translation initiation factor 4E-binding protein (4EBP) (60). When eIF4E is phosphorylated and activated, it dissociates from 4EBP, enabling the formation of a heterotrimeric eIF4F complex in conjunction with scaffold protein eIF4G and RNA helicase eIF4A, thus facilitating mRNA recruitment to the ribosome (61, 62). The role of AR in protein translation may elucidate a potential mechanism through which AR contributes to prostate cancer progression by promoting YAP translation.

AR not only upregulates protein levels by promoting the translation of YAP but also enhances the stability of the YAP protein. Studies have demonstrated that AR can enhance YAP protein stability and activity by modulating its post-translational modifications (63).

YAP, a core component of the Hippo pathway, is primarily localized in the cytoplasm, yet its primary transcription-dependent biological functions are typically executed in the nucleus. The translocation of YAP between the nucleus and the cytosol is regulated by multiple mechanisms, with post-translational modifications—especially phosphorylation—playing a crucial role in its nucleocytoplasmic transport (64, 65). YAP possesses multiple phosphorylation sites closely associated with its nuclear translocation, the most notable being the phosphorylation of serine-127. When the Hippo pathway is activated, MST1/2 and LATS1/2 kinases cascade to phosphorylate Ser-127, causing the phosphorylated YAP to bind with 14-3-3 proteins and sequester it in the cytoplasm, ultimately leading to its degradation via the ubiquitin-proteasome pathway (66, 67). Cinar et al. proposed the AR-STK4/MST1-PP2A axis as a regulatory mechanism for YAP nuclear localization (63), with serine/threonine phosphatases PP1/PP2A acting as effective inhibitors of STK4/3 (68). They demonstrated that androgen stimulation-induced AR activation in prostate cancer cells significantly reduced Ser-127 phosphorylation and increased both nuclear and total cellular YAP protein levels, thereby promoting the expression of YAP-dependent genes. These effects were abolished upon enzalutamide treatment or AR silencing (63). Additionally, ectopic expression of STK4/MST1 in cell lines confirmed that the reduction in Ser-127 phosphorylation levels resulted from antagonism between androgen signaling and STK4/MST1 (63). To determine whether androgen-induced inhibition of STK4/MST1 signaling was mediated by PP1/PP2A, Cinar et al. found that treatment with PP1/PP2A inhibitors restored Ser-127 phosphorylation levels (63). Therefore, it can be concluded that AR activation in prostate cancer cells may maintain YAP protein stability and activity by reducing Ser-127 phosphorylation levels.

In the progression of prostate cancer, evidence suggests that YAP can regulate AR, in addition to AR’s regulation of YAP. Bainbridge et al. demonstrated that inhibitor of nuclear factor kappa B kinase epsilon (IKBKE) could not only affect AR transcriptional levels through YAP but also regulate AR transcriptional activity, and proposed a regulatory strategy involving IKBKE-YAP-AR in prostate cancer (69). IKBKE, also known as IKK-I, is a mitogen-activated protein kinase and belongs to the non-classical IκB kinase family (70, 71). IKBKE plays a significant role in regulating inflammation, cellular immunity, and the progression of various metabolic diseases. It has been confirmed to be associated with multiple tumors, including prostate cancer (72–74). Bainbridge et al. found through WB and quantitative PCR experiments that silencing IKBKE not only inhibited the expression of AR target genes (such as PSA and TMPRSS2) in both androgen-sensitive and androgen-insensitive cells, but also significantly reduced the expression level of AR itself. Even with exogenous expression of AR-GFP, the expression of AR target genes was not restored. ChIP experiments further demonstrated that AR-GFP was still able to bind to the PSA enhancer, suggesting that IKBKE not only influences AR expression levels but also regulates its transcriptional activity (69). IKBKE has previously been shown to regulate YAP expression through its modulation of the Hippo pathway via LATS1/2 (75). Under IKBKE silencing conditions, Bainbridge et al. observed a significant reduction in the expression levels of both YAP and MYC. After treatment with the proteasome inhibitor MG132, the level of YAP was restored, further validating the role of IKBKE in regulating YAP expression (69). MYC, one of the most common oncogenes in humans, is a key transcription factor that plays an important role in various biological processes (76, 77). Dysregulation of MYC is often closely associated with the development and progression of multiple cancers (78, 79). Previous studies have indicated that MYC can directly bind to the AR gene, promoting AR mRNA synthesis (80). Bainbridge et al. observed a significant reduction in MYC binding to the AR gene in cells with silenced IKBKE, as shown by ChIP experiments. Moreover, ectopic expression of constitutively active YAP effectively restored the suppression of AR mRNA levels caused by IKBKE silencing (69). It is also noteworthy that MYC is considered a downstream target of YAP (81, 82), which further supports the conclusions of Bainbridge et al.’s study. Therefore, it can be concluded that in prostate cancer, YAP not only regulates AR expression but also does so in an IKBKE-dependent manner.

While mainstream perspectives suggest that the synergy between YAP and AR promotes the development of prostate cancer, a recent study presents a contrasting viewpoint. This study argues that YAP competes with AR for TEAD, thereby inhibiting the expression of AR target genes (83). The TEAD, also referred to as transcription enhancer factors (TEFs), plays a critical role in development (84). Additionally, it is well-established that TEAD is associated with YAP (85). Using RNA-seq analysis, Li et al. discovered that in PCa cells with ectopic expression of YAP or constitutively active YAP, while YAP target genes were significantly upregulated, AR target genes were notably downregulated, and PCa cell proliferation was inhibited. Similar results were observed in cells treated with Hippo pathway inhibitors: AR target genes were downregulated in a dose-dependent manner, and this downregulation could be reversed by YAP knockdown (83). Further ChIP experiments indicated that this inhibitory effect might result from significantly reduced binding between AR and its target gene promoters/enhancers. Co-IP and WB experiments revealed that AR and YAP each formed complexes with TEAD1/4, although no direct binding between AR and YAP was detected. Meanwhile, ChIP-seq analysis demonstrated co-occupancy of AR and TEAD at AR target gene loci (83). Based on these findings, Li et al. proposed that YAP might suppress PCa progression by competing with AR for TEAD binding, thereby downregulating AR target genes. To test this hypothesis, researchers performed ChIP-qPCR analysis in cells expressing a YAP mutant deficient in TEAD binding. The results showed that neither AR binding to its target genes nor AR target gene expression levels were affected, further supporting their hypothesis (83). These findings suggest that YAP has the potential to suppress prostate cancer progression through inhibition of AR signaling.

Current research has noted that YAP can function as both an oncogene and a tumor suppressor in various cancers (86), although the specific regulatory mechanisms remain unclear. In prostate cancer and other tumors, the prevailing view still considers YAP as an oncogenic factor (87). While Li et al.’s findings differ from this mainstream perspective, they provide new insights into YAP’s dual regulatory role in AR signaling. Whether promoting or suppressing cancer development, YAP demonstrates significant value as a potential therapeutic target.

Due to the absence of a DNA-binding domain, YAP cannot independently bind to DNA. Consequently, YAP must interact with DNA-binding transcription factors to associate with target gene promoters, thereby initiating the transcription of downstream genes and promoting cell proliferation, epithelial-mesenchymal transition (EMT), and the maintenance of stemness (89). Thus, the YAP-TEAD complex is considered central to the Hippo pathway and represents the most promising target for YAP inhibitors, particularly in comparison to the upstream proteins of the Hippo pathway. Verteporfin (VP), a benzoporphyrin derivative, was approved by the U.S. Food and Drug Administration (FDA) in 2000 for the treatment of macular degeneration (90). Recent studies have demonstrated that VP can selectively bind to YAP, inducing a conformational change that inhibits the interaction between YAP and TEAD (91). Glioblastoma (GBM) is recognized as the most aggressive and prevalent primary brain tumor, and it is largely regarded as an incurable disease. A recent analysis of chromatin accessibility in glioblastoma revealed that factors such as TEAD and YAP are associated with the migration and epithelial-mesenchymal transition of GBM (92). In the study conducted by Barrette et al., the effects of VP on tumor proliferation and migration in vitro were assessed using glioblastoma patient-derived cell lines, as well as its impact on tumor burden and survival rates in patient-derived xenograft (PDX) models (93).This study demonstrates that VP can disrupt the migration and epithelial-mesenchymal transition (EMT) of GBM by inhibiting YAP-TEAD activity. Furthermore, VP has been shown to reduce the burden of infiltrative tumors and improve survival rates in PDX models without causing systemic toxicity.

Gastric cancer (GC) is the fifth most commonly diagnosed cancer and the fourth most common cause of cancer death (94). Studies have clearly indicated that the Hippo signaling pathway is abnormally expressed in various solid tumors, including GC, and have suggested that YAP is closely related to tumor growth and metastasis; thus, YAP is a critical therapeutic target in the treatment of GC (95). Vestigial-like family member 4 (VGLL4) is a transcriptional cofactor belonging to the VGLL family. Similar to YAP, VGLL4 lacks a DNA-binding domain and requires pairing with TEAD through its C-terminal Tondu (TDU) domain to exert its transcriptional regulatory functions (96–98). Research has demonstrated that VGLL4 is capable of directly competing with YAP for binding to TEAD, creating a complex with TEAD through its two TDU domains, which ultimately hinders the growth and advancement of cancer cells (99). Jiao et al. designed a VGLL4 mimetic peptide, Super-TDU, which can directly target TEADs and reduce the interaction between YAP and TEAD, resulting in a dose-dependent downregulation of the expression of YAP downstream genes CTGF, CYR61, and CDX2 (100). Furthermore, Jiao et al. demonstrated that Super-TDU can inhibit cell viability and colony formation of GC cell lines in vitro, as well as suppress the growth of GC tumors in mouse models. This further supports the potential of Super-TDU as a therapeutic agent for human cancers by inhibiting the YAP-TEAD interaction.

The current development of YAP inhibitors primarily focuses on the relationship between Hippo-YAP dysregulation and tumor progression, which can be broadly categorized into two main strategies: 1. Activating the dysregulated Hippo pathway to inhibit YAP activation; 2. Directly targeting YAP/TEAD to prevent the formation of the YAP-TEAD complex (101). Although the strategy targeting the YAP-TEAD complex is more direct and effective, some studies have indicated that Hippo pathway-activating drugs show promise in gastrointestinal tumors. Dysregulation of the Hippo pathway is prevalent in tumors (102). Among the various mechanisms, the inactivation of MST1/2 and LATS1/2 is often implicated in the abnormal activation of YAP, leading to excessive proliferation and tumorigenesis (103). Tang et al. discovered that STRN3 functions as a modulator of PP2A, facilitating the recruitment of MST1/2 to the PP2A core enzyme complex, which results in the dephosphorylation of MST1/2 (104). Building on this finding, Tang et al. developed a Hippo-activating peptide (SHAP) derived from STRN3. SHAP disrupted the interaction between STRN3-mediated MST1/2 and PP2A in a dose-dependent manner. In PDX models of gastric cancer, significant tumor regression was observed, accompanied by a notable reduction in the expression of YAP target genes (CYR61, CTGF). Furthermore, RNA-seq transcriptome analysis and GSEA demonstrated that Hippo signaling is a target of SHAP. A recent study suggested that other drugs with similar mechanisms of action can reactivate the Hippo pathway, thereby inhibiting gastric cancer and enhancing chemotherapy sensitivity (105).MST1/2 activation drugs not only effectively inhibit the progression of gastric cancer but are also capable of significantly suppressing the metastasis of colorectal cancer. Tripartite motif-containing protein 21 (TRIM21) is a widely distributed RING-type E3 ubiquitin ligase (106). In colorectal cancer, TRIM21 can directly interact with and ubiquitinate the lysine 473 (K473) site of MST2 via K63 linkages (107). This ubiquitination enhances the formation of MST2 homodimers, which are critical for its autophosphorylation and activation (108, 109). Activated MST2 inhibits the epithelial-mesenchymal transition (EMT) of tumors by influencing the nuclear-cytoplasmic translocation of the YAP protein. Liu et al. identified Vilazodone as an ideal ligand for TRIM21 using the AlphaFold and ZINC15-DrugBank databases, demonstrating its effectiveness in inhibiting the migration and invasion of colorectal cancer (CRC) cells. In summary, the therapeutic strategy of reactivating the Hippo pathway has shown promise in the treatment of gastrointestinal tumors, providing further insights into the clinical potential of YAP inhibitors.

YAP serves as a critical factor in prostate cancer progression. Studies have shown that silencing YAP significantly inhibits tumor cell proliferation, metastasis, and castration resistance (26, 110, 111). However, there are currently no clinical studies on YAP inhibitors related to prostate cancer. Clemens Thoma suggested that targeting the AR-YAP interaction might be key to treating this disease (112). Kuser-Abali et al. silenced YAP expression in C4-2 cells using shRNA technology and found that compared to the control group, the growth and invasion abilities of the shYAP group were significantly reduced. Moreover, both androgen-induced cell proliferation and AR target gene expression were suppressed, and this result was confirmed in a mouse model (32). In VP-treated cells, besides inhibiting cell proliferation and invasion, apoptosis was also promoted. Immunofluorescence, Co-IP, and WB experiments confirmed that VP significantly inhibited YAP nuclear translocation and the AR-YAP interaction (32).YAP not only plays a crucial role in tumor cell proliferation and invasion but has also been demonstrated to induce cancer stemness in prostate cancer, thus promoting enzalutamide resistance (113). In a mouse model constructed with enzalutamide-resistant cells by Hsiu-Chi Lee et al., VP showed significantly better tumor growth inhibition than other AR inhibitors (113). In the tumor microenvironment, studies have found that YAP can promote the recruitment of myeloid-derived suppressor cells (MDSCs), thereby exerting immunosuppressive effects. Silencing YAP expression in a mouse model reduced MDSC infiltration and inhibited tumor growth (114). Recent studies have also proposed a therapeutic approach combining YAP inhibition, radiotherapy, and PD-1 blockade. Although this strategy was tested in a preclinical colon cancer model, it provides new evidence for the immunomodulatory effects of YAP inhibitors and presents new insights for combination therapy approaches (115).

In conclusion, the multiple roles of YAP in prostate cancer make it a potential therapeutic target, and future clinical studies are expected to reveal its practical efficacy as a treatment strategy.