Research progress on the role of AGC kinase family in bladder cancer

Abstract

Bladder cancer is a common malignant tumor worldwide, with rising incidence and mortality rates. In recent years, the role of the AGC kinase family (including PKA, PKC, PKG, etc.) in the initiation, progression, and metastasis of bladder cancer has attracted widespread attention. AGC kinases regulate various cellular processes such as proliferation, migration, metabolism, and apoptosis through phosphorylation of specific substrates. Aberrant activation or expression of these kinases is closely associated with the malignant progression of bladder cancer. This review summarizes the current research on the AGC kinase family in bladder cancer, focusing on the roles of PKA, PKC, and PKG in bladder cancer cell biology, and discusses key signaling pathways related to these kinases, such as the PI3K/Akt and MAPK/ERK pathways. Furthermore, the potential of AGC kinases as therapeutic targets has been extensively explored, with preclinical studies showing promising results for targeted inhibitors and combination therapies. Finally, we discuss future research directions, including molecular mechanisms of AGC kinases, the development of targeted therapies, and clinical trial design, aiming to provide a theoretical basis and strategy for bladder cancer treatment.

Introduction

Bladder cancer is one of the most common cancer types globally. According to the Global Burden of Disease Study, the global incidence and mortality rates of bladder cancer increased by 54.4% and 30.4%, respectively, in 2019. In 2022, there were 613,791 new cases of bladder cancer worldwide, ranking it as the ninth most common cancer. The development of bladder cancer is influenced by various factors, including age, gender, and genetic predisposition, which are considered high-risk factors. Recent studies have highlighted the crucial role of the AGC kinase family (including protein kinase A (PKA), protein kinase C (PKC), and protein kinase G (PKG), among others) in bladder cancer. These kinases are closely linked to tumorigenesis and cancer progression. The following is a detailed summary of the recent advancements in bladder cancer research involving the AGC kinase family.

The AGC kinase family currently includes 63 evolutionarily related serine/threonine kinases, including PKA (protein kinase A), which is cAMP-dependent; PKG (protein kinase G), which is cGMP-dependent; PKC (protein kinase C), which is activated by lipids; as well as Akt/protein kinase B, Aurora kinases, ribosomal protein S6 kinase, phosphoinositide-dependent kinases, and YANK kinases [1] (Fig. 1). These kinases play a significant role in cellular signal transduction. By phosphorylating specific substrates, they regulate various cellular processes, such as cell proliferation, survival, migration, and metabolism [2]. Dysregulation or abnormal expression of AGC family kinases has been implicated in the development of several cancers, including bladder cancer.

Fig. 1.

Schematic representation of the AGC kinase family members.

(adapted from cell signaling)

Role of AGC kinases in bladder cancer

AGC family kinases display distint and abnormal expression profiles in bladder cancer. Through analysis with the UALCAN online database, it has been observed that these kinases are not only aberrantly expressed in bladder cancer tissues but also correlated with patient survival outcomes, as depicted in Fig. 2. Extensive research, encompassing both in vitro and in vivo studies, has consistently shown that mutations within AGC kinases or the aberrant activation of their downstream signaling pathways can induce cellular transformation and drive tumor progression.

Fig. 2.

Expression and Survival Analysis of Representative AGC Kinases in Bladder Cancer Based on the UALCAN Database. A Expression and Survival Analysis of PKA. B Expression and Survival Analysis of PKC. C Expression and Survival Analysis of PKG. D Expression and Survival Analysis of SGK2. E Expression and Survival Analysis of STK32C. F Expression and Survival Analysis of PDK4

PKA (protein kinase A)

PKA is a critical enzyme involved in intracellular signal regulation. PKA activation is typically dependent on cyclic adenosine monophosphate (cAMP) levels, and PKA regulates vital cellular processes, including growth, proliferation, and apoptosis, through phosphorylation of various target proteins [3]. In bladder cancer cells, PKA activity may be modulated, influencing tumor growth and progression. Additionally, PKA can affect bladder cancer cell migration and invasion. Experimental studies have shown that PKA regulates migration by affecting cytoskeletal remodeling and the expression of cell adhesion molecules [4]. Moreover, PKA is not only a component of an independent signaling pathway but may also interact with other pathways, such as the PKCα pathway. For instance, PKCα activation may modulate PKA activity, affecting cancer cell behavior [5], which highlights the complex regulatory role of PKA in bladder cancer initiation and progression.

PKC (protein kinase C)

The PKC family consists of several isoforms, including PKCα, PKCβ, PKCγ, PKCδ, PKCε, and PKCζ, each having specific functions and regulatory mechanisms within cells [6]. PKC interacts with various signaling molecules to regulate cell processes like proliferation, apoptosis, migration, and invasion [7]. Studies on PKC suggest that it plays a crucial role in bladder cancer metastasis and drug resistance. Different PKC isoforms show distinct expression patterns in bladder cancer. For example, overexpression of PKCα is associated with the invasiveness of bladder cancer [8], while PKCδ may exert inhibitory effects [9]. PKCα expression is significantly higher in cancerous bladder tissues compared to normal tissues, supporting its role in bladder cancer pathogenesis [7]. Western blot analysis has revealed that PKCα expression correlates with tumor recurrence in bladder cancer samples. PKCα promotes resistance to apoptosis by stimulating nuclear translocation of NF-κB p65 [10]. NF-κB, a transcription factor, promotes tumor cell survival and proliferation, and the PKCα/NF-κB signaling axis is thought to play a key role in bladder cancer development and progression. Therefore, PKCα not only promotes bladder cancer cell survival but may also influence patient prognosis. The activation status of PKCα is considered a potential urinary biomarker for bladder cancer [11], providing new insights for early diagnosis and monitoring. In addition to PKCα, other PKC isoforms, such as PKCε and PKCζ, are also involved in tumorigenesis, though research on their roles remains limited and warrants further investigation.

PKG (protein kinase G)

PKG is a cGMP-dependent kinase that regulates various physiological functions by phosphorylating target proteins, including smooth muscle relaxation, cell proliferation, apoptosis, and metabolic regulation. Although studies on PKG in bladder cancer are relatively scarce, preliminary research suggests that PKG expression levels vary significantly in bladder cancer cells. In advanced stages of bladder cancer, PKG expression may be reduced, which is hypothesized to correlate with the malignancy of the tumor [12]. Thus, detecting PKG expression may provide valuable information for predicting bladder cancer prognosis. Furthermore, PKG activation has been shown to increase the expression of tumor-suppressor proteins, such as p21, p27, and HINT1, which play crucial roles in cell cycle regulation and apoptosis [13]. PKG’s interactions with other signaling molecules might influence bladder cancer metastasis, as studies suggest that PKG can regulate the expression of growth factors or cytokines that affect tumor cell migration. These interactions may present novel therapeutic targets for bladder cancer metastasis [14]. Additionally, PKG activity has been shown to enhance bladder cancer cell sensitivity to chemotherapy, with agents like etoposide triggering PKG activation and subsequent tumor cell apoptosis, thus inhibiting tumor growth [15].

Other AGC kinases in bladder cancer research

Other members of the AGC kinase family also exhibit potential importance in bladder cancer research.

SGK2

SGK2 is a kinase associated with growth factor signaling. Although its expression and function in bladder cancer remain unclear, preliminary studies suggest that it may promote bladder cancer cell proliferation, migration, and invasion by enhancing the β-catenin/c-Myc signaling pathway [16].

STK32C

STK32C, a member of the YANK family within the AGC superfamily, shows elevated expression in bladder tumors compared to adjacent normal tissues, indicating its potential role in bladder cancer progression [17].

PDK4

PDK4 plays a pivotal role in glucose metabolism and its function in bladder cancer warrants attention. Studies suggest that PDK4 may be involved in the metabolic reprogramming of bladder cancer cells, influencing tumor cell growth and survival [18, 19].

Table 1.

Summary of commonly implicated AGC kinases in bladder cancer

Name	p (expression of normal vs. primary)	p (effect of expression level on BLCA patient survival)	Function	Refs.
PKA	3.163400E-01	0.024	PKA affects the growth and progression of bladder cancer cells and regulates their migratory capacity	[4]
PKC	1.857510E-05	0.077	Overexpression of PKC-α is associated with the invasiveness of bladder cancer, while PKC-δ may exert inhibitory effects	[8, 9]
PKG	2.960000E-03	0.0045	Activation of PKG can enhance the sensitivity of bladder cancer cells to chemotherapeutic drugs and may serve as a prognostic indicator for bladder cancer	[16]
SGK2	4.1454000E-05	0.02	SGK2 promotes proliferation, migration, and invasion of bladder cancer cells by enhancing the β-catenin/c-Myc signaling pathway	[17]
STK32C	< 1E-12	0.9	STK32C influences the progression of bladder cancer	[18]
PDK4	1.037850E-04	0.29	PDK4 may be involved in metabolic reprogramming in bladder cancer, affecting its growth and survival	[19]

Key signaling pathways of AGC family kinases in regulating bladder cancer

AGC family kinases influence the biological characteristics of bladder cancer through multiple signaling pathways. The main signaling pathways involved include:

PI3K/Akt pathway

PI3K (phosphoinositide 3-kinase) and Akt (protein kinase B) are the primary components of this signaling pathway. The activation of PI3K is typically triggered by growth factors or cytokines, which then catalyze the production of PIP3 (phosphatidylinositol-3,4,5-trisphosphate), a molecule that further activates Akt. The activation of Akt promotes the phosphorylation of various downstream effectors, which are involved in regulating critical biological processes such as cell survival, proliferation, metabolism, and migration [20].

Cell proliferation

The PI3K/Akt pathway promotes cell proliferation by regulating the expression of cyclins and cell cycle inhibitors. For instance, CSTP1 interacts with Akt at the Ser473 site, dephosphorylating Akt and preventing cell cycle progression while promoting apoptosis in bladder cancer cells [21]. In bladder cancer, Akt activation facilitates cell cycle progression, further promoting tumor cell proliferation [22].

Cell survival

This pathway enhances tumor cell survival by modulating the expression of anti-apoptotic proteins such as Bcl-2 and Mcl-1 [23]. For example, RRM2 activation of the PI3K/Akt pathway accelerates bladder cancer progression; knockdown of RRM2 in bladder cancer cells significantly induces apoptosis and arrests the cell cycle at the G0/G1 phase [24]. Overactivation of Akt in bladder cancer cells inhibits apoptosis, thereby enhancing tumor cell resistance [25].

Metabolic reprogramming

The PI3K/Akt pathway is also involved in regulating tumor cell metabolism, promoting the Warburg effect (aerobic glycolysis). In bladder cancer cells, Akt activation enhances the phosphorylation of glucose transporters and associated metabolic enzymes, thereby increasing glucose uptake and utilization, supporting rapid tumor cell proliferation [26].

Metastasis and invasion

The PI3K/Akt pathway plays a crucial role in tumor metastasis and invasion [27, 28]. By regulating cytoskeletal remodeling [29] and epithelial-to-mesenchymal transition (EMT) [30], this pathway enhances tumor cell migration. In bladder cancer, Akt activation is closely associated with the downregulation of cell adhesion molecules and upregulation of matrix metalloproteinases, promoting tumor cell invasion [31].

Tumor microenvironment

Recent studies have highlighted the importance of the PI3K/Akt pathway in the tumor microenvironment (TME) of bladder cancer. The recruitment and activation of tumor-associated macrophages (TAMs) are tightly linked to the PI3K/Akt pathway. Tumor cells secrete cytokines that activate PI3K/Akt signaling, thereby modulating macrophage function and promoting tumor growth and metastasis [32]. Moreover, PI3K/Akt signaling can alter the overall characteristics of the tumor microenvironment, such as by modifying the function of stromal cells to further support tumor progression and metastasis [33, 34].

Fig. 3.

Schematic representation of the PI3K/Akt signaling pathway

MAPK/ERK pathway

The MAPK/ERK signaling pathway is a signal transduction network consisting of multiple enzymes, including RAF, MEK, and ERK. Activation of this pathway is typically induced by extracellular signals such as growth factors and cytokines, which then trigger a series of phosphorylation events that ultimately alter gene expression and cellular behavior.

Tumorigenesis and progression

The MAPK/ERK pathway plays a significant role in the initiation and progression of bladder cancer. YAP has been shown to be closely associated with the proliferation and progression of bladder cancer cells, primarily through the activation of the ERK/MAPK pathway [35]. ERK1/2 and MEK are key members of the MAPK/ERK pathway, regulating numerous cellular and physiological processes, including development, cell cycle control, proliferation, differentiation, survival, and apoptosis [36].

Cell proliferation and migration

LMTK3 (lemur tyrosine kinase 3) promotes the proliferation and migration of bladder cancer cells by activating the ERK/MAPK signaling pathway [37]. In bladder cancer cells, downregulation of IQGAP2 can activate the MAPK/ERK pathway, promoting epithelial-to-mesenchymal transition (EMT) and enhancing cell migration [38].

Tumor microenvironment

The MAPK signaling pathway not only influences the behavior of tumor cells but also affects bladder cancer progression by modulating the tumor microenvironment [39]. For instance, activation of the MAPK pathway can alter immune cell function, influencing tumor immune evasion mechanisms and promoting tumor growth and metastasis [40].

Gene mutations and aberrant activation of signaling pathways

Large-scale genomic studies have revealed frequent alterations in the MAPK signaling pathway, such as amplification of the RAF gene, which is considered one of the driving events in bladder cancer. Aberrant activation of this pathway may lead to sustained cell proliferation and survival, thus contributing to tumor development [41].

Fig. 4.

Schematic representation of the MAPK/ERK signaling pathway

(adapted from MedChemExpress)

Potential targets of the AGC kinase family in bladder cancer

Given the significance of AGC family kinases in bladder cancer, they are considered potential therapeutic targets. Aberrant activation of AGC kinases in bladder cancer is closely associated with tumor progression and prognosis. For instance, the overactivation of Akt has been found to correlate with the invasiveness and metastatic potential of bladder cancer [31]. Moreover, disruption of the CDK4/6 pathway is commonly observed in advanced bladder cancer [42, 43], underscoring the need for targeted therapies against these signaling pathways.

Small-molecule inhibitors

Inhibitors targeting AGC kinases primarily focus on PI3K/Akt inhibition, such as MK-2206, GDC-0068, and AZ7328. These agents have demonstrated efficacy in inhibiting cell growth and inducing cell cycle arrest in cancer cells [44]. AZ7328, in particular, interacts with autophagy inhibitors, inducing apoptosis in bladder cancer cell lines. Small-molecule inhibitors targeting PKA and PKC have also shown antitumor activity in preclinical models, slowing the growth and metastasis of bladder cancer cells by inhibiting these kinases [45–47]. Currently, many AGC kinase inhibitors are undergoing clinical trials to assess their effectiveness in bladder cancer treatment. For example, although several FDA-approved immune checkpoint inhibitors (ICIs) for advanced bladder cancer benefit only about 20% of patients, the international Phase II trials using erdafitinib achieved a response rate of 40% [48]. The Akt inhibitor perifosine (KRX-0401, Keryx Biopharmaceuticals) was studied in a Phase I trial that included bladder cancer patients, where it was found to be safe, and Phase II trials are currently underway [49, 50].

Combination therapy

Combining AGC kinase inhibitors with other anticancer agents may enhance therapeutic efficacy. Research suggests that combining Akt inhibitors with chemotherapy or immunotherapy could improve the treatment outcomes for bladder cancer. For example, the use of Akt inhibitors alongside immune checkpoint inhibitors in bladder cancer has been shown to block the Akt signaling pathway, reducing PD-L1 expression. This reduction may increase tumor cell sensitivity to immune therapy, thereby improving therapeutic efficacy [51]. Studies have also demonstrated that co-inhibition of the PI3K/Akt/mTOR pathway effectively suppresses tumor cell growth and proliferation. For instance, inhibiting both Akt and mTOR may prevent tumor cell metabolic reprogramming, leading to enhanced therapeutic effects [22, 52]. Targeting EGFR alone, either in its kinase-dependent or -independent functions, may be insufficient to effectively inhibit tumor growth. Therefore, combining AGC kinase inhibitors with EGFR inhibitors could enhance treatment efficacy [45, 53].

Future research directions

While research on the AGC kinase family in bladder cancer has made notable progress, many unanswered questions remain. Future studies may focus on the following areas.

Molecular mechanisms of AGC kinases

Understanding the molecular mechanisms by which AGC kinases regulate bladder cancer development and progression is crucial for future research. Investigating the functions of different AGC kinases, such as PKCα and AMPK, could elucidate how these kinases modulate the proliferation, invasion, and metastasis of bladder cancer cells. For example, research has shown that increased activation of PKCα in bladder cancer cells correlates with the malignancy of the tumor [7, 8]. Future studies could explore the specific roles of AGC kinases in bladder cancer cells through gene editing or small-molecule inhibitors.

Development of targeted therapies

As the role of AGC kinases in bladder cancer becomes more understood, targeted therapies against these kinases may represent a new treatment avenue. For instance, PKC inhibitors have shown promise in other cancers, with PKCα inhibition enhancing CD8 + T cell-mediated tumor evasion in breast cancer and abolishing antitumor activity in immune-deficient mice [54]. Clinical trials could evaluate the effectiveness of PKC inhibitors in bladder cancer patients. Furthermore, investigating the interactions between different AGC kinases and other signaling pathways [55] could provide a basis for developing novel targeted drugs.

Identification of biomarkers

The expression and activation status of AGC kinases may correlate with the prognosis of bladder cancer patients. For example, the upregulation of p70S6K in bladder cancer has been associated with poor prognosis, suggesting its potential as a prognostic biomarker [56]. Studies involving multiple bladder cancer patients have analyzed the relationship between AGC kinase expression levels and patient outcomes. Further research has found a negative correlation between AGC kinase expression and tumor malignancy and survival, indicating that AGC kinase activity could be an important indicator for assessing bladder cancer prognosis [53]. Future research could focus on identifying AGC kinase-related biomarkers that could aid in early diagnosis, prognosis assessment, and monitoring treatment responses.

Combination with immunotherapy

In recent years, immunotherapy has shown promise in the treatment of bladder cancer. Studies suggest that AGC kinases may play a key role in regulating immune responses [57, 58]. Future research could explore how AGC kinase inhibitors could be combined with immune checkpoint inhibitors to enhance immunotherapy effectiveness. This combination therapy might improve treatment responses in bladder cancer, particularly in patients resistant to existing therapies.

Interactions between signaling pathways

AGC kinases function not only within single signaling pathways but often interact with other signaling networks [55]. Future studies could investigate the interactions between AGC kinases and the PI3K/Akt/mTOR pathway, exploring their combined regulatory mechanisms in bladder cancer. A deeper understanding of these interactions could reveal the complexity of bladder cancer and provide new strategies for multi-targeted therapies.

Mechanisms of drug resistance

Bladder cancer patients often face challenges with drug resistance, and AGC kinases may play a critical role in this process [59, 60]. Future studies could focus on how AGC kinases contribute to drug resistance mechanisms, particularly in chemotherapy and targeted therapies. For example, exploring how PKC activation promotes resistance to chemotherapy drugs in bladder cancer cells could provide new strategies to overcome resistance.

Genomics and personalized medicine

With advancements in genomics, future studies could utilize genomic analysis to identify mutations and expression changes related to AGC kinases. This research could help understand the biological characteristics of individual bladder cancers and support the development of personalized treatment strategies. By identifying specific gene alterations associated with AGC kinases, clinicians could devise more precise treatment plans for patients.

Clinical trial design

Finally, designing effective clinical trials to evaluate the safety and efficacy of AGC kinase-targeted therapies remains a crucial research direction. Clinical studies based on laboratory findings should adopt randomized, controlled designs to validate the actual effects of AGC kinase inhibitors in bladder cancer treatment. These trials will provide robust evidence to support clinical applications.

Although research on the AGC kinase family has not yet been fully incorporated into current clinical guidelines for bladder cancer, its potential in predicting chemotherapy sensitivity, synergizing with immunotherapy, and combining with targeted therapies has already provided directions for the updating of these guidelines. In the future, with the advancement of more clinical trials (such as the phase II trial of the Akt inhibitor perifosine and the clinical translation of PKC inhibitors), AGC kinase-related targets, biomarkers, and combination regimens are expected to be gradually included in the guidelines. This will promote the evolution of bladder cancer treatment from “empirical selection” to “precision stratification”.

Limitations

The review primarily relied on existing literature, focusing extensively on several well-characterized members of the AGC kinase family, including protein kinase A (PKA), protein kinase C (PKC), and protein kinase G (PKG), as well as the PI3K/Akt and MAPK/ERK signaling pathways. However, our exploration of other AGC kinase members and their associated signaling pathways was relatively limited. This limitation restricted our ability to conduct a comprehensive evaluation of the AGC kinase family’s multifaceted role in bladder cancer and to elucidate their intricate interactions with other signaling pathways. Additionally, this review did not fully explore the potential of AGC kinases as biomarkers in bladder cancer. As a result, we were unable to comprehensively delineate the correlation between AGC kinase expression levels and clinical characteristics, such as tumor staging, grading, and prognosis. Future research should aim to address these limitations by incorporating a broader range of AGC kinase family members and conducting more in-depth analyses of their associations with clinical outcomes in bladder cancer.

Conclusion

The AGC kinase family has demonstrated significant biological significance and clinical potential in bladder cancer research, particularly in terms of its roles in tumorigenesis, metastasis, and drug resistance. In-depth research into AGC kinases will not only elucidate their specific molecular mechanisms in bladder cancer progression but also drive the development of targeted therapies and combination treatments. Future studies, focusing on biomarker identification and the integration of AGC kinases with immunotherapy, may offer new approaches for early diagnosis, prognosis evaluation, and precision treatment in bladder cancer.

Author contributions

Peng Su and Hong Zheng conceived of the study, Peng Su, Ying Yang, XiuLan Luo, Neng Zhang and Hong Zheng participated in its design and coordination. Peng Su, Ying Yang helped to draft the manuscript. Ying Yang, Xiulan Luo and Neng Zhang prepared Figs. 1, 2 and 3. Neng Zhang and Hong Zheng revised the article. All authors reviewed and approved the final manuscript. Peng Su and Ying Yang are co-first authors, Neng Zhang and Hong Zheng is the co-corresponding author.

Funding

This work was supported by the Zunyi Municipal Science and Technology Program [Grant Number: Zunshi Kehe HZ [2024] 189] and the Science and Technology Fund Project of Guizhou Provincial Health Commission of China [Grant Number: gzwkj2025-063].

Data availability

Expression profiles and survival analyses of representative AGC kinases in bladder cancer were generated using the publicly available UALCAN database (http://ualcan.path.uab.edu). All other data generated or analyzed during this study are included in the article.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.