The molecular aspects in prostatic cancer, unraveling the role of CDKs in prostate cancer progression

Background

Prostate cancer (PCa) is one of the leading causes of morbidity and mortality in the world, it becomes imperative to investigate the molecular mechanisms underlying practical therapeutic approaches. Cyclin-dependent kinases (CDKs) are critical regulators of cell cycle progression, and their dysregulation has been implicated in PCa. Methods: This review synthesizes recent findings on CDK-mediated signaling pathways and their role in PCa progression. The overall goal of this review article is to elucidate the molecular mechanisms by which CDK molecules contribute to the development, progression, and metastasis of PCa. Results: Elevated expression of CDK1, CDK3, CDK5, and CDK16 is associated with poorer outcomes, and specifically, high CDK3 expression correlates with shorter progression-free survival. Conclusions: Targeting CDKs, particularly CDK3 and CDK4/6, may represent a promising therapeutic strategy for PCa.

Introduction

Prostate cancer (PCa), a significant health concern among men worldwide, underscores the importance of early detection and effective treatment. Understanding this condition empowers individuals to take proactive steps toward their health. Prostate cancer develops in the prostate gland, a small walnut-shaped gland located below the bladder, in men. It occurs when the cells in the prostate gland mutate, leading to abnormal cell growth. These cancerous cells can accumulate, monopolize resources, and damage surrounding tissues. If left untreated, the cancer can spread to other parts of the body. Additionally, there is growing interest in integrating radiomics into the multi-omics framework, linking biomolecular-level data with imaging features (linking biomolecular-level data with imaging features such as Magnetic Resonance Imaging (MRI), Transrectal Ultrasound (TRUS), Computed Tomography (CT), and Positron Emission Tomography/Computed Tomography (PET/CT) with radiolabeled Prostate-Specific Membrane Antigen (PSMA) tracers) [1].

PCa is a prevalent disease in Western countries, with a significantly higher incidence compared to Asian countries [2]. With about 350,000 yearly deaths worldwide, prostate cancer is one of the most significant causes of cancer-related deaths [3]. The substantial rates of morbidity and mortality of PCa and its excessively harmful impact on public health have made this disease a significant health problem[4]. The incidence of PCa is geographically variable, and the incidence of the disease differs between different ethnic populations and geographical locations [2, 5]. For instance, it is over 7 times more likely to be diagnosed with prostate cancer by the age of 79 in countries with a high sociodemographic index than in countries with a low–middle sociodemographic index [4]. This disease is the second most common cancer in the Northern American male population, while Asian countries have the lowest rate of incidence [2, 3]. It is speculated that the accessibility and sustainability of healthcare in North America are contributing factors to the high number of reported cases in this region [6, 7]. Notably, the number of cases has increased in all countries over the past few years, and researchers have focused on better understanding this disease [5, 6].

Prostate cancer predominantly affects individuals with male reproductive organs. However, certain risk factors can increase the likelihood of developing this condition, including age, environment, lifestyle, culture, and genetics [7]. Age is a significant factor, as PCa is more prevalent in older men [8], and it is uncommon for men younger than 40 to be diagnosed with PCa [4]. Consequently, regular prostate screenings are recommended as men age to ensure early detection [8]. In addition, research suggests that Black men have a higher risk of developing PCa compared to other races or ethnicities [8]. Furthermore, being overweight or obese may also contribute to an elevated risk [8]. Moreover, genetics can play a critical role, and a family history of PCa or certain types of breast cancer can increase the likelihood of a diagnosis [8].

While certain risk factors may increase the chances of developing PCa, individuals can take proactive steps to reduce their risk. Adopting a healthy diet, engaging in regular exercise, and maintaining overall well-being can significantly lower the likelihood of developing PCa. This empowering knowledge empowers individuals to take control of their health and offers hope for prevention.

Pathogenesis of prostate cancer

The prostate gland, a complex organ, is composed of a stroma that primarily contains smooth muscles, as well as ducts and acini covered by a layer of basal and luminal epithelial cells [9, 10]. It has been established that both primary human basal and luminal cells are the origin of PCa (PCa). However, luminal cells are considered the preferred cell of origin for PCa [11, 12]. The stroma, often overlooked, plays a significant role in PCa progression [9]. Research has unveiled that the development and progression of PCa involve intricate molecular pathways. In the normal state, the glandular secretory epithelium of the prostate is regulated by paracrine factors from the stroma, including hormones and proteins [13]. However, in PCa, the epithelium becomes independent from stromal influence and androgens, making it prone to metastasis [13].

Several key molecular alterations contribute to this transformation proto-oncogenes, such as growth factor genes, become activated, while tumor suppressors are inactivated [14]. The hyperexpression of specific growth factors and the apoptosis inhibitor B-Cell CLL/Lymphoma 2 (Bcl-2) further promote the progression of PCa [14]. Additionally, the inactivation of the androgen receptor, a crucial component in androgen signaling, plays a significant role in the development and progression of PCa [14].

The role of genetic and environmental factors

Epidemiological studies have suggested that both genetic and environmental factors, each with its unique influence, contribute to the development of PCa [14]. While genetic predisposition accounts for a small fraction of cases, environmental factors, particularly dietary components, have been identified as significant risk factors [15]. The interplay between these factors and the prostate gland’s response to genome-damaging stresses is crucial in understanding the pathogenesis of this disease.

Genome-damaging stresses and prostate cancer

Normal and neoplastic prostate cells are constantly exposed to a barrage of genome-damaging stresses [16]. These stresses, arising from various sources such as environmental toxins, oxidative damage, and hormonal imbalances [17, 18], underscore the need for preventive measures. Accumulating evidence suggests that dietary components and male sex steroids may modulate the level of these insults, influencing the transformation of normal prostate cells into neoplastic ones [16]. Importantly, patients with Breast Cancer gene 2 (BRCA2) pathogenic variants often present with higher Prostatic Specific Antigen (PSA) levels, high-grade tumors, and increased risk of nodal/distant metastasis, in addition, It is indicated that both germline and somatic BRCA alterations correlate with poorer prognosis but also uniquely predict responsiveness to PARP inhibitors such as olaparib, rucaparib, and niraparib [19].

The role of GSTP1 in prostate carcinogenesis

One of the most intriguing discoveries in recent years has been the somatic inactivation of the Glutathione S-transferase pi 1 (GSTP1) gene in PCa cells [16]. This gene, which encodes the enzyme glutathione S-transferase pi, plays a crucial role in detoxifying carcinogens. The somatic inactivation of GSTP1 is believed to be an initiating genome lesion in prostatic carcinogenesis, leading to chronic genome damage in normal prostate cells [16] (Fig. 1).

Fig. 1.

Some of the most important molecular alterations in PCa. It includes genetic mutation, overexpression of transcriptional factors, and modifications in the renin-angiotensin system. GTSP Glutathione S-transferase P, HER2/neu human epidermal growth factor receptor 2, NKX3.1 Homeobox protein NK3 homolog A, FOXA1 Forkhead box protein A1, AR androgen receptor, ETS family Erythroblast Transformation Specific, MKI67 Marker Of Proliferation Ki-67, MMP: Matrix metalloproteinases, ESR Estrogen Receptor 1

Opportunities for prostate cancer prevention strategies

The critical role of GSTP1 inactivation in the early stages of prostatic carcinogenesis opens up several avenues for PCa prevention strategies [16]. These strategies include the restoration of GSTP1 function, compensation for inadequate GSTP1 activity through the use of therapeutic inducers of other glutathione S-transferases (GST), and the abrogation or attenuation of genome-damaging stresses [16]. By targeting these mechanisms, we may be able to develop effective preventive measures for PCa.

In addition to GSTP1 inactivation, numerous other molecular alterations have been identified in the pathogenesis of PCa. These alterations involve key molecular pathways that regulate proliferation, angiogenesis, neuroendocrine differentiation, apoptosis, and nuclear structure [20]. Understanding these pathways is crucial for unraveling the complex nature of PCa and identifying potential therapeutic targets [20].

Genetic and molecular markers in prostate cancer prognosis

Several genetic and molecular markers have shown promise in predicting the prognosis of PCa. For instance, the human epidermal growth factor receptor 2 Her-2/neu oncogene has been identified as a significant prognostic indicator, indicating poor progression-free survival (PFS) in patients [20]. Additionally, alterations in nuclear structure, as measured by nuclear roundness variance (NRV), are highly predictive of [20]. The integration of these markers with traditional pathological parameters not only enhances the accuracy of prognosis in PCa patients [20] (Fig. 2) but also offers a hopeful outlook for the future of PCa management. Moreover, circulating microRNAs have emerged as prognostic biomarkers. For instance, miR-21 contributes to castration resistance, while miR-200 and miR-17 families are associated with PSA response and survival in CRPC patients receiving docetaxel [21].

Fig. 2.

The schematic illustration of CDK and association of cyclin molecules. The addition of the phosphate group to the larger C-terminal lobe led to the activation of the CDK/cyclin complex by serine/threonine kinase family members

The role of transcription factors in prostate cancer

Transcription factors play a crucial role in regulating gene expression and cellular processes in PCa [22]. Onenotable example is the Erythroblast Transformation Specific (ETS) family of transcription factors, which has been implicated in PCa through chromosomal translocation events [22]. The transcriptional activation of truncated ETS transcription factors ETS-related gene (ERG) and ETS Variant Transcription Factor 1 (ETV1), resulting from these chromosomal translocations, represents the first identified dominant oncogenes in PCa [23]. This insight into the role of transcription factors in PCa enlightens us about the complexity of the disease.

Understanding the molecules and signaling pathways that initiate PCa and progression into Castration-resistant prostate cancer (CPRC) is crucial. This understanding not only helps researchers find an effective and durable treatment strategy for patients with advanced PCa [9] but also motivates them to continue their research. Key genes involved in the maturation and development of the prostate include NK3 Homeobox 1 (NKX3.1), Forkhead box A1 (FOXA1), and Androgen Receptor (AR); abnormal activation of these genes may lead to increased proliferation and contribute to PCa [9].

Nkx3.1

Nkx3.1 is an androgen-related homeobox gene. NKX3.1 is a transcriptional repressor required in all stages of prostate development and organogenesis [9, 24]. A loss in specific regions of chromosome 8p has been reported in about 80% of prostate tumors. NKX3.1 is located in the 8p1-21 locus and was speculated to be responsible for this mutation [25]. Mutations in NKX3.1 improve the possibility of PCa development [9]. It has been shown that in case a disruption in the Nkx3.1 gene occurs in mice, various incidents like morphogenesis of prostate branching, differentiation of epithelial cells, growth, and protein secretion are disturbed. Additionally, Nkx3.1 deficiency in mice results in prostatic epithelial hyperplasia and Prostatic intraepithelial neoplasia [24].

FOXA1

Forkhead Box A1 (FOXA1) plays a crucial role in PCa by regulating factors that induce the open formation of chromatin, thereby increasing the binding of transcription factors, like AR, to the DNA. Its significant role in AR signaling and the regulation of Epithelial-Mesenchymal transition further underscores its importance. FOXA1 mutations in PCa, including coding sequence mutations, cis-regulatory elements (CREs), or post-translational mutations, can be both coding and non-coding, with over half occurring in the Forkhead DNA binding domain.

The complex network of the Renin-Angiotensin system (RAS)

In recent years, researchers have also explored the role of the renin-angiotensin system (RAS) in the development and progression of PCa [26]. Angiotensin 1–7 (Ang1-7), a bioactive component of the renin-angiotensin system (RAS), has been of particular interest due to its cardiovascular properties and potential anti-proliferative and anti-angiogenic effects [27].

Studies have shown that Ang1-7 can reduce cell proliferation in PCa cell lines, such as DU-145, and induce a decrease in the expression of Antigen Kiel 67 (MKI67) in LNCaP cells [27]. Additionally, Ang1-7 has been found to modulate gene expression related to apoptosis, growth factors, and transcription factors [27]. It also affects cell adhesion and matrix metalloproteinase (MMP) activity, which play crucial roles in cancer progression [27]. Ang1-7 has been found to modulate the expression of estrogen receptor genes (ESR1 and ESR2) and androgen receptor (AR) genes in PCa cells [27]. These effects not only shed light on the disease mechanisms but also hold promise for the development of novel hormonal therapies and the overall improvement of treatment outcomes [27].

Androgen

The androgen receptor, a type of nuclear receptor, plays a significant role in the development and function of the prostate. An AR signaling pathway is crucial in the development of the prostate during fetal stages and continues to have a significant role in its function. If prostate growth is dependent on androgen conserves, it could lead to BPH, PCa, or premalignant prostatic intraepithelial neoplasia (PIN). Although AR mutations are rare in the early stages of PCa, these mutations are seen in the advanced stages of the disease. A genetic alteration in the androgen signaling pathway is responsible for most cases of PCa. Clinically, abiraterone plus prednisolone with Androgen Deprivation Therapy (ADT) is now considered standard of care in high-risk non-metastatic PCa. In metastatic disease, enzalutamide and abiraterone should not be combined when initiating ADT. Long-term survival benefits of abiraterone + ADT extend beyond 7 years [28].

Furthermore, it has been mentioned before that recurrent activity of Androgen Receptors (AR) progresses the disease toward the castrate-resistant stage. Several mechanisms control the reactivation of androgen receptors [9]. These cases include:

1. Overexpression or amplification of Androgen Receptors: The reactivation of androgen receptors is typically caused by gene overexpression resulting from gene amplification or increased protein levels. PCa Cells with AR overexpression are sensitive to low levels of androgen and are resilient to AR antagonists [9].

2. AR mutations: AR mutations are rare in the early stages of PCa but become more common in advanced disease. AR mutations are rare along the length of AR, and some areas are mutation-dense. For example, exon 1 is a high-density mutation area, and most mutations in this region cause complete androgen insensitivity syndrome [29]. The majority of AR mutations are gain-of-function and provide a growth advantage by reducing ligand specificity or androgen hypersensitivity [9, 29].

3. AR splice variants: Several AR splice variants were found as a result of gene splicing. AR antagonists attack the androgen receptor ligand-binding domain. Splice variants lack this domain, which renders cells resistant to ADT [9, 27]. This finding highlights the need for more effective treatment strategies that can overcome the resistance posed by these splice variants.

Furthermore, minimally invasive liquid biopsies (Circulating Tumor Cells, Cell-free DNA, exosomes in blood/urine/semen) are emerging as powerful tools for diagnosis and treatment monitoring in PCa [30]. Among all possible pathways, the interception of the Cyclin-Dependent Kinase (CDK) pathway in PCa is of utmost importance. Understanding this could be the key to developing urgently needed, targeted therapies and significantly improving treatment outcomes for patients with PCa.

General properties of CDK

CDKs in normal biology

The Cyclin-Dependent Kinase pathway is a marvel of complexity, a network of proteins that intricately regulates the cell cycle and ensuresproper cell division [31]. CDKs, a family of serine/threonine kinases, work in harmony with their regulatory subunits, known as cyclins, to control the progression through various phases of the cell cycle [31]. The dysregulation of this intricate system has been implicated in various types of cancer, including [31]. CDKs require activation by their subunits, cyclins [31]. Furthermore, for the complete activation of the CDK-cyclin complex, the CDK-activating kinase must phosphorylate the threonine residues at the CDK active sites [32] (Fig. 2). The backbone of CDKs consists of two lobes: a smaller N-terminal lobe that holds the G loop inhibitory component and a larger C-terminal lobe that contains phosphorylation residues (serine or threonine) [31].

Out of the 20 known CDK subfamilies, CDK1, CDK2, CDK3, CDK4, CDK6, and CDK7 are involved in cell cycle transitions [33]. However, it has been shown that only CDK1, due to its unique ability to phosphorylate a wide range of substrates, is adequate for cell cycle progression in the case of loss of CDK2, 3, 4, and 6 [34].

The CDK pathway, a crucial process in molecular biology, can be divided into several steps: initiation, promoter escape, transcription, and termination [35]. During initiation, the preinitiation complex, consisting of CDKs and basal transcription factors, as well as the CDK8/Mediator complex, is assembled [35] (Fig. 3a). These complex anchors the preinitiation complex to gene-specific upstream enhancers, enabling gene transcription [35] (Fig. 3b). Promoter escape is facilitated by the phosphorylation of the Pol II C-terminal domain (CTD) by CDK7, which recruits pre-mRNA 5 ′ capping enzymes and promotes transcriptional elongation.

Fig. 3.

CDK activation steps; A and B: Initial, the formation of the pre-initial complex itself includes CDKs and TFs, which should be separated from the site and at a distance from DNA. The desired gene should be connected after the promoter. C: Stage of escape from the promoter: Here, the phosphorylation of Pol II enzyme sequences by CDK7 occurs, and the result will be the addition of CAP in the 5” mRNA. CDK Cyclin Depended Kinase, TFs Transcription factors, Pol II RNA Polymerase two, CTD C-terminal domain, MAT 1 Methionine Adenosyltransferase 1, Ser Serine

After promoter escape, transcription is temporarily paused downstream of the transcription start site by negative factors, such as DRB sensitivity-inducing factor (DSIF) and negative elongation factor (NELF) [35] (Fig. 4a). To overcome this pause, the recruitment of bromodomain-containing protein 4 (BRD4) and the positive transcription elongation factor (P-TEFb), consisting of CDK9 and cyclin T, to acetylated chromatin is a key part of the process [35] (Fig. 4a). CDK7 activates CDK9 through T-loop phosphorylation, which in turn phosphorylates DSIF, NELF, and the Pol II CTD at serine 2 (Ser2), leading to the release of the pause and allowing for productive elongation [35] (Fig. 4a).

Fig. 4.

CDK controlling mechanisms: A temporary stop stage and pre-elongation stage, after the previous stage (Fig. 6c), transcription stops temporarily due to the influence of controlling factors such as DRB sensitivity-inducing factor (DSIF) and negative elongation factor (NELF). For elongation, two groups of activation factors, including bromodomain-containing protein 4 (BRD4) and CDK9 and cyclin T, as the positive transcription elongation factor (P-TEFb), are utilized to resolve cell arrest. These factors can activate transcription through three pathways: first, by acetylating chromatin, they cause the conversion of heterochromatin into euchromatin; second, by phosphorylating inhibitory factors such as DSIF, NELF; and third, by phosphorylating the CTD of Pol II, they activate transcription. B The elongation stage, facilitated by the activity of other CDK factors such as CDK9 and CDK12/CDK13 and the phosphorylation of Pol II CTD at Ser2, Ser5, and Ser7, enables the continuation of transcription. C At the end stage, by activating CDK12 and calling poly A tail, adding factors, transcription stops, and mRNA is released

Progression through the gene body and productive elongation are maintained by the differential phosphorylation of the Pol II CTD at Ser2, Ser5, and Ser7 by CDK9 and CDK12/CDK13(Fig. 4b). The pattern of CTD phosphorylation contributes to the recruitment of splicing and chromatin remodeling factors [35]. Finally, a significant part of the process is the regulation of termination by CDK12, which promotes the use of distal 3′ transcription termination sites and recruits cleavage and polyadenylation (Cleavage and Poly A) factors [35] (Fig. 4c).

CDK primary cell cycle regulator and activator

CDK1: CDK1 binds to cyclin B1 and controls cell transition from G2 phase into mitosis. Other checkpoint kinases also regulate CDK1 to ensure that the DNA is appropriately distributed to daughter cells [36] (Fig. 5). CDK1 is essential for cell-cycle progression, and many factors regulate its activity, including WEE1 G2 checkpoint kinase (WEE1), the membrane-associated tyrosine- and threonine-specific cdc2-inhibitory kinase myelin transcription factor 1 (MYT1, also known as PKMYT1), and the phosphorylation of CDC25C phosphatases [33].

Fig. 5.

Cell cycle activation by CDK function: A Illustrates the activation of CDK4 and CDK6 by mitogenic factors, which drive quiescent cells to re-enter the cell cycle and transition from the G0/G1 phase into the S phase. B Shows the phosphorylation of the retinoblastoma protein (RB) by the CDK4/6-cyclin D complex. This phosphorylation leads to RB inactivation, allowing the E2F transcription factor to activate the transcription of key cell cycle genes, including Cyclins E and A, Essential for CDK2 activation in the late G1 and S phases. C Depicts the formation of the cyclin E/CDK2 complex in the late G1 phase. This complex phosphorylates proteins required for cell cycle progression, facilitating the transition from the G1 to S phase. D Shows the degradation of cyclin E and the formation of the cyclin A/CDK2 complex in the final stages of the S phase. This complex phosphorylates E2F1, halting cell cycle progression at the S phase and allowing for the transition into mitosis. E The precise timing and execution of the cell cycle are crucial for maintaining genomic integrity and proper development. Two key players in this intricate dance are CDK1/Cyclin B and the Anaphase Promoting Complex/Cyclosome (APC). These proteins work in concert to orchestrate the transition from metaphase to anaphase, marking the pivotal step where sister chromatids separate, leading to the formation of two daughter cells. APC Activation and Cyclin B Degradation: As the cell progresses through mitosis, the spindle checkpoint mechanism is satisfied, and APC activity is stimulated. APC then ubiquitinates Cyclin B, leading to its degradation by the proteasome

CDK2: CDK2 is a significant cell cycle regulator that binds to Cyclin E and controls the transition of cells from G1 to S and from S to G2. Additionally, CDK2 regulates the phosphorylation of various transcription factors, thereby controlling the cell cycle during transitions [36].

CDK4/6: CDK4/6 regulates the cell cycle transition from G1 to S by binding to cyclin D. When activated, CDK4/6 complexes phosphorylate retinoblastoma gene product (Rb). This leads to E2F Transcription Factor 1 (E2F) factors detaching from Rb/E2F complexes and, subsequently, the transcription of E2F target genes, such as E-type cyclins. Cyclin E then hyperphosphorylates RB, increasing the activity of E2F target genes, which are necessary for the initiation of DNA synthesis and the S phase [36].

CDK5: In addition to its role in cell division, cell differentiation, and gene expression, CDK5 plays a vital role in the central nervous system and insulin secretion. It regulates various processes, including neuronal migration, synaptic plasticity, and neurotransmitter release. Fos, cyclic AMP Response Element Bindingprotein (CREB), and δFosB are transcription factors that regulate the expression of the CDK5 gene. Unlike other CDKs, cyclin binding and T-loop phosphorylation are not required for CDK5 activation [36]. CDK5 also regulates cell-cycle progression by targeting the retinoblastoma protein [36].

CDK7: For a stable binding between CDK 7 and cyclin H, threonine phosphorylation in the CDK7 activation loop is necessary. CDK7 plays a crucial role in regulating the cell cycle transition from the G1 to the S phase. It does so by phosphorylating the CDK2/cyclin E complex, thereby activating it. Similarly, CDK7 activates CDK1/cyclin B, helping cells initiate mitosis. RING finger protein Methionine Adenosyltransferase 1 (MAT1) will form a trimer with CDK7 and cyclin H and provide CDK activating kinase (CAK) [36].

CDK8, CDK19: CDK8 and CDK19 form a subcomplex with their regulatory subunits, cyclin C, Mediator of RNA polymerase II transcription (MED12), and MED13. This four-subunit complex is part of the Mediator complex, a key regulator of RNA polymerase II transcription. The Mediator complex inhibits the activity of RNA polymerase II and general transcription factors, thereby regulating gene expression.

At the start of mitosis, the formation of cyclin B and CDK1 initiates a series of events. It leads to the breakdown of the nuclear envelope, the reorganization and condensation of chromosomes, and the formation of the mitotic spindle (Fig. 5E). As the anaphase stage progresses, cyclin B is degraded, leading to a reduction in CDK1 activity. CDK1 plays a crucial role in the completion of mitosis and cytokinesis. The cyclin C/CDK3 complex, in contrast, can phosphorylate RB, prompting cells to enter the S phase or exit the cell cycle into the permanent G0 phase [33].

Urgent need for Understanding genomic alterations leading to CDK4/6 hyperactivation

The hyperactivation of CDK4/6 in cancer cells, a key area of our research, is often attributed to a multitude of genomic alterations [37]. One common mechanism involves the amplification or overexpression of cyclin D1, which forms complexes with CDK4/6, thereby enhancing their activity [37]. This amplification is frequently observed in breast cancer, where cyclin D1 is overexpressed in approximately 50% of cases [37]. Our findings in this area could potentially lead to novel treatment strategies. Notably, while cyclin D1 amplification occurs in ~ 50% of breast cancers, it is relatively rare in PCa, indicating distinct tumor-type mechanisms of CDK4/6 dysregulation [38].

In addition to cyclin D1 alterations, mutations in CDK4 itself have been identified in specific cancer types such as melanoma and glioblastoma [37]. These mutations can result in constitutively active CDK4, thereby bypassing standard regulatory mechanisms and driving uncontrolled cell proliferation [37].

Overcoming tumor suppression mechanisms: senescence and apoptosis

Tumor suppression mechanisms play a crucial role in preventing uncontrolled cell growth and the development of cancer [39]. Senescence, a state of irreversible cell cycle arrest, acts as a barrier against tumorigenesis by halting the proliferation of damaged or abnormal cells [39]. Apoptosis, the programmed cell death process, eliminates cells that have sustained irreparable DNA damage or genetic abnormalities [39].

CDK4/6, through its activation of the cell cycle, effectively counteracts these tumor-suppression mechanisms [39]. By promoting the G1-S phase transition, CDK4/6 enables cells to bypass senescence and continue proliferating, even in the presence of DNA damage or other cellular stressors [39]. Additionally, CDK4/6-mediated phosphorylation of pRb inhibits apoptosis by activating pro-survival signaling pathways, such as the Phosphoinositide 3-kinase- Protein Kinase B (PI3K-Akt) pathway and the Mitogen-activated protein kinase kinase (MAPK) pathway, which promote cell survival and proliferation [39].

CDK and signaling pathways

As central nodes in cellular signaling networks, cyclin D-dependent kinases integrate a multitude of mitogenic signals to orchestrate specific transcriptional programs [40]. These signals can originate from growth factors, hormones, and other extracellular cues, ultimately influencing cell fate and behavior [40].

The signaling pathways converging on CDK4/6 activation are diverse and interconnected [40]. For instance, the activation of receptor tyrosine kinases (RTKs) can initiate signaling cascades involving the PI3K/AKT pathway, which promotes CDK4/6 activity [40]. Additionally, MAPK signaling pathways, such as the Ras/Raf/ (MEK)/ERK pathway, can also stimulate CDK4/6 activation [40].

Comprehending the intricate architecture and dynamics of the cellular signaling network regulated by CDK4/6 is not just a scientific pursuit but a crucial step toward developing targeted therapies and identifying new therapeutic targets.

Wnt signaling

Wnt signaling, a key player in the regulation of the cell cycle, upregulates the transcription of genes like bHLH transcription factor (cMyc) or Cyclin D. The components of Wnt, such as b-Catenin, Axis inhibition protein 1 (Axin 1), Axin2/conduction, and Adenomatous polyposis coli (APC), are crucial for the formation of the mitotic spindle, thereby playing a significant role in the process of mitosis.

Neurogenic locus Notch homolog protein (Notch) pathway

The notch pathway, a significant player in cell cycle regulation, increases the activity of CDK2 and cyclin D1. More importantly, this pathway induces the transcription of S phase kinase-associated protein 2 (SKP2). SKP2 then degrades the p27 and p16 CDK inhibitors, facilitating the cell’s entry into the S phase.

Post-translational modification of CDKs

Post-translational modifications, such as phosphorylation, play a crucial role in the regulation of CDK activity [33]. Phosphorylation of CDKs can either activate or inhibit their kinase activity, depending on the specific residue being phosphorylated and the context of the cell cycle phase [33]. The phosphorylation of CDKs is mediated by various kinases and phosphatases,which act in a coordinated manner to control CDK activity [33].

One of the key regulators of CDK phosphorylation is the cyclin subunit itself. Cyclins can phosphorylate specific residues on CDKs, leading to their activation or inactivation. For example, cyclin D-CDK4/6 complexes phosphorylate the retinoblastoma protein (Rb), leading to its inactivation and the release of E2F transcription factors [33]. On the other hand, cyclin-dependent kinase inhibitors (CKIs) can bind to cyclin-CDK complexes and inhibit their activity. CKIs, such as p21 and p27, play a crucial role in the regulation of CDK activity during the cell cycle [33].

Furthermore, the phosphorylation status of CDKs can also be regulated by other signaling pathways. For instance, the MAPK pathway has been shown to regulate CDK activity through the phosphorylation of specific residues on CDKs and cyclins. This cross-talk between different signaling pathways ensures the tight regulation of CDK activity and cell cycle progression [41, 42].

Protein degradation and CDK regulation

Protein degradation plays a critical role in the regulation of CDK activity. The ubiquitin-proteasome system is responsible for the targeted degradation of proteins, including cyclins and CDK inhibitors [43]. The degradation of cyclins is essential for terminating their function and allowing the cell to progress through the cell cycle [43].

The ubiquitin-proteasome system involves the covalent attachment of ubiquitin molecules to target proteins, marking them for proteasomal degradation [44]. The addition of ubiquitin molecules to cyclins and CDK inhibitors is mediated by a cascade of enzymes, including E1 ubiquitin-activating enzymes, E2 ubiquitin-conjugating enzymes, and E3 ubiquitin ligases [44]. The specific E3 ligases involved in the degradation of cyclins and CDK inhibitors vary depending on the cell cycle phase [44].

The degradation of cyclins and CDK inhibitors is tightly regulated and can be influenced by various factors, including phosphorylation, subcellular localization, and protein-protein interactions. For example, phosphorylation of cyclins can promote their recognition by specific E3 ligases, leading to their degradation. Conversely, dephosphorylation of cyclins can stabilize them and inhibit their degradation [41, 45, 46].

Subcellular localization of CDKs

The subcellular localization of CDKs is another critical mechanism that regulates their activity during the cell cycle. CDKs are predominantly localized in the nucleus, where they phosphorylate target proteins involved in cell cycle progression. However, the precise localization of CDKs can vary depending on the cell cycle phase and specific cellular [47, 48] context.

During the G1 phase, CDKs are predominantly localized in the cytoplasm, where they are inactive. The translocation of CDKs from the cytoplasm to the nucleus is essential for their activation and the initiation of DNA synthesis during the S phase [49]. This translocation is regulated by phosphorylation events and the binding of specific regulatory proteins.

In addition to the nucleus and cytoplasm, CDKs can also be localized to other cellular compartments, such as the centrosomes and the mitotic spindle during mitosis. The precise localization of CDKs to these subcellular structures is critical for their function and ensures the proper progression through the cell cycle.

The fundamental mechanisms of CDK activation and regulation are not just essential for normal cell cycle control, but also provide the molecular framework for understanding their role in cancer. In PCa, dysregulation of these same CDK-driven checkpoints translates into uncontrolled proliferation, tumor progression, and therapeutic resistance. Therefore, the transition from basic CDK biology to their involvement in PCa is critical, as it highlights how alterations in universal cell cycle regulators are specifically co-opted in the pathogenesis of PCa.

CDK and prostate cancer

Prostate cancer is a common malignancy that affects the prostate gland in men. The progression and prognosis of PCa are influenced by various factors, including the dysregulation of cell cycle-associated cyclin-dependent kinases (ccCDKs) [50]. ccCDKs are essential regulators that control cell division and have been implicated in tumorigenesis and cancer progression [50]. In the context of PCa, the dysregulation of ccCDKs can lead to uncontrolled cell division and tumor growth. While previous studies have highlighted the role of ccCDKs in different types of cancer, there is currently limited research on their specific functions in PCa [50].

Expression of CcCDKs in prostate cancer

To determine the expression patterns of ccCDKs in prostate cancer, several databases and tools were utilized [50]. The University of ALabama CANcer portal (UALCAN) databases revealed that CDK1, 3, 4, 5, 6, and 16 were expressed at relatively higher levels in PCa tissues compared to normal tissues [50]. This suggests that these ccCDKs may play a significant role in PCa progression [50]. The expression of CDKs was further verified using the Human Protein Atlas, which confirmed their presence in PCa tissues [50].

Prognostic value of CcCDKs in prostate cancer

The research on the prognostic value of ccCDKs in PCa, as assessed using the Gene Expression Profiling Interactive Analysis (GEPIA) database [50], has potential clinical implications. Patients with low expression of CDK1/3/5/16 showed significantly better disease-free survival compared to those with high expression [50]. These findings suggest that the expression levels of ccCDKs could serve as prognostic markers for PCa [50], offering hope for improved patient outcomes and personalized treatment strategies.

Gene functions of CcCDKs in prostate cancer

To gain insights into the biological functions of ccCDKs in PCa, Gene Multiple Association Network Integration Algorithm (GeneMANIA), and Metascape analyses were employed [50]. The results indicated that ccCDKs were enriched in the IL-18 signaling pathway, which has been implicated in cancer progression and immune responses [50]. Additionally, the correlation between ccCDKs and immune infiltration in PCa was evaluated using the TIMER database [50]. The results suggested that ccCDKs may play a role in modulating the tumor microenvironment and immune responses in PCa, potentially by influencing the expression of immune-related genes or the recruitment of immune cells to the tumor site [50].

Mutation analysis of CcCDKs in prostate cancer

The cBioPortal platform was used to analyze the mutation status of ccCDKs in prostate cancer [50]. The results showed that genetic alterations in ccCDKs were relatively rare in PCa, with only a small percentage of patients exhibiting mutations in these genes. This rarity suggests that the dysregulation of ccCDKs in PCa may primarily occur at the transcriptional level rather than through genetic mutations, highlighting the importance of studying their expression levels and regulatory mechanisms [50].

Validation of CcCDK expression in prostate cancer tissues

To validate the findings from the databases, a cohort study was conducted to examine the expression of ccCDKs in prostate cancer tissues compared to those in benign prostate hyperplasia tissues [50]. The results confirmed significantly higher expression levels of CDK1/3/5/16 in prostate cancer tissues, further supporting their potential role in PCa progression [50].

Focus on CDK3 and its prognostic value in prostate cancer

Among the ccCDKs, CDK3 was identified as an exciting candidate for prostate cancer [50]. Immunohistochemistry and Western blot analysis revealed positive expression of CDK3 in PCa cells and tissues [50]. Further analysis suggested that high expression of CDK3 was associated witha shorter progression-free survival for biochemical recurrence in PCa patients [50]. These findings highlight the potential prognostic value of CDK3 in prostate cancer [50].

Functional role of CDK3 in prostate cancer progression

To investigate the functional role of CDK3 in PCa progression, in vitro experiments were conducted using the C4-2 cell line [50]. Silencing CDK3 expression was found to inhibit the proliferation, migration, and invasion of PCa cells [50]. These results suggest that CDK3 may act as an oncogene in PCa, promoting its aggressive behavior and potentially influencing treatment strategies [50].

Precision oncology and CDK pathway in prostate cancer

The CDK pathway, a potential target for precision oncology, represents a promising avenue for individualized treatment [51]. By identifying the genetic alterations and biomarkers associated with CDK pathway dysregulation, clinicians can select the most appropriate treatment options for individual patients, offering a brighter future for cancer patients.

Several studies have investigated the genomic alterations and gene expression profiles associated with CDK pathway dysregulation in PCa (PCa). For example, one study identified genetic alterations in CDK12, a key regulator of transcriptional elongation, in a subset of metastatic castration-resistant PCa patients [52]. These alterations were associated with increased genomic instability and sensitivity to CDK inhibitors [52, 53]. Another study found that the expression of CDKs was inversely correlated with the activation of the signal transducer and activator of transcription factor 3 (STAT3) in [54]. This finding suggests that CDKs may play a role in modulating the immune response in PCa. Those study results are summarized in Table 1.

Table 1.

A summary of studies on the role of CDK in the progression and prediction of prostate cancer

The correlation of ccCDKs and PCa	Methods of data collection and analysis	Dysregulations of ccCDKs	The possible outcome regarding PCa
Expression of ccCDKs in PCa	The UALCAN database	Higher expression levels of CDK1/3/4/5/6/16	PCa progression
Prognostic value of ccCDKs in PCa	The GEPIA database	Low expression levels of CDK1/3/5/16	Better chance of disease-free survival
The biological functions of ccCDKs in PCa	Gene MANIA and Meta escape analysis	Enriched ccCDKs in the IL-18 signaling pathway	Cancer progression
ccCDKs and immune infiltration in PCa	TIMER database	Higher levels of ccCDKs	Tumor diagnosis due to immune infiltration
CDK3 and its prognostic value in PCa	Immunohistochemistry and western blot analysis	High expression of CDK3	PCa prognosis factor
Molecular alterations in PCa targeted with precision oncology	Review of clinical data	CDK Pathway dysregulation	Potential PCa indicator
Mutation analysis of ccCDKs in PCa	A study concerning genetic alterations	Genetic alterations of CDK12	Potential PCa indicator
Genomic instability and sensitivity to CDK inhibitors in PCa	multi-institutional, retrospective study	Genetic alteration of CDK12	Potential PCa indicator

The integration of genomic and transcriptomic data with clinical information holds great promise for enhancing treatment outcomes in PCa. A key aspect of this approach is the identification of specific biomarkers associated with CDK pathway dysregulation. These biomarkers play a crucial role in more accurately predicting treatment responses and tailoring therapies, thereby enhancing the precision and individualization of the treatment. This approach may help overcome the heterogeneity of PCa and improve patient outcomes.

CDKs as therapeutic targets in prostate cancer

Given the critical role of CDKs in cell cycle regulation, it is not surprising that dysregulation of this pathway can contribute to the development and progression of cancer, including [55]. Several CDK inhibitors have been developed and are currently being investigated in clinical trials for various cancer types, including [55]. The most extensively studied CDK inhibitors in PCa are abemaciclib, palbociclib, and ribociclib, which specifically target CDK4 and CDK6 [55].

Preclinical studies have shown promising results, demonstrating that CDK4/6 inhibitors can effectively inhibit PCa cell proliferation and induce cell cycle arrest [56]. These inhibitors work by blocking the activity of CDK4 and CDK6, thereby preventing the phosphorylation of the retinoblastoma protein (pRb) and inhibiting the transition from the G1 to the S phase of the cell cycle [56]. In addition to their direct effects on cancer cells, CDK4/6 inhibitors have also been shown to modulate the tumor microenvironment and enhance anti-tumor immune responses [56]. This potential of CDK4/6 inhibitors in prostate cancer brings hope for the future of cancer therapies. Clinical evaluation is ongoing: the NCT03706365 trial (abemaciclib in mCRPC) and NCT02905318 (palbociclib combination study) have reported early evidence of manageable safety and biological activity. Additionally, biallelic CDK12 inactivation defines a subset of PCa with genomic instability, creating a therapeutic vulnerability to CDK4/6 inhibitors and immune checkpoint blockade [38].

Clinical trials evaluating the efficacy of CDK4/6 inhibitors in PCa are currently underway [56]. These trials, which are part of an ongoing and dynamic research process, aim to determine the potential role of CDK4/6 inhibitors in the treatment of early and advanced-stage prostate cancer, both in hormone-sensitive and castration-resistant states [56]. Preliminary results from some of these trials have shown promising anti-tumor activity with manageable side effects [56]. However, further research is needed to fully understand the optimal patient selection, combination therapies, and potential resistance mechanisms associated with CDK4/6 inhibitors [56].

Conclusion

Prostate cancer has emerged as one of the most frequently diagnosed cancers in the United States, contributing significantly to cancer-related mortality inNorth America and Western Europe. Over the years, extensive research has shed light on the complex molecular mechanisms underlying the development and progression of this disease. The integration of precision oncology approaches, such as genomic profiling and biomarker-guided treatment selection, holds immense promise for improving the efficacy of CDK inhibitors in PCa. By identifying specific genetic alterations and gene expression profiles associated with CDK pathway dysregulation, clinicians can more effectively personalize treatment strategies and enhance patient outcomes. This emphasis on precision oncology instills a sense of optimism about the future of cancer treatment.

Targeting the CDK pathway represents a promising therapeutic approach in PCa. CDK inhibitors have demonstrated promising anti-tumor activity in preclinical and clinical studies, and ongoing trials will further elucidate their potential role in PCa treatment. Precision oncology approaches, including the identification of predictive biomarkers, the study of resistance mechanisms, and the integration of genomic profiling, will be crucial for maximizing the therapeutic potential of CDK inhibitors in PCa. However, the need for further research in this field is urgent and cannot be overstated. It will undoubtedly contribute to improving patient outcomes and advancing our understanding of PCa biology.

Additionally, the expression patterns of ccCDKs in PCa tissues, their prognostic value, and potential gene functions have been investigated. Specifically, CDK3 has emerged as a promising candidate with prognostic implications and functional significance in PCa. Further research is warranted to understand the role of ccCDKs in prostate cancer fully and to develop targeted therapies that can effectively modulate their activity.

Author contributions

All authors contributed to the study conception, and Z.F. and M.M.A. designed this review. Z.F. , Z.M. , M.M.A. , B.S. , and M.T.N. performed data collection and analysis. M.M.A. wrote the first draft of the manuscript, and all authors commented on previous versions. M.M.A. , Z.M. , and Z.F. edited the final draft. All authors read and approved the final manuscript.

Funding

This study did not receive any financial support.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Ethics and Consent to Participate declarations: not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.