Plk1-dependent microtubule dynamics promotes androgen receptor signaling in prostate cancer

Mitotic kinase inhibitors as Therapeutic Interventions for Prostate Cancer: Evidence from In vitro Studies

Prostate cancer is one of the devastating diseases characterized by genetic changes leading to uncontrolled growth and metastasis of the cells of the prostate gland and affects men worldwide. Conventional hormonal and chemotherapeutic agents are effective in mitigating the disease if diagnosed at an early stage. All dividing eukaryotic cells require mitotic progression for the maintenance of genomic integrity in progeny populations. The protein kinases, upon activation and de-activation in an ordered fashion, lead to spatial and temporal regulation of the cell division process. The entry into mitosis along with the progression into sub-phases of mitosis is ensured due to the activity of mitotic kinases. These kinases include Polo-Like-Kinase 1 (PLK1), Aurora kinases, and Cyclin-Dependent-Kinase 1 (CDK1), among others. The mitotic kinases, among others, are usually overexpressed in many cancers and can be targeted using small molecule inhibitors to reduce the effects of these regulators on mechanisms, such as regulation of genomic integrity and mitotic fidelity. In this review, we attempted to discuss the appropriate functions of mitotic kinases revealed through cell culture studies and the impact of their respective inhibitors derived in pre-clinical studies. The review is designed to elucidate the growing field of small molecule inhibitors and their functional screening or mode of action at the cellular and molecular level in the context of Prostate Cancer. Therefore, studies performed specifically on cells of Prostatic-origin are narrated in this review, culminating in a comprehensive view of the specific field of mitotic kinases that can be targeted for therapy of Prostate cancer.

Plk1 Inhibitors and Abiraterone Synergistically Disrupt Mitosis and Kill Cancer Cells of Disparate Origin Independently of Androgen Receptor Signaling

Abiraterone is a standard treatment for metastatic castrate-resistant prostate cancer (mCRPC) that slows disease progression by abrogating androgen synthesis and antagonizing the androgen receptor (AR). Here we report that inhibitors of the mitotic regulator polo-like kinase-1 (Plk1), including the clinically active third-generation Plk1 inhibitor onvansertib, synergizes with abiraterone in vitro and in vivo to kill a subset of cancer cells from a wide variety of tumor types in an androgen-independent manner. Gene-expression analysis identified an AR-independent synergy-specific gene set signature upregulated upon abiraterone treatment that is dominated by pathways related to mitosis and the mitotic spindle. Abiraterone treatment alone caused defects in mitotic spindle orientation, failure of complete chromosome condensation, and improper cell division independently of its effects on AR signaling. These effects, although mild following abiraterone monotherapy, resulted in profound sensitization to the antimitotic effects of Plk1 inhibition, leading to spindle assembly checkpoint-dependent mitotic cancer cell death and entosis. In a murine patient-derived xenograft model of abiraterone-resistant metastatic castration-resistant prostate cancer (mCRPC), combined onvansertib and abiraterone resulted in enhanced mitotic arrest and dramatic inhibition of tumor cell growth compared with either agent alone. Overall, this work establishes a mechanistic basis for the phase II clinical trial (NCT03414034) testing combined onvansertib and abiraterone in mCRPC patients and indicates this combination may have broad utility for cancer treatment.

SIGNIFICANCE

Abiraterone treatment induces mitotic defects that sensitize cancer cells to Plk1 inhibition, revealing an AR-independent mechanism for this synergistic combination that is applicable to a variety of cancer types.

Comprehensive kinomic study via a chemical proteomic approach reveals kinome reprogramming in hepatocellular carcinoma tissues

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. Kinases are attractive therapeutic targets since they are commonly altered in cancers. Here, to identify kinases of potential therapeutic interest in HCC, a quantitative kinomic study of tumour and adjacent non-tumour liver tissues was performed using a chemical proteomics approach. In total, 124 kinases were found differentially expressed and they were distributed over all nine kinase groups. Exploration of The Cancer Genome Atlas (TCGA) data showed that the dysregulation of 45 kinases was correlated with poor prognosis in HCC patients. We then tested 11 inhibitors targeting 12 crucial protein kinases alone or in combination for their ability to inhibit cell growth in Hep3B and PLC/PRF/5 cell lines. Six inhibitors significantly reduced viability in both cell lines. Combination inhibition of polo-like kinase 1 (PLK1) and casein kinase 1 epsilon (CSNK1E) significantly induced growth arrest in both cell lines synergistically. In summary, our analysis presents the most complete view of kinome reprogramming in HCC and provides novel insight into crucial kinases in HCC and potential therapeutic targets for HCC treatment. Moreover, the identification of hundreds of differentially expressed kinases forms a rich resource for novel drug targets or diagnostic biomarker discovery. Data are available via ProteomeXchange (identifier PXD023806).

Non-mitotic functions of polo-like kinases in cancer cells

Inhibitors of mitotic protein kinases are currently being developed as non-neurotoxic alternatives of microtubule-targeting agents (taxanes, vinca alkaloids) which provide a substantial survival benefit for patients afflicted with different types of solid tumors. Among the mitotic kinases, the cyclin-dependent kinases, the Aurora kinases, the kinesin spindle protein and Polo-like kinases (PLKs) have emerged as attractive targets of cancer therapeutics. The functions of mammalian PLK1-5 are traditionally linked to the regulation of the cell cycle and to the stress response. Especially the key role of PLK1 and PLK4 in cellular growth and proliferation, their overexpression in multiple types of human cancer and their druggability, make them appealing targets for cancer therapy. Inhibitors for PLK1 and PLK4 are currently being tested in multiple cancer trials. The clinical success of microtubule-targeting agents is attributed not solely to the induction of a mitotic arrest in cancer cells, but also to non-mitotic effects like targeting intracellular trafficking on microtubules. This raises the question whether new cancer targets like PLK1 and PLK4 regulate critical non-mitotic functions in tumor cells. In this article we summarize the important roles of PLK1-5 for the regulation of non-mitotic signaling. Due to these functions it is conceivable that inhibitors for PLK1 or PLK4 can target interphase cells, which underscores their attractive potential as cancer drug targets. Moreover, we also describe the contribution of the tumor-suppressors PLK2, PLK3 and PLK5 to cancer cell signaling outside of mitosis. These observations highlight the urgent need to develop highly specific ATP-competitive inhibitors for PLK4 and for PLK1 like the 3rd generation PLK-inhibitor Onvansertib to prevent the inhibition of tumor-suppressor PLKs in- and outside of mitosis. The remarkable feature of PLKs to encompass a unique druggable domain, the polo-box-domain (PBD) that can be found only in PLKs offers the opportunity for the development of inhibitors that target PLKs exclusively. Beyond the development of mono-specific ATP-competitive PLK inhibitors, the PBD as drug target will support the design of new drugs that eradicate cancer cells based on the mitotic and non-mitotic function of PLK1 and PLK4.

Inhibition of polo-like kinase 1 suppresses microtubule dynamics in MCF-7 cells

Polo-like kinase 1 (Plk1) is a mitotic serine/threonine kinase implicated in spindle formation and cytokinesis in mammalian cells. Here, purified Plk1 was found to bind to reconstituted microtubules in vitro. Further, Plk1 was found to co-localize with interphase microtubules in MCF-7 cells and to co-immunoprecipitate with polymerized tubulin. The binding of Plk1 to interphase microtubules appeared to increase with an increase in the level of tubulin acetylation in MCF-7 cells. Interestingly, Plk1 inhibitor III, an inhibitor of Plk1 kinase activity, treatment increased the association of Plk1 with the interphase microtubules in MCF-7 cells. Therefore, the effect of inhibition of Plk1 kinase activity on the dynamic instability of microtubules was determined by time-lapse imaging in MCF-7 cells. Plk1 inhibitor III dampened the dynamic instability of microtubules. For example, Plk1 inhibitor III (3 μM) reduced the rate and extent of the growing phase by 28 and 48%, respectively, and inhibited the dynamicity of microtubules by 53% as compared to the microtubules in control MCF-7 cells. Plk1 inhibitor III treatment also increased the level of acetylated microtubules, indicating that it stabilizes microtubules. The findings indicated that Plk1 interacts with microtubules and Plk1 may have a role in the regulation of microtubule dynamics.

Androgen receptor splicing variant 7: Beyond being a constitutively active variant

In prostate cancer development, the androgen receptor (AR) signaling plays a crucial role during both formation of early prostate lesions and progression to the lethal, incurable castration resistant stage. Accordingly, numerous approaches have been developed to inhibit AR activity including androgen deprivation therapy, application of the AR antagonists as well as the use of taxanes. However, these treatments, although effective initially, resistance inevitably occur for most of the patients within several years and limiting the therapeutic efficacy. Of note, alterations and reactivation of the AR signaling pathway have been demonstrated as the major reasons for the observed resistance. Accumulating evidences have suggested that synthesis of AR splicing variants, in particular, the constitutively active AR-V7, is one of the most important mechanisms that contribute to the abnormal AR signaling. In addition, clinical data also highlight the potential of using AR-V7 as a predictive biomarker and a therapeutic target in metastatic castration resistant prostate cancer (mCRPC). In this review, we summarize the recent findings concerning the specific role of AR-V7 in CRPC progression, drug resistance and its potential value in clinical assessment.

Cotargeting Plk1 and androgen receptor enhances the therapeutic sensitivity of paclitaxel-resistant prostate cancer

Backgrounds:

Despite the clinical success of taxanes, they still have limitations, such as chemoresistance. To overcome the limitations of paclitaxel, genetic alterations and targeting effects of altered genes were observed in paclitaxel-resistant cancer. Because paclitaxel-resistant cancer shows high levels of Plk1, a promising target in chemotherapy, the effectiveness of Plk1 inhibitors in paclitaxel-resistant cancer cells has been investigated.

Methods:

Paclitaxel-resistant cancer cells were developed by exposure of stepwise escalating levels of paclitaxel. Genetic alterations were detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunoblotting. Using a cell viability assay, combined targeting effects for Plk1 and androgen receptor (AR) were determined. Clinical data were analyzed to understand the relationship between Plk1 and AR in prostate cancer patients.

Results:

Treatment with Plk1 inhibitors markedly reduced the expression of MDR1, MRP1, and Plk1 in the paclitaxel-resistant cancer. Among Plk1 inhibitors, genistein, recently found as a direct Plk1 inhibitor, tended to be more effective in the paclitaxel-resistant prostate cancer than the parental cancer cells, which was related to the suppression of the AR, as well as inhibition of Plk1 activity. A combination of Plk1 inhibitors and AR antagonist bicalutamide exhibited a synergistic effect in LNCaP^TXR, as well as LNCaP cells, by inhibiting Plk1 and AR. Analysis of clinical data provides evidence for the relevance between Plk1 and AR in prostate cancer patients, showing that Plk1 and AR are strong predictors of poor survival rates.

Conclusions:

We suggest that cotargeting Plk1 and AR would be effective in advanced chemoresistant prostate cancer cells to overcome the limitations associated with paclitaxel.

Chronic low dose arsenic exposure preferentially perturbs mitotic phase of the cell cycle

Environmental pollution is a big challenge for human survival. Arsenic compounds are well-known biohazard, the exposure of which is closely linked to onsets of various human diseases, particularly cancers. Upon chronically exposing to arsenic compounds, genomic integrity is often disrupted, leading to tumor development. However, the underlying mechanisms by which chronic, low dose arsenic exposure targets genetic stability to initiate carcinogenesis still remain not fully understood. In this study, human lung epithelial BEAS-2B cells and keratinocytes were treated with 0.5 μM of sodium arsenite for one month (designated as BEAS-2B-SA cells or keratinocytes-SA), and its effect on cell cycle responses was analyzed. After being arrested in mitotic phase of the cell cycle by nocodazole treatment, BEAS-2B-SA cells or keratinocytes-SA were delayed to enter next cytokinesis. The lagging exit of the cells from mitosis was accompanied by a sustained Plk1 phosphorylation, which led to a persistent activation of the mitotic regulators BubR1 and Cdc27. As the result, cyclin B1 (clnB1) degradation was attenuated. BEAS-2B-SA cells or keratinocytes-SA also expressed a constitutively active Akt. The cytogenetic analysis showed an increased numbers of aneuploidy in these cells. The suppression of Akt reversed the aberrant expressions of the mitotic regulators, delay of mitotic exit as well as chromosomal aberrations. Our findings suggest that a long-term exposure to low dose sodium arsenite aberrantly retains the catenation of mitosis, which facilitates establishing genetic instability and predisposes the cells to tumorigenesis.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

Plk1 Inhibition Enhances the Efficacy of BET Epigenetic Reader Blockade in Castration-Resistant Prostate Cancer

Polo-like kinase 1 (Plk1), a crucial regulator of cell-cycle progression, is overexpressed in multiple types of cancers and has been proven to be a potent and promising target for cancer treatment. In case of prostate cancer, we once showed that antineoplastic activity of Plk1 inhibitor is largely due to inhibition of androgen receptor (AR) signaling. However, we also discovered that Plk1 inhibition causes activation of the β-catenin pathway and increased expression of c-MYC, eventually resulting in resistance to Plk1 inhibition. JQ1, a selective small-molecule inhibitor targeting the amino-terminal bromodomains of BRD4, has been shown to dramatically inhibit c-MYC expression and AR signaling, exhibiting antiproliferative effects in a range of cancers. Because c-MYC and AR signaling are essential for prostate cancer initiation and progression, we aim to test whether targeting Plk1 and BRD4 at the same time is an effective approach to treat prostate cancer. Herein, we show that a combination of Plk1 inhibitor GSK461364A and BRD4 inhibitor JQ1 had a strong synergistic effect on castration-resistant prostate cancer (CRPC) cell lines, as well as in CRPC xenograft tumors. Mechanistically, the synergistic effect is likely due to two reasons: (i) Plk1 inhibition results in the accumulation of β-catenin in the nucleus, thus elevation of c-MYC expression, whereas JQ1 treatment directly suppresses c-MYC transcription; (ii) Plk1 and BRD4 dual inhibition acts synergistically in inhibition of AR signaling. Mol Cancer Ther; 17(7); 1554-65. ©2018 AACR.

Plk1-Mediated Phosphorylation of TSC1 Enhances the Efficacy of Rapamycin

The AKT/TSC/mTOR axis is an important pathway controlling cell growth, survival, and proliferation in response to extracellular cues. Recently, it was reported that AKT activity fluctuates across the cell cycle. However, it remains unclear whether downstream targets of AKT are also regulated by the cell cycle. Here, we report that mTORC1 activity inversely correlates with AKT activity during the cell cycle. Mechanistically, Plk1 phosphorylation of TSC1 at S467 and S578 interfered with TSC1/TSC2 binding, destabilized TSC1, promoted dissociation of the TSC complex from the lysosome, and eventually led to mTORC1 activation. Tumors derived from cancer cells expressing the TSC1-S467E/S578E mutant exhibited greater sensitivity to rapamycin than those expressing WT TSC1. Collectively, our data support a model in which Plk1, instead of AKT, regulates the TSC/mTORC1 pathway during mitosis, eventually regulating the efficacy of rapamycin.Significance: This seminal report shows that activation of mTORC1 can be independent of AKT during mitosis. Cancer Res; 78(11); 2864-75. ©2018 AACR.

MicroRNAs as potential therapeutics to enhance chemosensitivity in advanced prostate cancer

Docetaxel and cabazitaxel are taxane chemotherapy treatments for metastatic castration-resistant prostate cancer (CRPC). However, therapeutic resistance remains a major issue. MicroRNAs are short non-coding RNAs that can silence multiple genes, regulating several signalling pathways simultaneously. Therefore, synthetic microRNAs may have therapeutic potential in CRPC by regulating genes involved in taxane response and minimise compensatory mechanisms that cause taxane resistance. To identify microRNAs that can improve the efficacy of taxanes in CRPC, we performed a genome-wide screen of 1280 microRNAs in the CRPC cell lines PC3 and DU145 in combination with docetaxel or cabazitaxel treatment. Mimics of miR-217 and miR-181b-5p enhanced apoptosis significantly in PC3 cells in the presence of these taxanes. These mimics downregulated at least a thousand different transcripts, which were enriched for genes with cell proliferation and focal adhesion functions. Individual knockdown of a selection of 46 genes representing these transcripts resulted in toxic or taxane sensitisation effects, indicating that these genes may be mediating the effects of the microRNA mimics. A range of these genes are expressed in CRPC metastases, suggesting that these microRNA mimics may be functional in CRPC. With further development, these microRNA mimics may have therapeutic potential to improve taxane response in CRPC patients.

miR-129-3p controls centrosome number in metastatic prostate cancer cells by repressing CP110

The centrosome plays a key role in cancer invasion and metastasis. However, it is unclear how abnormal centrosome numbers are regulated when prostate cancer (PCa) cells become metastatic. CP110 was previously described for its contribution of centrosome amplification (CA) and early development of aggressive cell behaviour. However its regulation in metastatic cells remains unclear. Here we identified miR-129-3p as a novel metastatic microRNA. CP110 was identified as its target protein. In PCa cells that have metastatic capacity, CP110 expression was repressed by miR-129-3p. High miR-129-3p expression levels increased cell invasion, while increasing CP110 levels decreased cell invasion. Overexpression of CP110 in metastatic PCa cells resulted in a decrease in the number of metastasis. In tissues of PCa patients, low CP110 and high miR-129-3p expression levels correlated with metastasis, but not with the expression of genes related to EMT. Furthermore, overexpression of CP110 in metastatic PCa cells resulted in excessive-CA (E-CA), and a change in F-actin distribution which is in agreement with their reduced metastatic capacity. Our data demonstrate that miR-129-3p functions as a CA gatekeeper in metastatic PCa cells by maintaining pro-metastatic centrosome amplification (CA) and preventing anti-metastatic E-CA.

Polo-like kinase 1 (Plk1) overexpression enhances ionizing radiation-induced cancer formation in mice

Polo-like kinase 1 (Plk1), a serine/threonine protein kinase normally expressed in mitosis, is frequently up-regulated in multiple types of human tumors regardless of the cell cycle stage. However, the causal relationship between Plk1 up-regulation and tumorigenesis is incompletely investigated. To this end, using a conditional expression system, here we generated Plk1 transgenic mouse lines to examine the role of Plk1 in tumorigenesis. Plk1 overexpression in mouse embryonic fibroblasts prepared from the transgenic mice led to aberrant mitosis followed by aneuploidy and apoptosis. Surprisingly, Plk1 overexpression had no apparent phenotypes in the mice. Given that no malignant tumor formation was observed even after a long period of Plk1 overexpression, we reasoned that additional factors are required for tumorigenesis in Plk1-overexpressing mice. Because Plk1 can directly participate in the regulation of the DNA damage response (DDR) pathway, we challenged Plk1-overexpressing mice with ionizing radiation (IR) and found that Plk1-overexpressing mice are much more sensitive to IR than their wild-type littermates. Analysis of tumor development in the Plk1-overexpressing mice indicated a marked decrease in the time required for tumor emergence after IR. At the molecular level, Plk1 overexpression led to reduced phosphorylation of the serine/threonine kinases ATM and Chk2 and of histone H2AX after IR treatment both in vivo and in vitro Furthermore, RNA-Seq analysis suggested that Plk1 elevation decreases the expression of several DDR genes. We conclude that Plk1 overexpression may contribute to tumor formation by both inducing chromosomal instability and suppressing the DDR pathway.

lncRNA HOXD-AS1 Regulates Proliferation and Chemo-Resistance of Castration-Resistant Prostate Cancer via Recruiting WDR5

Castration-resistant prostate cancer (CRPC) that occurs after the failure of androgen deprivation therapy is the leading cause of deaths in prostate cancer patients. Thus, there is an obvious and urgent need to fully understand the mechanism of CRPC and discover novel therapeutic targets. Long noncoding RNAs (lncRNAs) are crucial regulators in many human cancers, yet their potential roles and molecular mechanisms in CRPC are poorly understood. In this study, we discovered that an lncRNA HOXD-AS1 is highly expressed in CRPC cells and correlated closely with Gleason score, T stage, lymph nodes metastasis, and progression-free survival. Knockdown of HOXD-AS1 inhibited the proliferation and chemo-resistance of CRPC cells in vitro and in vivo. Furthermore, we identified several cell cycle, chemo-resistance, and castration-resistance-related genes, including PLK1, AURKA, CDC25C, FOXM1, and UBE2C, that were activated transcriptionally by HOXD-AS1. Further investigation revealed that HOXD-AS1 recruited WDR5 to directly regulate the expression of target genes by mediating histone H3 lysine 4 tri-methylation (H3K4me3). In conclusion, our findings indicate that HOXD-AS1 promotes proliferation, castration resistance, and chemo-resistance in prostate cancer by recruiting WDR5. This sheds a new insight into the regulation of CRPC by lncRNA and provides a potential approach for the treatment of CRPC.

Targeting Plk1 to Enhance Efficacy of Olaparib in Castration-Resistant Prostate Cancer

Olaparib is an FDA-approved PARP inhibitor (PARPi) that has shown promise as a synthetic lethal treatment approach for BRCA-mutant castration-resistant prostate cancer (CRPC) in clinical use. However, emerging data have also shown that even BRCA-mutant cells may be resistant to PARPi. The mechanistic basis for these drug resistances is poorly understood. Polo-like kinase 1 (Plk1), a critical regulator of many cell-cycle events, is significantly elevated upon castration of mice carrying xenograft prostate tumors. Herein, by combination with Plk1 inhibitor BI2536, we show a robust sensitization of olaparib in 22RV1, a BRCA1-deficient CRPC cell line, as well as in CRPC xenograft tumors. Mechanistically, monotherapy with olaparib results in an override of the G1-S checkpoint, leading to high expression of Plk1, which attenuates olaparib's overall efficacy. In BRCA1 wild-type C4-2 cells, Plk1 inhibition also significantly increases the efficacy of olaparib in the presence of p53 inhibitor. Collectively, our findings not only implicate the critical role of Plk1 in PARPi resistance in BRCA-mutant CRPC cells, but also shed new light on the treatment of non-BRCA-mutant patient subgroups who might also respond favorably to PARPi. Mol Cancer Ther; 16(3); 469-79. ©2017 AACR.

Plk1 Inhibitors in Cancer Therapy: From Laboratory to Clinics

Polo-like kinase 1 (Plk1) overexpression has been shown to occur in a wide range of tumors, prompting research and development of Plk1 inhibitors as a means of cancer treatment. This review discusses recent advances in the development of Plk1 inhibitors for cancer management. Plk1 inhibition has been shown to cause mitotic block and apoptosis of cells with higher mitotic index and therefore higher Plk1 expression. The potential of Plk1 inhibitors as cancer therapeutics has been widely investigated. However, a complete understanding of Plk1 biology/mechanism is yet to be fully achieved. Resistance to certain chemotherapeutic drugs has been linked to Plk1 overexpression, and Plk1-mediated mitotic events such as microtubule rearrangement have been found to reduce the efficacy of chemotherapeutic agents. The Plk1 inhibitor volasertib has shown considerable promise in clinical studies, having reached phase III trials. However, preclinical success with Plk1 inhibitors has not translated well into clinical success. In our view, combined therapies targeting other relevant pathways together with Plk1 may be vital to combat issues observed with monotherapy, especially resistance. In addition, research should also be directed toward understanding the mechanisms of Plk1 and designing additional next generations of specific, potent Plk1 inhibitors to target cancer. Mol Cancer Ther; 15(7); 1427-35. ©2016 AACR.

Cross Talk between Wnt/β-Catenin and CIP2A/Plk1 Signaling in Prostate Cancer: Promising Therapeutic Implications

Aberrant activation of the Wnt/β-catenin pathway and polo-like kinase 1 (Plk1) overexpression represent two common events in prostate cancer with relevant functional implications. This minireview analyzes their potential therapeutic significance in prostate cancer based on their role as androgen receptor (AR) signaling regulators and the pivotal role of the tumor suppressor protein phosphatase 2A (PP2A) modulating these pathways.

Cotargeting Polo-Like Kinase 1 and the Wnt/β-Catenin Signaling Pathway in Castration-Resistant Prostate Cancer

The Wnt/β-catenin signaling pathway has been identified as one of the predominantly upregulated pathways in castration-resistant prostate cancer (CRPC). However, whether targeting the β-catenin pathway will prove effective as a CRPC treatment remains unknown. Polo-like kinase 1 (Plk1) is a critical regulator in many cell cycle events, and its level is significantly elevated upon castration of mice carrying xenograft prostate tumors. Indeed, inhibition of Plk1 has been shown to inhibit tumor growth in several in vivo studies. Here, we show that Plk1 is a negative regulator of Wnt/β-catenin signaling. Plk1 inhibition or depletion enhances the level of cytosolic and nuclear β-catenin in human prostate cancer cells. Furthermore, inhibition of Wnt/β-catenin signaling significantly potentiates the antineoplastic activity of the Plk1 inhibitor BI2536 in both cultured prostate cancer cells and CRPC xenograft tumors. Mechanistically, axin2, a negative regulator of the β-catenin pathway, serves as a substrate of Plk1, and Plk1 phosphorylation of axin2 facilitates the degradation of β-catenin by enhancing binding between glycogen synthase kinase 3β (GSK3β) and β-catenin. Plk1-phosphorylated axin2 also exhibits resistance to Cdc20-mediated degradation. Overall, this study identifies a novel Plk1-Wnt signaling axis in prostate cancer, offering a promising new therapeutic option to treat CRPC.

Osteopontin-c mediates the upregulation of androgen responsive genes in LNCaP cells through PI3K/Akt and androgen receptor signaling

Androgen receptor (AR) signaling is a key pathway modulating prostate cancer (PCa) progression. Several steps in this pathway have been investigated in order to propose novel treatment strategies for advanced PCa. Total osteopontin (OPN) has been described as a biomarker for PCa, in addition to its role in activating the progression of this tumor. Based on the known effects of the OPNc splice variant on PCa progression, the present study investigated whether this isoform can also modulate AR signaling. In order to test this, an in vitro model was used in which LNCaP cells were cultured in the presence of conditioned medium (CM) secreted by PCa cells overexpressing OPNc (OPNc-CM). The activation of AR signaling was evaluated by measuring the expression levels of AR-responsive genes (ARGs) using quantitative polymerase chain reaction and specific oligonucleotides. The data demonstrated that all nine tested ARGs (Fgf8, TMPRSS2, Greb1, Cdk2, Ndrg1, Cdk1, Pmepa1, Psa and Ar) are significantly upregulated in response to OPNc-CM compared with LNCaP cells cultured in CM secreted by control cells transfected with empty expression vector. The specific involvement of OPNc was demonstrated by depleting OPNc from OPNc-CM using an anti-OPNc neutralizing antibody. In addition, by using a phosphoinositide 3-kinase (PI3K)-specific inhibitor and AR antagonists, such as flutamide and bicalutamide, it was also observed that upregulation of ARGs in response to OPNc-CM involves PI3K signaling and depends on the AR. In conclusion, these data indicated that OPNc is able to activate AR signaling through the PI3K pathway and the AR. These data further corroborate our previous data, revealing the OPNc splice variant to be a key molecule that is able to modulate key signaling pathways involved in PCa progression.

Inhibition of polo-like kinase 1 (Plk1) enhances the antineoplastic activity of metformin in prostate cancer

The widely used anti-diabetic drug metformin has been shown to exert strong antineoplastic actions in numerous tumor types, including prostate cancer (PCa). In this study, we show that BI2536, a specific Plk1 inhibitor, acted synergistically with metformin in inhibiting PCa cell proliferation. Furthermore, we also provide evidence that Plk1 inhibition makes PCa cells carrying WT p53 much more sensitive to low-dose metformin treatment. Mechanistically, we found that co-treatment with BI2536 and metformin induced p53-dependent apoptosis and further activated the p53/Redd-1 pathway. Moreover, we also show that BI2536 treatment inhibited metformin-induced glycolysis and glutamine anaplerosis, both of which are survival responses of cells against mitochondrial poisons. Finally, we confirmed the cell-based observations using both cultured cell-derived and patient-derived xenograft studies. Collectively, our findings support another promising therapeutic strategy by combining two well tolerated drugs against PCa proliferation and the progression of androgen-dependent PCa to the castration-resistant stage.

Low-dose arsenic-mediated metabolic shift is associated with activation of Polo-like kinase 1 (Plk1)

Arsenic is a well-established human carcinogen associated with cancers of the skin, liver, lung, kidney, and bladder. Although numerous carcinogenic pathways have been proposed, the molecular mechanisms underlying arsenic-associated cancer etiology are still elusive. The cellular responses to arsenic exposure are dose dependent. It was recently shown that low-dose arsenic leads to a metabolic shift from mitochondrial respiration to aerobic glycolysis via inactivation of tumor suppressor p53 and activation of NF-κB. However, how inactivation of p53, activation of NF-κB, and metabolic change are coordinated in response to low-dose arsenic exposure is still not completely understood. Polo-like kinase 1 (Plk1) is a well- documented regulator in many cell cycle-related events. Herein, we showed that low-dose arsenic leads to elevation of Plk1 in an NF-κB-dependent manner and that elevation of Plk1 contributes to the metabolic change from oxidative phosphorylation to glycolysis via activation of the PI3K/AKT/mTOR pathway. Furthermore, we showed that inhibition/depletion of Plk1 reverses low-dose arsenic-associated phenotypes, including enhanced cell proliferation, activation of the PI3K/AKT/mTOR pathway, and increased glycolysis. Finally, inhibition of the PI3K/AKT/mTOR pathway also antagonizes the enhanced glycolytic influx due to low-dose arsenic exposure. Our studies support the notion that Plk1 likely plays a critical role in cellular responses to low-dose arsenic.

Inhibition of Plk1 represses androgen signaling pathway in castration-resistant prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer-related death in males in the United States. Majority of prostate cancers are originally androgen-dependent and sensitive to androgen-deprivation therapy (ADT), however, most of them eventually relapse and progress into incurable castration-resistant prostate cancer (CRPC). Of note, the activity of androgen receptor (AR) is still required in CRPC stage. The mitotic kinase polo-like kinase 1 (Plk1) is significantly elevated in PCa and its expression correlates with tumor grade. In this study, we assess the effects of Plk1 on AR signaling in both androgen-dependent and androgen-independent PCa cells. We demonstrate that the expression level of Plk1 correlated with tumorigenicity and that inhibition of Plk1 caused reduction of AR expression and AR activity. Furthermore, Plk1 inhibitor BI2536 down-regulated SREBP-dependent expression of enzymes involved in androgen biosynthesis. Of interest, Plk1 level was also reduced when AR activity was inhibited by the antagonist MDV3100. Finally, we show that BI2536 treatment significantly inhibited tumor growth in LNCaP CRPC xenografts. Overall, our data support the concept that Plk1 inhibitor such as BI2536 prevents AR signaling pathway and might have therapeutic potential for CRPC patients.

Plk1 inhibition enhances the efficacy of androgen signaling blockade in castration-resistant prostate cancer

Prostate cancer is thought to be driven by oxidative stress, lipid metabolism, androgen receptor (AR) signaling, and activation of the PI3K-AKT-mTOR pathway, but it is uncertain how they may become coordinated during progression to castration-resistant disease that remains incurable. The mitotic kinase polo-like kinase 1 (Plk1) is elevated in prostate cancer, where its expression is linked to tumor grade. Notably, Plk1 signaling and lipid metabolism were identified recently as two of the top five most upregulated pathways in a mouse xenograft model of human prostate cancer. Herein, we show that oxidative stress activates both the PI3K-AKT-mTOR pathway and AR signaling in a Plk1-dependent manner in prostate cells. Inhibition of the PI3K-AKT-mTOR pathway prevented oxidative stress-induced activation of AR signaling. Plk1 modulation also affected cholesteryl ester accumulation in prostate cancer via the SREBP pathway. Finally, Plk1 inhibition enhanced cellular responses to androgen signaling inhibitors (ASI) and overcame ASI resistance in both cultured prostate cancer cells and patient-derived tumor xenografts. Given that activation of AR signaling and the PI3K-AKT-mTOR pathway is sufficient to elevate SREBP-dependent expression of key lipid biosynthesis enzymes in castration-resistant prostate cancer (CRPC), our findings argued that Plk1 activation was responsible for coordinating and driving these processes to promote and sustain the development of this advanced stage of disease. Overall, our results offer a strong mechanistic rationale to evaluate Plk1 inhibitors in combination drug trials to enhance the efficacy of ASIs in CRPC.

Targeting CDC25C, PLK1 and CHEK1 to overcome Docetaxel resistance induced by loss of LZTS1 in prostate cancer

Docetaxel is used as a standard treatment in patients with metastatic castration-resistant prostate cancer. However, a large subset of patients develops resistance. Understanding resistance mechanisms, which are largely unknown, will allow identification of predictive biomarkers and therapeutic targets. We established resistant IGR-CaP1 prostate cancer cell lines for different doses of Docetaxel. We investigated gene expression profiles by microarray analyses in these cell lines and generated a signature of 99 highly differentially expressed genes potentially implicated in chemoresistance. We focused on the role of the cell cycle regulator LZTS1, which was under-expressed in the Docetaxel-resistant cell lines, its inhibition resulting from the promoter methylation. Knockdown of LZTS1 in parental cells with siRNA showed that LZTS1 plays a role in the acquisition of the resistant phenotype. Furthermore, we observed that targeting CDC25C, a partner of LZTS1, with the NSC663284 inhibitor specifically killed the Docetaxel-resistant cells. To further investigate the role of CDC25C, we used inhibitors of the mitotic kinases that regulate CDC25C. Inhibition of CHEK1 and PLK1 induced growth arrest and cell death in the resistant cells. Our findings identify an important role of LZTS1 through its regulation of CDC25C in Docetaxel resistance in prostate cancer and suggest that CDC25C, or the mitotic kinases CHEK1 and PLK1, could be efficient therapeutic targets to overcome Docetaxel resistance.