Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells

Integrative mRNA profiling comparing cultured primary cells with clinical samples reveals PLK1 and C20orf20 as therapeutic targets in cutaneous squamous cell carcinoma

Identifying therapeutic targets for cancer treatment relies on consistent changes within particular types or sub-types of malignancy. The ability to define either consistent changes or sub-types of malignancy is often masked by tumor heterogeneity. To elucidate therapeutic targets in cutaneous squamous cell carcinoma (cSCC), the most frequent skin neoplasm with malignant potential, we have developed an integrated approach to gene expression profiling beginning with primary keratinocytes in culture. Candidate drivers of cSCC development were derived by first defining a set of in vitro cancer genes and then comparing their expression in a range of clinical data sets containing normal skin, cSCC and the benign hyper-proliferative condition psoriasis. A small interfering RNA (siRNA) screen of the resulting 21 upregulated genes has yielded targets capable of reducing xenograft tumor volume in vivo. Small-molecule inhibitors for one target, Polo-like kinase-1 (PLK1), are already in clinical trials for other malignancies, and our data show efficacy in cSCC. Another target, C20orf20, is identified as being overexpressed in cSCC, and siRNA-mediated knockdown induces apoptosis in vitro and reduces tumor growth in vivo. Thus, our approach has shown established and uncharacterized drivers of tumorigenesis with potent efficacy as therapeutic targets for the treatment of cSCC.

Polo-like kinase 1 is a potential therapeutic target in human melanoma

Exploration of the human melanoma cell-cycle pathway can lead to identification of new therapeutic targets. By gene set enrichment analysis, we identified the cell-cycle pathway and its member polo-like kinase 1 (Plk-1) to be significantly overexpressed in primary melanomas and in melanoma metastases. In vitro expression of Plk-1 was peaked at the G2/M phase of the cell cycle. Plk-1 knockdown/inhibition led to induction of apoptosis, which was caspase-3/8-dependent and p53-independent, and involved BID and Bcl-2 proteins. Comparative genomic hybridization/single-nucleotide polymorphism arrays showed no genetic alteration in the Plk-1 locus. Previous suggestions and significant enrichment of the mitogen-activated protein kinase (MAPK) signaling pathway pointed to potential regulation of Plk-1 by MAPK signaling. Inhibition of this pathway resulted in decreased Plk-1 expression as a consequence of G1 cell-cycle arrest rather than direct regulation of Plk-1. Inhibition of MAPK and Plk-1 had an additive effect on reduced cell viability. This study shows that in human melanoma, Plk-1 expression is dynamically regulated during the cell cycle, knockdown of Plk-1 leads to apoptotic cell death, and that a combination of Plk-1 and MAPK inhibition has an additive effect on melanoma cell viability. We conclude that combined inhibition of Plk-1 and MAPK could be a potentially attractive strategy in melanoma therapy.

Efficient inhibition of human colorectal carcinoma growth by RNA interference targeting polo-like kinase 1 in vitro and in vivo

Polo-like kinase 1 (PLK1) showing a high expression in various kinds of tumors is considered a candidate target for cancer therapy. The aim of our study was to explore the effects of silencing PLK1 gene on human colorectal carcinoma cell line HCT-116 in vitro and in vivo. In vitro, the plasmids generating short hairpin RNA (shRNA)-targeting PLK1 were transfected into HCT-116 by using FugeneHD reagent, and the silencing potency was measured by RT-PCR, western blot, flow cytometry, and Caspase-Glo 3/7 assay, respectively. In vivo, the growth inhibition capacity of PLK1-shRNA on HCT-116 xenograft was measured in nude mice. Then, the silencing effect of PLK1 was analyzed by RT-PCR, western blot, and immunohistochemistry, respectively. Apoptosis, angiogenesis, and proliferation in tumor tissues were measured by TUNEL, CD31, and PCNA stain, respectively. The RNA interference targeting PLK1 significantly decreased the expression of PLK1 in vitro. More importantly, anti-PLK1 treatment in HCT-116 xenograft decreased tumor weight by 81.58% compared with the control group (p<0.001), accompanied with decreased PLK1 mRNA and protein expression, increased cell apoptosis, and reduced angiogenesis and proliferation (p<0.001). Our study showed that knockdown of PLK1 by shRNA might be the potential therapeutic approach against human colorectal carcinoma.

PLK1 as an oncology target: current status and future potential

The Polo-like kinases (PLKs) have been investigated as oncology targets for several years; however, only recently have potent inhibitors been described. Here, we report on progress in the clinical validation of the PLKs as antitumor drug targets as well as recent understanding gained regarding their synergistic roles in the context of other molecular defects occurring in tumors. Also relevant to the development of PLK inhibitors as therapeutics are the putative roles of other members of this family as tumor suppressors. The resulting potential drawbacks of non-isoform selective compounds are presented. As an alternative approach to achieving PLK1 specificity, we discuss prospects for developing small molecule inhibitors of the crucial regulatory and subcellular targeting domain containing the Polo-boxes.

Polo-like kinase 1 (Plk1) in cutaneous T-cell lymphoma

BACKGROUND

Polo-like kinase 1 (Plk1) has multiple functions throughout mitosis. Plk1 levels are high in a number of cancers and haematological malignancies while being low in most differentiated tissues.

OBJECTIVES

To assess the immunoreactivity of Plk1 in cutaneous T-cell lymphoma (CTCL) as a potential therapeutic target, to differentiate Plk1 levels among lesion types and to compare the detection level of Plk1 in fresh frozen (f) vs. paraffin-embedded (p) tissue.

METHODS

Immunohistochemical staining of CTCL skin lesions with anti-Plk1 antibody was performed in a total of 65 biopsies from 49 patients with CTCL. Both f and p tissue was available for comparison in 46 biopsies.

RESULTS

Tumour-stage CTCL lesions displayed significantly more Plk1 (mean f 7·7%, p 8·8%) than patch (mean f 0·7%, p 2·0%) and plaque-stage lesions (mean f 1·1%, p 2·0%) (P < 0·05). Plk1 ranged from 0% to 18% in f and 0% to 24% in p samples. p tissue revealed a higher mean Plk1 detection rate of 4·4% compared with 2·9% in f tissue with no statistical significance.

CONCLUSIONS

Our results indicate that in CTCL, Plk1 is increased mainly in advanced lesions. Several Plk1 inhibitors have already shown promising results in preclinical and clinical phase I and II trials for different types of cancers with low adverse effects. Immunohistochemical detection of high Plk1 levels in patients with CTCL could help select individuals who might benefit from treatment with small molecule Plk1 inhibitors.

A signature inferred from Drosophila mitotic genes predicts survival of breast cancer patients

Introduction

The classification of breast cancer patients into risk groups provides a powerful tool for the identification of patients who will benefit from aggressive systemic therapy. The analysis of microarray data has generated several gene expression signatures that improve diagnosis and allow risk assessment. There is also evidence that cell proliferation-related genes have a high predictive power within these signatures.

Methods

We thus constructed a gene expression signature (the DM signature) using the human orthologues of 108 Drosophila melanogaster genes required for either the maintenance of chromosome integrity (36 genes) or mitotic division (72 genes).

Results

The DM signature has minimal overlap with the extant signatures and is highly predictive of survival in 5 large breast cancer datasets. In addition, we show that the DM signature outperforms many widely used breast cancer signatures in predictive power, and performs comparably to other proliferation-based signatures. For most genes of the DM signature, an increased expression is negatively correlated with patient survival. The genes that provide the highest contribution to the predictive power of the DM signature are those involved in cytokinesis.

Conclusion

This finding highlights cytokinesis as an important marker in breast cancer prognosis and as a possible target for antimitotic therapies.

Essential role of microphthalmia transcription factor for DNA replication, mitosis and genomic stability in melanoma

Malignant melanoma is an aggressive cancer known for its notorious resistance to most current therapies. The basic helix-loop-helix microphthalmia transcription factor (MITF) is the master regulator determining the identity and properties of the melanocyte lineage, and is regarded as a lineage-specific 'oncogene' that has a critical role in the pathogenesis of melanoma. MITF promotes melanoma cell proliferation, whereas sustained supression of MITF expression leads to senescence. By combining chromatin immunoprecipitation coupled to high throughput sequencing (ChIP-seq) and RNA sequencing analyses, we show that MITF directly regulates a set of genes required for DNA replication, repair and mitosis. Our results reveal how loss of MITF regulates mitotic fidelity, and through defective replication and repair induces DNA damage, ultimately ending in cellular senescence. These findings reveal a lineage-specific control of DNA replication and mitosis by MITF, providing new avenues for therapeutic intervention in melanoma. The identification of MITF-binding sites and gene-regulatory networks establish a framework for understanding oncogenic basic helix-loop-helix factors such as N-myc or TFE3 in other cancers.

Polo-like kinase1 (Plk1) knockdown enhances cisplatin chemosensitivity via up-regulation of p73α in p53 mutant human epidermoid squamous carcinoma cells

Polo-like kinase 1 (Plk1), a critical regulator of mitotic entry, progression and exit, has been shown to be involved in a variety of cancers and thus is becoming an attractive target for cancer management. In case of DNA damage, Plk1 not only inhibits p53 independent apoptosis by dysfunctioning p73α but also allows cells to recover from growth arrest. Here, we showed the effects of knocking down plk1 gene through small interference RNA (siRNA) on cell cycle progression, proliferation and chemosensitivity of p53 mutant A431 cells to cisplatin (CDDP). The expression of Plk1 was measured by RT-PCR and Western blotting. Anti-proliferative response accompanied with cell cycle arrest in G(2)/M phase and induction of cell death was recorded following Plk1 knockdown. Furthermore, cells following knockdown of Plk1, which induced increase of Cyclin B1, p-Cdc2 and p73α with a decrease in p-Cdc25C, were more sensitive to CDDP. CDDP treatment induced nuclear translocation and co-localization of Plk1 with p73α whereas combination of CDDP and Plk1siRNA upregulated the expression of p73α protein in a synergistic manner thereby leading to an increase up to ∼5 folds in CDDP-induced cell death. The increase in caspase-3 activity indicated apoptosis as a contributor in the total cell death. Conclusively, plk1 gene silencing can enhance the sensitivity of A431 cells to low doses of CDDP by upregulating p73α expression and thus can be a revolutionary approach in cancer chemotherapy.

Validating cancer drug targets through chemical genetics

Targeted therapies for cancer promise to revolutionize treatment by specifically inactivating pathways needed for the growth of tumor cells. The most prominent example of such therapy is imatinib (Gleevec), which targets the BCR-ABL kinase and provides an effective low-toxicity treatment for chronic myelogenous leukemia. This success has spawned myriad efforts to develop similarly targeted drugs for other cancers. Unfortunately, the high expectations of these efforts have not yet been realized, likely due to the genetic diversity among and within tumors, as well as the complex and largely unpredictable interactions of drug-like compounds with innumerable targets that affect cellular and organismal metabolism. While improvements in sequencing technologies are beginning to address the first problem, solving the second problem requires methods for linking specific features of the cancer genome to their optimally targeted therapies. One approach, referred to as chemical genetics, accomplishes this by genetic control of chemical susceptibility. Chemical genetics is a crucial tool for the rational development of cancer drugs.

Modulating polo-like kinase 1 as a means for cancer chemoprevention

Naturally occurring agents have always been appreciated for their medicinal value for both their chemopreventive and therapeutic effects against cancer. In fact, the majority of the drugs we use today, including the anti-cancer agents, were originally derived from natural compounds, either in their native form or modified to enhance their bioavailability or specificity. It is believed that for maximum effectiveness, it will useful to design novel target-based agents for chemoprevention as well as the treatment of cancer. Recent studies have shown that the serine/threonine kinase polo-like kinase (Plk) 1 is widely overexpressed in a variety of cancers and is being increasingly appreciated as a target for cancer management. Additionally, several chemopreventive agents have been shown to inhibit Plk1 in cancer cells. In this review, we will discuss if Plk1 could also be a target for designing novel strategies for cancer chemoprevention.

Identification of 4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline derivatives as a new class of orally and selective Polo-like kinase 1 inhibitors

Polo-like kinase 1 (Plk1) is a fundamental regulator of mitotic progression whose overexpression is often associated with oncogenesis and therefore is recognized as an attractive therapeutic target in the treatment of proliferative diseases. Here we discuss the structure-activity relationship of the 4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline class of compounds that emerged from a high throughput screening (HTS) campaign as potent inhibitors of Plk1 kinase. Furthermore, we describe the discovery of 49, 8-{[2-methoxy-5-(4-methylpiperazin-1-yl)phenyl]amino}-1-methyl-4,5-dihydro-1H-pyrazolo[4,3-h]quinazoline-3-carboxamide, as a highly potent and specific ATP mimetic inhibitor of Plk1 (IC(50) = 0.007 microM) as well as its crystal structure in complex with the methylated Plk1(36-345) construct. Compound 49 was active in cell proliferation against different tumor cell lines with IC(50) values in the submicromolar range and active in vivo in the HCT116 xenograft model where it showed 82% tumor growth inhibition after repeated oral administration.

Polo-like kinase 1 (Plk1) in non-melanoma skin cancers

Polo-like kinase 1 (Plk1) is becoming an increasingly attractive target for cancer management. Plk1 has been shown to be overexpressed in a variety of cancers; however its role in skin cancers is not well-understood. We recently demonstrated that Plk1 is overexpressed in human melanoma and gene-knockdown as well as chemical-inhibition of Plk1 resulted in a significant decrease in melanoma cell viability and growth without affecting the growth of the normal human epidermal melanocytes (NHEMs). Further, the observed anti-proliferative response of Plk1 was found to be accompanied with a significant G(2)/M cell cycle arrest, mitotic catastrophe and induction of apoptosis in melanoma cells. In this study, we determined the expression profile of Plk1 in non-melanoma skin cancers viz. basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Our data demonstrated that like melanoma, Plk1 is significantly overexpressed in BCC and SCC samples. Further, we also found that compared to normal human epidermal keratinocytes (NHEKs), Plk1 was overexpressed at both the protein and mRNA levels in squamous A253 and A431 cells. In addition, a similar protein expression pattern was found for the downstream targets of Plk1, viz. Cdk1, Cyclin B1 and Cdc25C. We believe that the expression pattern of Plk1 in the various skin cancers, the observed insusceptibility of normal cells to Plk1 inhibition and the easy accessibility for topical applications lends the skin as an attractive tissue for Plk1 based cancer chemoprevention and chemotherapeutic applications.