Downregulation of Polo-like kinase 1 induces cellular senescence in human primary cells through a p53-dependent pathway

Increased ANKRD1 Levels in Early Senescence Mediated by RBMS1-Elicited ANKRD1 mRNA Stabilization

Cellular senescence is a dynamic biological process triggered by sublethal cell damage and driven by specific changes in gene expression programs. We recently identified ANKRD1 (ankyrin repeat domain 1) as a protein strongly elevated after triggering senescence in fibroblasts. Here, we set out to investigate the mechanisms driving the elevated production of ANKRD1 in the early stages of senescence. Our results indicated that the rise in ANKRD1 levels after triggering senescence using etoposide (Eto) was the result of moderate increases in transcription and translation, and robust mRNA stabilization. Antisense oligomer (ASO) pulldown followed by mass spectrometry revealed a specific interaction of the RNA-binding protein RBMS1 with ANKRD1 mRNA that was confirmed by ribonucleoprotein immunoprecipitation analysis. RBMS1 abundance decreased in the nucleus and increased in the cytoplasm during Eto-induced senescence; in agreement with the hypothesis that RBMS1 may participate in post-transcriptional stabilization of ANKRD1 mRNA, silencing RBMS1 reduced, while overexpressing RBMS1 enhanced ANKRD1 mRNA half-life after Eto treatment. A segment proximal to the ANKRD1 coding region was identified as binding RBMS1 and conferring RBMS1-dependent increased expression of a heterologous reporter. We propose that RBMS1 increases expression of ANKRD1 during the early stages of senescence by stabilizing ANKRD1 mRNA.

Temporal regulation of gene expression and pathways in chemotherapy-induced senescence in HeLa cervical cancer cell line

Cellular senescence is the state of permanent growth arrest. Chemotherapeutic drugs induce senescence, known as therapy-induced senescence. Although there are studies deciphering processes in senescence, more studies providing detailed information on therapy-induced senescence at the transcriptome level are needed. In order to understand temporal molecular changes of doxorubicin treatment in the course of senescence formation, two data sets from HeLa cells at 16 h and 72 h doxorubicin treatment were analyzed. GO BP enrichment, KEGG pathways and hub genes specific to or shared between 16 h and 72 h doxorubicin treated HeLa cells were identified. Genes functioning in p53 signaling were upregulated only in 16 h, while genes functioning in extracellular matrix organization were upregulated only in 72 h doxorubicin treated HeLa cells. Wound healing genes were gradually upregulated from 16 h to 72 h doxorubicin treatment and metabolic pathways were downregulated at both. ncRNA processing and ribosome biogenesis GO BP terms were enriched in upregulated genes at 16 h, while these terms were enriched in downregulated genes at 72 h senescent HeLa cells. According to our results, genes functioning in p53 signaling may be involved in the induction of senescence, but may not be required to maintain senescence in HeLa cells.

Transcriptomic changes during the replicative senescence of human articular chondrocytes

Osteoarthritis (OA) is a degenerative joint disease and a leading cause of disability worldwide. Aging is a major risk factor for OA, but the specific mechanisms underlying this connection remain unclear. Although chondrocytes rarely divide in adult articular cartilage, they undergo replicative senescence in vitro which provides an opportunity to study changes related to aging under controlled laboratory conditions. In this pilot study, we performed bulk RNA sequencing on early- and late-passage human articular chondrocytes to identify transcriptomic changes associated with cellular aging. Chondrocytes were isolated from the articular cartilage of three donors, two with OA (age 70-80 years) and one with healthy cartilage (age 26 years). Chondrocytes were serially passaged until replicative senescence and RNA extracted from early- and late-passage cells. Principal component analysis of all genes showed clear separation between early- and late-passage chondrocytes, indicating substantial age-related differences in gene expression. Differentially expressed genes (DEGs) analysis confirmed distinct transcriptomic profiles between early- and late-passage chondrocytes. Hierarchical clustering revealed contrasting expression patterns between the two isolates from osteoarthritic samples and the healthy sample. Focused analysis of DEGs on transcripts associated with turnover of the extra-cellular matrix and the senescence-associated secretory phenotype (SASP) showed consistent downregulation of Col2A1 and ACAN, and upregulation of MMP19, ADAMTS4, and ADAMTS8 in late passage chondrocytes across all samples. SASP components including IL-1α, IL-1β, IL-6, IL-7, p16INK4A (CDKN2A) and CCL2 demonstrated significant upregulation in late passage chondrocytes originally isolated from OA samples. Pathway analysis between sexes with OA revealed shared pathways such as extracellular matrix (ECM) organization, collagen formation, skeletal and muscle development, and nervous system development. Sex-specific differences were observed, with males showing distinctions in ECM organization, regulation of the cell cycle process as well as neuron differentiation. In contrast, females exhibited unique variations in the regulation of the cell cycle process, DNA metabolic process, and the PID-PLK1 pathway.

Assessment of cell cycle progression and mitotic slippage by videomicroscopy

Senescence is a state of irreversible cell cycle arrest accompanied by the acquisition of the senescence-associated secretory phenotype (SASP), which is activated in response to a variety of damaging stimuli, including genotoxic therapy. Accumulating evidence indicates that mitotic stress also promotes entry into senescence. This occurs via a mechanism involving defective mitoses and mitotic arrest, followed by abortion of cell division and slippage in the G1 phase. In this process, mitotic slippage leads to the generation of senescent cells characterized by a large cell body and a multinucleated and/or enlarged nuclear size. Here, we provide a detailed protocol for the assessment of cell proliferation and mitotic slippage in colorectal cancer cells upon pharmacological inhibition of the mitotic kinesin KIF11, best known as EG5. This approach can be used for preliminary characterization of senescence induction by therapeutics, but requires validation with standard senescence assays.

Shared and unique phosphoproteomics responses in skeletal muscle from exercise models and in hyperammonemic myotubes

Skeletal muscle generation of ammonia, an endogenous cytotoxin, is increased during exercise. Perturbations in ammonia metabolism consistently occur in chronic diseases, and may blunt beneficial skeletal muscle molecular responses and protein homeostasis with exercise. Phosphorylation of skeletal muscle proteins mediates cellular signaling responses to hyperammonemia and exercise. Comparative bioinformatics and machine learning-based analyses of published and experimentally derived phosphoproteomics data identified differentially expressed phosphoproteins that were unique and shared between hyperammonemic murine myotubes and skeletal muscle from exercise models. Enriched processes identified in both hyperammonemic myotubes and muscle from exercise models with selected experimental validation included protein kinase A (PKA), calcium signaling, mitogen-activated protein kinase (MAPK) signaling, and protein homeostasis. Our approach of feature extraction from comparative untargeted "omics" data allows for selection of preclinical models that recapitulate specific human exercise responses and potentially optimize functional capacity and skeletal muscle protein homeostasis with exercise in chronic diseases.

Exploiting senescence for the treatment of cancer

Senescence is a cellular response to a variety of stress signals that is characterized by a stable withdrawal from the cell cycle and major changes in cell morphology and physiology. While most research on senescence has been performed on non-cancer cells, it is evident that cancer cells can also mount a senescence response. In this Review, we discuss how senescence can be induced in cancer cells. We describe the distinctive features of senescent cancer cells and how these changes in cellular physiology might be exploited for the selective eradication of these cells (senolysis). We discuss activation of the host immune system as a particularly attractive way to clear senescent cancer cells. Finally, we consider the challenges and opportunities provided by a 'one-two punch' sequential treatment of cancer with pro-senescence therapy followed by senolytic therapy.

Reversed Senescence of Retinal Pigment Epithelial Cell by Coculture With Embryonic Stem Cell via the TGFβ and PI3K Pathways

Retinal pigment epithelium (RPE) cellular senescence is an important etiology of age-related macular degeneration (AMD). Aging interventions based on the application of stem cells to delay cellular senescence have shown good prospects in the treatment of age-related diseases. This study aimed to investigate the potential of the embryonic stem cells (ESCs) to reverse the senescence of RPE cells and to elucidate its regulatory mechanism. The hydrogen peroxide (H₂O₂)-mediated premature and natural passage-mediated replicative senescent RPE cells were directly cocultured with ESCs. The results showed that the proliferative capacity of premature and replicative senescent RPE cells was increased, while the positive rate of senescence-associated galactosidase (SA-β-GAL) staining and levels of reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) were decreased. The positive regulatory factors of cellular senescence (p53, p21^WAF1/CIP1, p16^INK4a) were downregulated, while the negative regulatory factors of cellular senescence (Cyclin A2, Cyclin B1, Cyclin D1) were upregulated. Furthermore, replicative senescent RPE cells entered the S and G₂/M phases from the G₀/G₁ phase. TGFβ (TGFB1, SMAD3, ID1, ID3) and PI3K (PIK3CG, PDK1, PLK1) pathway-related genes were upregulated in premature and replicative senescent RPE cells after ESCs application, respectively. We further treated ESCs-cocultured premature and replicative senescent RPE cells with SB531542 and LY294002 to inhibit the TGFβ and PI3K pathways, respectively, and found that p53, p21^WAF1/CIP1 and p16^INK4a were upregulated, while Cyclin A2, Cyclin B1, Cyclin D1, TGFβ, and PI3K pathway-related genes were downregulated, accompanied by decreased proliferation and cell cycle transition and increased positive rates of SA-β-GAL staining and levels of ROS and MMP. In conclusion, we demonstrated that ESCs can effectively reverse the senescence of premature and replicative senescent RPE cells by a direct coculture way, which may be achieved by upregulating the TGFβ and PI3K pathways, respectively, providing a basis for establishing a new therapeutic option for AMD.

Radiosensitization of Non-Small Cell Lung Cancer Cells by the Plk1 Inhibitor Volasertib Is Dependent on the p53 Status

Polo-like kinase 1 (Plk1), a master regulator of mitotic cell division, is highly expressed in non-small cell lung cancer (NSCLC) making it an interesting drug target. We examined the in vitro therapeutic effects of volasertib, a Plk1 inhibitor, in combination with irradiation in a panel of NSCLC cell lines with different p53 backgrounds. Pretreatment with volasertib efficiently sensitized p53 wild type cells to irradiation. Flow cytometric analysis revealed that significantly more cells were arrested in the G₂/M phase of the cell cycle after the combination therapy compared to either treatment alone (p < 0.005). No significant synergistic induction of apoptotic cell death was observed, but, importantly, significantly more senescent cells were detected when cells were pretreated with volasertib before irradiation compared to both monotherapies alone (p < 0.001), especially in cells with functional p53. Consequently, while most cells with functional p53 showed permanent growth arrest, more p53 knockdown/mutant cells could re-enter the cell cycle, resulting in colony formation and cell survival. Our findings assign functional p53 as a determining factor for the observed radiosensitizing effect of volasertib in combination with radiotherapy for the treatment of NSCLC.

In vitro study of the Polo-like kinase 1 inhibitor volasertib in non-small-cell lung cancer reveals a role for the tumor suppressor p53

Polo-like kinase 1 (Plk1), a master regulator of mitosis and the DNA damage response, is considered to be an intriguing target in the research field of mitotic intervention. The observation that Plk1 is overexpressed in multiple human malignancies, including non-small-cell lung cancer (NSCLC), gave rise to the development of several small-molecule inhibitors. Volasertib, presently the most extensively studied Plk1 inhibitor, has been validated to efficiently reduce tumor growth in preclinical settings. Unfortunately, only modest antitumor activity against solid tumors was reported in clinical trials. This discrepancy prompted research into the identification of predictive biomarkers. In this study, we investigated the therapeutic effect of volasertib monotherapy (i.e., cytotoxicity, cell cycle distribution, apoptotic cell death, cellular senescence, and migration) in a panel of NSCLC cell lines differing in p53 status under both normal and reduced oxygen tension (<0.1% O2 ). A strong growth inhibitory effect was observed in p53 wild-type cells (A549 and A549-NTC), with IC50 values significantly lower than those in p53 knockdown/mutant cells (A549-920 and NCI-H1975) (P < 0.001). While mitotic arrest was significantly greater in cells with nonfunctional p53 (P < 0.005), apoptotic cell death (P < 0.026) and cellular senescence (P < 0.021) were predominantly induced in p53 wild-type cells. Overall, the therapeutic effect of volasertib was reduced under hypoxia (P < 0.050). Remarkably, volasertib inhibited cell migration in all cell lines tested (P < 0.040), with the exception of for the NCI-H1975 p53 mutant cell line. In conclusion, our results show an important difference in the therapeutic effect of Plk1 inhibition in NSCLC cells with versus without functional p53. Overall, the p53 wild-type cell lines were more sensitive to volasertib treatment, suggesting that p53 might be a predictive biomarker for Plk1 inhibition in NSCLC. Moreover, our results pave the way for new combination strategies with Plk1 inhibitors to enhance antitumor activity.

Targeting PLK1 as a novel chemopreventive approach to eradicate preneoplastic mucosal changes in the head and neck

Head and neck squamous cell carcinomas (HNSCC) and local relapses thereof develop in preneoplastic fields in the mucosal linings of the upper aerodigestive tract. These fields are characterized by tumor-associated genetic changes, are frequently dysplastic and occasionally macroscopically visible. Currently, no adequate treatment options exist to prevent tumor development. Array-based screening with a panel of tumor-lethal small interfering RNAs (siRNAs) identified Polo-like kinase 1 (PLK1) as essential for survival of preneoplastic cells. Inhibition of PLK1 caused cell death of preneoplastic and HNSCC cells, while primary cells were hardly affected. Both siRNAs and small molecule inhibitors caused a strong G2/M cell cycle arrest accompanied by formation of monopolar spindles. In a xenografted mouse model PLK1 caused a significant tumor growth delay and cures, while chemoradiation had no effect. Thus, PLK1 seems to be a promising target for chemopreventive treatment of preneoplastic cells, and could be applied to prevent HNSCC and local relapses.

Small molecule inhibition of polo-like kinase 1 by volasertib (BI 6727) causes significant melanoma growth delay and regression in vivo

The objective of this study was to determine the therapeutic potential of polo-like kinase 1 (Plk1) inhibition in melanoma, in vivo. Employing Vectra technology, we assessed the Plk1 expression profile in benign nevi, malignant (stages I-IV) and metastatic melanomas. We found a significant elevation of Plk1 immunostaining in melanoma tissues. Further, a second generation small molecule Plk1 inhibitor, BI 6727, resulted in reductions in growth, viability and clonogenic survival, as well as an increase in apoptosis of A375 and Hs 294T melanoma cells. BI 6727 treatment also resulted in a G2/M-as well as S-phase cell cycle arrest in melanoma cells. Importantly, BI 6727 (intravenous injection; 10 and 25 mg/kg body weight) treatment resulted in significant tumor growth delay and regression in vivo in A375-and Hs 294T-implanted xenografts in athymic nude mice. These anti-melanoma effects were accompanied with a decreased cellular proliferation (Ki-67 staining) and induction of apoptosis (caspase 3 activation). In addition, BI 6727 treatment caused a marked induction of p53 and p21 in vitro as well as in vivo. Overall, we suggest that Plk1 inhibition may be a useful approach as a monotherapy as well as in combination with other existing therapeutics, for melanoma management.

Impact of Polo-like kinase 1 inhibitors on human adipose tissue-derived mesenchymal stem cells

Polo-like kinase 1 (Plk1) has been established as one of the most promising targets for molecular anticancer intervention. In fact, various Plk1 inhibitors have been identified and characterized. While the data derived from the bench are prospective, the clinical outcomes are less encouraging by showing modest efficacy. One of the explanations for this discrepancy could be unintendedly targeting of non-malignant cells by Plk1 inhibitors. In this work, we have addressed the effect of Plk1 inhibition in adipose tissue-derived mesenchymal stem cells (ASCs). We show that both visceral and subcutaneous ASCs display monopolar spindles, reduced viability and strong apoptosis induction upon treatment with BI 2536 and BI 6727, the Plk1 kinase domain inhibitors, and with Poloxin, the regulatory Polo-box domain inhibitor. While Poloxin triggers quickly apoptosis, BI 2536 and BI 6727 result in mitotic arrest in ASCs. Importantly, survived ASCs exhibit DNA damage and a pronounced senescent phenotype. In addition, Plk1 inhibition impairs ASCs' motility and homing ability. These results show that Plk1 inhibitors target slowly proliferating ASCs, an important population of anti-inflammation and immune modulation. The toxic effects on primary cells like ASCs could be partially responsible for the reported moderate antitumor activity in patients treated with Plk1 inhibitors.

Conserved genes and pathways in primary human fibroblast strains undergoing replicative and radiation induced senescence

Background

Cellular senescence is induced either internally, for example by replication exhaustion and cell division, or externally, for example by irradiation. In both cases, cellular damages accumulate which, if not successfully repaired, can result in senescence induction. Recently, we determined the transcriptional changes combined with the transition into replicative senescence in primary human fibroblast strains. Here, by γ-irradiation we induced premature cellular senescence in the fibroblast cell strains (HFF and MRC-5) and determined the corresponding transcriptional changes by high-throughput RNA sequencing.

Results

Comparing the transcriptomes, we found a high degree of similarity in differential gene expression in replicative as well as in irradiation induced senescence for both cell strains suggesting, in each cell strain, a common cellular response to error accumulation. On the functional pathway level, “Cell cycle” was the only pathway commonly down-regulated in replicative and irradiation-induced senescence in both fibroblast strains, confirming the tight link between DNA repair and cell cycle regulation. However, “DNA repair” and “replication” pathways were down-regulated more strongly in fibroblasts undergoing replicative exhaustion. We also retrieved genes and pathways in each of the cell strains specific for irradiation induced senescence.

Conclusion

We found the pathways associated with “DNA repair” and “replication” less stringently regulated in irradiation induced compared to replicative senescence. The strong regulation of these pathways in replicative senescence highlights the importance of replication errors for its induction.

Electronic supplementary material

The online version of this article (doi:10.1186/s40659-016-0095-2) contains supplementary material, which is available to authorized users.

Ionizing Radiation-Induced Endothelial Cell Senescence and Cardiovascular Diseases

Exposure to ionizing radiation induces not only apoptosis but also senescence. While the role of endothelial cell apoptosis in mediating radiation-induced acute tissue injury has been extensively studied, little is known about the role of endothelial cell senescence in the pathogenesis of radiation-induced late effects. Senescent endothelial cells exhibit decreased production of nitric oxide and expression of thrombomodulin, increased expression of adhesion molecules, elevated production of reactive oxygen species and inflammatory cytokines and an inability to proliferate and form capillary-like structures in vitro. These findings suggest that endothelial cell senescence can lead to endothelial dysfunction by dysregulation of vasodilation and hemostasis, induction of oxidative stress and inflammation and inhibition of angiogenesis, which can potentially contribute to radiation-induced late effects such as cardiovascular diseases (CVDs). In this article, we discuss the mechanisms by which radiation induces endothelial cell senescence, the roles of endothelial cell senescence in radiation-induced CVDs and potential strategies to prevent, mitigate and treat radiation-induced CVDs by targeting senescent endothelial cells.

Meeting Report: International Symposium on the Genetics of Aging and Life History II

The second International Symposium on the Genetics of Aging and Life History was held at the campus of Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu, South Korea, from May 14 to 16, 2014. Many leading scientists in the field of aging research from all over the world contributed to the symposium by attending and presenting their recent work and thoughts. The aim of the symposium was to stimulate international collaborations and interactions among scientists who work on the biology of aging. In the symposium, the most recent and exciting work on aging research was presented, covering a wide range of topics, including the genetics of aging, age-associated diseases, and cellular senescence. The work was conducted in various organisms, includingC. elegans, mice, plants, and humans. Topics covered in the symposium stimulated discussion of novel directions for future research on aging. The meeting ended with a commitment for the third International Symposium on the Genetics of Aging and Life History, which will be held in 2016.

Loss of p21Cip1/CDKN1A renders cancer cells susceptible to Polo-like kinase 1 inhibition

The deregulation of Polo-like kinase 1 is inversely linked to the prognosis of patients with diverse human tumors. Targeting Polo-like kinase 1 has been widely considered as one of the most promising strategies for molecular anticancer therapy. While the preclinical results are encouraging, the clinical outcomes are rather less inspiring by showing limited anticancer activity. It is thus of importance to identify molecules and mechanisms responsible for the sensitivity of Polo-like kinase 1 inhibition. We have recently shown that p21Cip1/CDKN1A is involved in the regulation of mitosis and its loss prolongs the mitotic duration accompanied by defects in chromosome segregation and cytokinesis in various tumor cells. In the present study, we demonstrate that p21 affects the efficacy of Polo-like kinase 1 inhibitors, especially Poloxin, a specific inhibitor of the unique Polo-box domain. Intriguingly, upon treatment with Polo-like kinase 1 inhibitors, p21 is increased in the cytoplasm, associated with anti-apoptosis, DNA repair and cell survival. By contrast, deficiency of p21 renders tumor cells more susceptible to Polo-like kinase 1 inhibition by showing a pronounced mitotic arrest, DNA damage and apoptosis. Furthermore, long-term treatment with Plk1 inhibitors induced fiercely the senescent state of tumor cells with functional p21. We suggest that the p21 status may be a useful biomarker for predicting the efficacy of Plk1 inhibition.

PLK1 inhibition down-regulates polycomb group protein BMI1 via modulation of the miR-200c/141 cluster

The polycomb group protein BMI1 is an important regulator of cancer stem cell (CSC) phenotype and is often overexpressed in cancer cells. Its overexpression leads to increase in CSC fraction and therapy resistance in tumors. BMI1 functions via polycomb repressive complex 1 (PRC1)-mediated gene silencing and also via PRC1-independent transcriptional activities. At present, very little is known about the therapy reagents that can efficiently inhibit BMI1 expression, and the CSC phenotype. Here, we report that the polo-like kinase 1 (PLK1) regulates BMI1 expression, and that its inhibition can efficiently down-regulate BMI1 expression and PRC1 activity, and induce premature senescence in breast cancer cells. We also show that the exogenous BMI1 overexpression mitigates anti-oncogenic effects of PLK1 inhibition and overcomes senescence induction by PLK1 inhibitors. We further show that PLK1 inhibition down-regulates BMI1 by upregulating the miRNA-200c/141 cluster, which encodes miR-200c and miR-141, both of which are known to post-transcriptionally downregulate BMI1 expression. Thus, our data suggest that PLK1 inhibitors can be successfully used to inhibit growth of tumors in which PcG protein BMI1 is overexpressed or the PRC1 activity is deregulated.

Pharmacological inhibition of Polo-like kinase 1 (PLK1) by BI-2536 decreases the viability and survival of hamartin and tuberin deficient cells via induction of apoptosis and attenuation of autophagy

The mechanistic target of rapamycin complex 1 (mTORC1) increases translation, cell size and angiogenesis, and inhibits autophagy. mTORC1 is negatively regulated by hamartin and tuberin, the protein products of the tumor suppressors TSC1 and TSC2 that are mutated in Tuberous Sclerosis Complex (TSC) and sporadic Lymphangioleiomyomatosis (LAM). Hamartin interacts with the centrosomal and mitotic kinase polo-like kinase 1 (PLK1). Hamartin and tuberin deficient cells have abnormalities in centrosome duplication, mitotic progression, and cytokinesis, suggesting that the hamartin/tuberin heterodimer and mTORC1 signaling are involved in centrosome biology and mitosis. Here we report that PLK1 protein levels are increased in hamartin and tuberin deficient cells and LAM patient-derived specimens, and that this increase is rapamycin-sensitive. Pharmacological inhibition of PLK1 by the small-molecule inhibitor BI-2536 significantly decreased the viability and clonogenic survival of hamartin and tuberin deficient cells, which was associated with increased apoptosis. BI-2536 increased p62, LC3B-I and GFP-LC3 punctae, and inhibited HBSS-induced degradation of p62, suggesting that PLK1 inhibition attenuates autophagy. Finally, PLK1 inhibition repressed the expression and protein levels of key autophagy genes and proteins and the protein levels of Bcl(-)2 family members, suggesting that PLK1 regulates both autophagic and apoptotic responses. Taken together, our data point toward a previously unrecognized role of PLK1 on the survival of cells with mTORC1 hyperactivation, and the potential use of PLK1 inhibitors as novel therapeutics for tumors with dysregulated mTORC1 signaling, including TSC and LAM.

Plk1 inhibition causes post-mitotic DNA damage and senescence in a range of human tumor cell lines

Plk1 is a checkpoint protein whose role spans all of mitosis and includes DNA repair, and is highly conserved in eukaryotes from yeast to man. Consistent with this wide array of functions for Plk1, the cellular consequences of Plk1 disruption are diverse, spanning delays in mitotic entry, mitotic spindle abnormalities, and transient mitotic arrest leading to mitotic slippage and failures in cytokinesis. In this work, we present the in vitro and in vivo consequences of Plk1 inhibition in cancer cells using potent, selective small-molecule Plk1 inhibitors and Plk1 genetic knock-down approaches. We demonstrate for the first time that cellular senescence is the predominant outcome of Plk1 inhibition in some cancer cell lines, whereas in other cancer cell lines the dominant outcome appears to be apoptosis, as has been reported in the literature. We also demonstrate strong induction of DNA double-strand breaks in all six lines examined (as assayed by γH2AX), which occurs either during mitotic arrest or mitotic-exit, and may be linked to the downstream induction of senescence. Taken together, our findings expand the view of Plk1 inhibition, demonstrating the occurrence of a non-apoptotic outcome in some settings. Our findings are also consistent with the possibility that mitotic arrest observed as a result of Plk1 inhibition is at least partially due to the presence of unrepaired double-strand breaks in mitosis. These novel findings may lead to alternative strategies for the development of novel therapeutic agents targeting Plk1, in the selection of biomarkers, patient populations, combination partners and dosing regimens.

The role of polo-like kinase 1 in carcinogenesis: cause or consequence?

Polo-like kinase 1 (Plk1) is a well-established mitotic regulator with a diverse range of biologic functions continually being identified throughout the cell cycle. Preclinical evidence suggests that the molecular targeting of Plk1 could be an effective therapeutic strategy in a wide range of cancers; however, that success has yet to be translated to the clinical level. The lack of clinical success has raised the question of whether there is a true oncogenic addiction to Plk1 or if its overexpression in tumors is solely an artifact of increased cellular proliferation. In this review, we address the role of Plk1 in carcinogenesis by discussing the cell cycle and DNA damage response with respect to their associations with classic oncogenic and tumor suppressor pathways that contribute to the transcriptional regulation of Plk1. A thorough examination of the available literature suggests that Plk1 activity can be dysregulated through key transformative pathways, including both p53 and pRb. On the basis of the available literature, it may be somewhat premature to draw a definitive conclusion on the role of Plk1 in carcinogenesis. However, evidence supports the notion that oncogene dependence on Plk1 is not a late occurrence in carcinogenesis and it is likely that Plk1 plays an active role in carcinogenic transformation.