Structural analysis of the polo-box domain of human Polo-like kinase 2

Specific inhibition of an anticancer target, polo-like kinase 1, by allosterically dismantling its mechanism of substrate recognition

Polo-like kinase 1 (Plk1) is considered an attractive target for anticancer therapy. Over the years, studies on the noncatalytic polo-box domain (PBD) of Plk1 have raised the expectation of generating highly specific protein-protein interaction inhibitors. However, the molecular nature of the canonical PBD-dependent interaction, which requires extensive water network-mediated interactions with its phospholigands, has hampered efforts to identify small molecules suitable for Plk1 PBD drug discovery. Here, we report the identification of the first allosteric inhibitor of Plk1 PBD, called Allopole, a prodrug that can disrupt intracellular interactions between PBD and its cognate phospholigands, delocalize Plk1 from centrosomes and kinetochores, and induce mitotic block and cancer cell killing. At the structural level, its unmasked active form, Allopole-A, bound to a deep Trp-Phe-lined pocket occluded by a latch-like loop, whose adjoining region was required for securely retaining a ligand anchored to the phospho-binding cleft. Allopole-A binding completely dislodged the L2 loop, an event that appeared sufficient to trigger the dissociation of a phospholigand and inhibit PBD-dependent Plk1 function during mitosis. Given Allopole's high specificity and antiproliferative potency, this study is expected to open an unexplored avenue for developing Plk1 PBD-specific anticancer therapeutic agents.

Plk2-mediated phosphorylation and translocalization of Nrf2 activates anti-inflammation through p53/Plk2/p21cip1 signaling in acute kidney injury

The Plk2 is a cellular stress-responsive factor that is induced in response to oxidative stress. However, the roles of Plk2 in acute kidney injury (AKI) have not been clarified. We previously found that Plk2 is an interacting factor of Nrf2 in response to cellular stress, since Plk2 is upregulated in the Nrf2-dependent network. Here, we show that the levels of p53, Plk2, p21cip1, and chromatin-bound Nrf2 were all upregulated in kidney tissues of mice or NRK52E cells treated with either cisplatin or methotrexate. Upregulation of Plk2 by p53 led to an increase of Nrf2 in both soluble and chromatin fractions in cisplatin-treated NRK52E cells. Consistently, depletion of Plk2 suppressed the levels of Nrf2. Of note, Plk2 directly phosphorylated Nrf2 at Ser40, which facilitated its interaction with p21cip1 and translocation into the nuclei for the activation of anti-oxidative and anti-inflammatory factors in response to AKI. Together, these findings suggest that Plk2 may serve as an anti-oxidative and anti-inflammatory regulator through the phosphorylation and activation of Nrf2 to protect kidney cells from kidney toxicants and that Plk2 and Nrf2 therefore work cooperatively for the protection and survival of kidney cells from harmful stresses.

Polo-Like Kinase 2: From Principle to Practice

Polo-like kinase (PLK) 2 is an evolutionarily conserved serine/threonine kinase that shares the n-terminal kinase catalytic domain and the C-terminal Polo Box Domain (PBD) with other members of the PLKs family. In the last two decades, mounting studies have focused on this and tried to clarify its role in many aspects. PLK2 is essential for mitotic centriole replication and meiotic chromatin pairing, synapsis, and crossing-over in the cell cycle; Loss of PLK2 function results in cell cycle disorders and developmental retardation. PLK2 is also involved in regulating cell differentiation and maintaining neural homeostasis. In the process of various stimuli-induced stress, including oxidative and endoplasmic reticulum, PLK2 may promote survival or apoptosis depending on the intensity of stimulation and the degree of cell damage. However, the role of PLK2 in immunity to viral infection has been studied far less than that of other family members. Because PLK2 is extensively and deeply involved in normal physiological functions and pathophysiological mechanisms of cells, its role in diseases is increasingly being paid attention to. The effect of PLK2 in inhibiting hematological tumors and fibrotic diseases, as well as participating in neurodegenerative diseases, has been gradually recognized. However, the research results in solid organ tumors show contradictory results. In addition, preliminary studies using PLK2 as a disease predictor and therapeutic target have yielded some exciting and promising results. More research will help people better understand PLK2 from principle to practice.

Plk2 Regulated by miR-128 Induces Ischemia-Reperfusion Injury in Cardiac Cells

Ischemia-reperfusion (I/R) injury occurs during cardiac surgery and is the major factor leading to heart dysfunction and heart failure. Our previous study showed that gene and microRNA expression profiles are altered in heart grafts with extended I/R injury. In this study, we, for the first time, demonstrated that I/R injury upregulates the expression of Polo-like kinase 2 (Plk2) but decreases miR-128 expression in heart cells both in vitro and in vivo. Silencing Plk2 using small interfering RNA (siRNA) protects cells from Antimycin A-induced cell apoptosis/death. Silencing Plk2 also decreases phosphorylated p65 expression but increases Angiopoietin 1 expression. In addition, Plk2 is negatively regulated by miR-128. miR-128 exerts a protective effect on cell apoptosis similar to Plk2 siRNA in response to I/R stress. Methylation inhibitor 5-azacytidine (5-AZ) increases the expression of miR-128 and subsequently reduces Plk2 expression and cell apoptosis. In conclusion, this study demonstrated that Plk2 regulated by miR-128 induces cell apoptosis/death in response to I/R stress through activation of the nuclear factor κB (NF-κB) signal pathway. miR-128 and Plk2 are new targets for preventing cardiac I/R injury or oxidative stress-mediated injury.

A selective inhibitor of the Polo-box domain of Polo-like kinase 1 identified by virtual screening

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We were able to identify a novel anticancer molecule that targets PLK1.

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The compound shows good activity in leukaemia-sensitive and -resistant cells.

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Compound (1) induces mitotic arrest and interferes with normal spindle formation.

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We showed the importance of virtual screening for facilitating drug discovery.

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Compound (1) is a good scaffold for further chemical development.

Polo-like kinase 1 (PLK1), a member of the Polo-like kinase family, plays an important regulatory role in mitosis and cell cycle progression. PLK1 overexpression is correlated with tumourigenesis and poor prognosis in cancer patients. Therefore, the identification of novel compounds that inhibit PLK1 would provide attractive therapeutic approaches. Although some PLK1 kinase inhibitors have been developed, their application has been limited by off-target effects. PLK1 contains a regulatory domain named the Polo-box domain (PBD), which is characteristic only for the Polo-like kinase family. This domain represents an alternative therapeutic target with higher selectivity for PLK1. In this study, we applied in silico virtual drug screening, fluorescence polarization and microscale thermophoresis to identify new scaffolds targeting the PBD of PLK1. One compound, 3-{[(1R,9S)-3-(naphthalen-2-yl)-6-oxo-7,11-diazatricyclo[7.3.1.0²,⁷]trideca-2,4-dien-11-yl]methyl}benzonitrile (designated compound (1)), out of a total of 30,793 natural product derivatives, inhibited the PLK1 PBD with high selectivity (IC50: 17.9 ± 0.5 µM). This compound inhibited the growth of cultured leukaemia cells (CCRF-CEM and CEM/ADR5000) and arrested the cell cycle in the G2/M phase, which is characteristic for PLK1 inhibitors. Immunofluorescence analyses showed that treatment with compound (1) disrupted spindle formation due to the aberrant localization of PLK1 during the mitotic process, leading to G2/M arrest and ultimately cell death. In conclusion, compound (1) is a selective PLK1 inhibitor that inhibits cancer cell growth. It represents a chemical scaffold for the future synthesis of new selective PLK1 inhibitors for cancer therapy.

Plk2 Loss Commonly Occurs in Colorectal Carcinomas but not Adenomas: Relationship to mTOR Signaling

Plk2 is a target of p53. Our previous studies demonstrated that with wild-type p53, Plk2 impacts mTOR signaling in the same manner as TSC1, and Plk2-deficient tumors grew larger than control. Other investigators have demonstrated that Plk2 phosphorylates mutant p53 in a positive feedback loop. We investigated Plk2’s tumor suppressor functions in relationship to mTOR signaling. Archival specimens from 12 colorectal adenocarcinomas were stained for markers including Plk2, phosphorylated mTOR (serine 2448) and ribosomal S6 (Serine 235/236). We show that Plk2 is expressed in normal colon, with a punctate staining pattern in supranuclear cytoplasm. In colorectal adenocarcinoma, Plk2 demonstrates complete or partial loss of expression. Strong expression of phosphorylated mTOR is observed in the invasive front. Phosphorylated S6 expression partially correlates with phosphorylated mTOR expression but appears more diffuse in some cases. p53 and Ki67 expression is diffuse, in the subset of cases examined. In order to determine whether Plk2 is lost prior to the development of invasive cancer, 8 colon polyps from 6 patients were evaluated for Plk2 expression. All polyps are positive for Plk2. A Cancer Genome Atlas search identified Plk2 mutations to be infrequent in colorectal adenocarcinomas. Neither Plk2 methylation (in the gene body) nor copy number variations correlated with changes in mRNA expression levels. Loss of Plk2 expression along with accentuated expression of phosphorylated mTOR and phosphorylated S6 at the invasive front in some colorectal carcinomas is consistent with previous findings that an interaction between Plk2 and TSC1 / mTOR signaling molecules plays a role in tumor suppression. Plk2 protein expression is lost at the same stage in colorectal carcinogenesis as p53. The p53 dependence of Plk2 loss and tumor suppressor function in relationship to mTOR signaling may have therapeutic implications.

Enhancing polo-like kinase 1 selectivity of polo-box domain-binding peptides

An important goal in the development of polo-like kinase 1 (Plk1) polo-box domain (PBD) binding inhibitors is selectivity for Plk1 relative to Plk2 and Plk3. In our current work we show that Plk1 PBD selectivity can be significantly enhanced by modulating interactions within a previously discovered "cryptic pocket" and a more recently identified proximal "auxiliary pocket."

Recent Advances and New Strategies in Targeting Plk1 for Anticancer Therapy

Polo-like kinase 1 (Plk1) plays key roles in regulating mitotic processes that are crucial for cellular proliferation. Overexpression of Plk1 is tightly associated with the development of particular cancers in humans, and a large body of evidence suggests that Plk1 is an attractive target for anticancer therapeutic development. Drugs targeting Plk1 can potentially be directed at two distinct sites: the N-terminal catalytic kinase domain (KD), which phosphorylates substrates, and the C-terminal polo-box domain (PBD) which is essential for protein-protein interactions. In this review we summarize recent advances and new challenges in the development of Plk1 inhibitors targeting these two domains. We also discuss novel strategies for designing and developing next-generation inhibitors to effectively treat Plk1-associated human disorders.