Polo-like kinase (Plk)1 depletion induces apoptosis in cancer cells

Abstract

Elevated expression of mammalian polo-like kinase (Plk)1 occurs in many different types of cancers, and Plk1 has been proposed as a novel diagnostic marker for several tumors. We used the recently developed vector-based small interfering RNA technique to specifically deplete Plk1 in cancer cells. We found that Plk1 depletion dramatically inhibited cell proliferation, decreased viability, and resulted in cell-cycle arrest with 4 N DNA content. The formation of dumbbell-like chromatin structure suggests the inability of these cells to completely separate the sister chromatids at the onset of anaphase. Plk1 depletion induced apoptosis, as indicated by the appearance of subgenomic DNA in fluorescence-activated cellsorter (FACS) profiles, the activation of caspase 3, and the formation of fragmented nuclei. Plk1-depletion-induced apoptosis was partially reversed by cotransfection of nondegradable mouse Plk1 constructs. In addition, the p53 pathway was shown to be involved in Plk1-depletion-induced apoptosis. DNA damage occurred in Plk1-depleted cells and inhibition of ATM strongly potentiated the lethality of Plk1 depletion. Although p53 is stabilized in Plk1-depleted cells, DNA damage also occurs in p53 ؊/؊ cells. These data support the notion that disruption of Plk1 function could be an important application in cancer therapy. T he polo kinase family includes mammalian polo-like kinase (Plk)1, Snk, Fnk, Xenopus laevis Plx1, Drosophila polo, fission yeast Plo1, and budding yeast Cdc5 (1). Genetic and biochemical experiments in various organisms indicate that polo-like kinases are important regulators of many cell-cycle-related events, including activation of Cdc2, chromosome segregation, centrosome maturation, bipolar spindle formation, regulation of anaphase-promoting complex, and execution of cytokinesis (1, 2).To investigate the functions of Plk1 in mammalian cells, we previously directly transfected 21-nucleotide double-stranded RNA into the cells to deplete Plk1 (3). We found that Plk1 depletion results in elevated Cdc2 protein kinase activity and thus attenuates cell-cycle progression. About 45% of cells treated with Plk1 small interfering RNA (siRNA) show the formation of a dumbbell-like DNA organization, suggesting that sister chromatids are not completely separated. About 15% of these cells do complete anaphase but do not complete cytokinesis. Finally, Plk1 depletion significantly reduces centrosome amplification in hydroxyurea-treated U2OS cells (3).A close correlation between mammalian Plk1 expression and carcinogenesis was recently documented. Mammalian Plk1 was found to be overexpressed in various human tumors, including head and neck squamous cell carcinomas, oropharyngeal carcinomas, non-small cell lung cancer, melanomas, and ovarian and endometrial carcinomas (4). It was proposed that Plk1 could be used as a novel diagnostic marker for several types of cancers (4-6). Furthermore, constitutive expression of Plk1 in NIH 3T3 cells causes oncogenic focus formation and induces tumor growth in nude mice (7)...

Effect of rosemary polyphenols on human colon cancer cells: transcriptomic profiling and functional enrichment analysis

“…Cancer cells with depleted PLK1 activity show inability to completely separate the sister chromatids during mitosis. It has been shown that PLK1-depleted cancer cells eventually undergo induction of G2/M arrest and apoptosis (Liu and Erikson 2003). Coincident with downregulation of PLK1, several members involved in the mitotic checkpoint function remained also down-regulated in treated cells.…”

“…Owing to the strong association of this kinase with cell proliferation and elevated expression in a variety of tumors, including colon tumors, inhibition of PLK1 function has been suggested as a potential alternative for cancer therapy (Chopra et al 2010). Moreover, depletion of PLK1 is associated with the decrease in cell viability and induction of apoptosis in various cancerous cells (Liu and Erikson 2003). Cancer cells with depleted PLK1 activity show inability to completely separate the sister chromatids during mitosis.…”

Polo-like kinase isoform expression is a prognostic factor in ovarian carcinoma

“…While early works in this field concentrated mainly on the identification of Polo homologues in different species and on the role of this enzyme family in normal tissue development and mitosis, the focus of interest has changed towards the functional and prognostic role of PLK isoenzymes in human malignancies. The central role of PLK isoenzymes in tumorigenesis has been emphasised lately by studies showing that PLK1 inhibition, either by antisense or siRNA, leads to dramatic antiproliferative effects on tumour cells in vitro (Spänkuch-Schmitt et al, 2002a, b;Elez et al, 2003;Liu and Erikson, 2003), pointing at a potential therapeutic use of inhibitory strategies targeting PLK isoenzymes. In-depth expression analysis of members of the PLK family in various cancers and correlation with other tumour characteristics may thus provide the translational basis for such approaches in clinical practice.…”

Plk1 depletion in nontransformed diploid cells activates the DNA-damage checkpoint

“…Thus, Plk1 has been proposed as a new prognostic marker for many types of malignancies (Eckerdt et al, 2005;Takai et al, 2005). Moreover, inhibition of Plk1 with various techniques, such as RNA interference (RNAi) and small-molecule inhibitor, results in growth arrest or apoptosis in cancer cells and inhibits xenograft tumor growth in nude mice (Liu and Erikson, 2003;Spankuch et al, 2004;Gumireddy et al, 2005). Therefore, Plk1 is an intriguing target for therapeutic intervention against cancer (Strebhardt and Ullrich, 2006).…”