Mammalian Polo-like kinase 1 (Plk1) promotes proper chromosome segregation by phosphorylating and delocalizing the PBIP1·CENP-Q complex from kinetochores.

Mammalian Plk1 is critically required for proper M phase progression. Plk1 is self-recruited to prekinetochores/kinetochores by phosphorylating and binding to the Thr-78 motif of a kinetochore scaffold protein, PBIP1 (also called CENP-U/50), which forms a stable complex with another kinetochore component, CENP-Q. However, the mechanism regulating Plk1 localization to this site remains largely unknown. Here, we demonstrate that the PBIP1·CENP-Q complex became hyperphosphorylated and rapidly delocalized from kinetochores as cells entered mitosis. Plk1 phosphorylated the CENP-Q subunit of the PBIP1·CENP-Q complex at multiple sites, and mutation of nine Plk1-dependent phosphorylation sites to Ala (9A) enhanced CENP-Q association with chromatin and prolonged CENP-Q localization to kinetochores. Conversely, mutation of the nine sites to phospho-mimicking Asp/Glu (9D/E) residues dissociated CENP-Q from chromatin and kept the CENP-Q(9D/E) mutant from localizing to interphase prekinetochores. Strikingly, both the 9A and 9D/E mutants induced a defect in proper chromosome segregation, suggesting that both timely localization of the PBIP1·CENP-Q complex to prekinetochores and delocalization from kinetochores are critical for normal M phase progression. Notably, although Plk1 did not alter the level of PBIP1 and CENP-Q ubiquitination, Plk1-dependent phosphorylation and delocalization of these proteins from kinetochores appeared to indirectly lead to their degradation in the cytosol. Thus, we propose that Plk1 regulates the timing of the delocalization and ultimate destruction of the PBIP1·CENP-Q complex and that these processes are important not only for promoting Plk1-dependent mitotic progression, but also for resetting the timing of Plk1 recruitment to prekinetochores in the next cell cycle.

Mechanisms underlying Plk1 polo-box domain-mediated biological processes and their physiological significance.

Mammalian polo-like kinase 1 (Plk1) has been studied intensively as a key regulator of various cell cycle events that are critical for proper M-phase progression. The polobox domain (PBD) present in Plk1's C-terminal noncatalytic region has been shown to play a central role in targeting the N-terminal kinase domain of Plk1 to specific subcellular locations. Subsequent studies reveal that PBD binds to a phosphorylated motif generated by one of the two mechanisms-self-priming by Plk1 itself or non-selfpriming by a Pro-directed kinase, such as Cdc2. Here, we comparatively review the differences in the biochemical steps of these mechanisms and discuss their physiological significance. Considering the diverse functions of Plk1 during the cell cycle, a better understanding of how the catalytic activity of Plk1 functions in concert with its cisacting PBD and how this coordinated process is intricately regulated to promote Plk1 functions will be important for providing new insights into different mechanisms underlying various Plk1-mediated biological events that occur at the multiple stages of the cell cycle.

Lysyl oxidase G473A polymorphism is closely associated with susceptibility to gastric cancer in a South Korean population.

The aim of this study was to determine whether a single nucleotide polymorphism at G473A (rs1800449) within the LOX-propeptide is associated with susceptibility to gastric cancer. We investigated the genotype and allele frequencies of this gene in tissue specimens from 458 gastric cancer patients and 282 healthy individuals. Polymorphism analysis was performed by amplifying the propeptide region of LOX and digestion with NotI followed by sequencing of the products. The frequencies of the LOX G473A G/G, G/A, and A/A genotypes were 54.4% (249/458), 34.3% (157/458), and 11.3% (52/458), respectively, in gastric cancer patients and 58.9% (166/300), 35.5% (100/282), and 5.7% (16/282), respectively, in the healthy controls. Statistically significant differences in the genotype and allele frequency of LOX rs1800449 were observed between the healthy controls and gastric cancer patients (p = 0.0294 and p = 0.0339). When the data were stratified according to gastric cancer histologic subtype, the risk of diffuse-type gastric cancer in carriers with an A allele (G/A or A/A genotypes) was statistically higher compared to that of carriers with the G/G genotype (p = 0.0001). Our findings suggest that G473A polymorphism of the LOX gene may be closely associated with susceptibility to the development and differentiation of gastric cancer in South Korean patients.

Gastrokine 1 functions as a tumor suppressor by inhibition of epithelial-mesenchymal transition in gastric cancers.

PURPOSE: Gastrokine 1 (GKN1) plays an important role in the gastric mucosal defense mechanism and also acts as a functional gastric tumor suppressor. The specific aim of this study was to determine the molecular mechanisms underlying GKN1 tumor suppressor activity in the progression of gastric cancers. METHODS: We examined the effect of GKN1 on epithelial-mesenchymal transition (EMT) and cell migration in GKN1-transfected and recombinant GKN1-treated AGS gastric cancer cells using in vitro wound healing, microchemotaxis, and invasion assays. RESULTS: In GKN1-transfected AGS cells, we observed inhibition of cell migration and invasion in wound healing, transwell and Matrigel assay. Also, GKN1-transfected and recombinant GKN1-treated AGS cells showed decreased levels of ROS and expression of phosphatidylinositol 3-kinase (PI3K)/Akt pathway proteins, concomitant with re-expression of E-cadherin and decreased expression of cytoplasmic and nuclear expression of ß-catenin, slug, snail, fibronectin, and vimentin. CONCLUSIONS: These data suggest that the GKN1 gene may play an important role in the progression of sporadic gastric cancers via inhibition of EMT and cancer cell migration.

Inactivation of the Gastrokine 1 gene in gastric adenomas and carcinomas.

Gastrokine 1 (GKN1) plays a role in the gastric mucosal defence mechanism and may be a gastric tumour suppressor. We have investigated whether inactivation of the GKN1 gene is involved in the development and/or progression of gastric cancers. GKN1 protein expression was examined in gastric adenomas and cancer and we also analysed GKN1 mutation and epigenetic alteration, DNA copy number change and mRNA transcript expression. The effect of GKN1 on cell proliferation and death was examined in wild-type GKN1-transfected AGS gastric cancer cells. Reduced or loss of GKN1 expression was detected in 36 (90%) and 170 (89.5%) of 40 adenomas and 190 gastric cancers, respectively. Statistically, there was no significant relationship between altered expression of GKN1 protein and clinicopathological parameters, including depth of invasion, location and lymph node metastasis (χ(2) test, p > 0.05). In western blot analysis, absence or reduced expression was found in 21 (84.0%) of 25 gastric carcinomas. No mutation was detected in gastric tumours, and hypermethylation of GKN1 gene was found in two tumours. DNA copy number and mRNA transcript of GKN1 were significantly decreased in gastric cancers. In functional analysis, AGS gastric cancer cells transfected with GKN1 wild-type showed marked inhibition of cell proliferation and induction of cell death. These data suggest that inactivation of the GKN1 gene may play an important role in the development of sporadic gastric cancers, as an early event.

Requirement for the budding yeast polo kinase Cdc5 in proper microtubule growth and dynamics.

In many organisms, polo kinases appear to play multiple roles during M-phase progression. To provide new insights into the function of the budding yeast polo kinase Cdc5, we generated novel temperature-sensitive cdc5 mutants by mutagenizing the C-terminal noncatalytic polo box domain, a region that is critical for proper subcellular localization. One of these mutants, cdc5-11, exhibited a temperature-sensitive growth defect with an abnormal spindle morphology. Strikingly, provision of a moderate level of benomyl, a microtubule-depolymerizing drug, permitted cdc5-11 cells to grow significantly better than the isogenic CDC5 wild type in a FEAR (cdc Fourteen Early Anaphase Release)-independent manner. In addition, cdc5-11 required MAD2 for both cell growth and the benomyl-remedial phenotype. These results suggest that cdc5-11 is defective in proper spindle function. Consistent with this view, cdc5-11 exhibited abnormal spindle morphology, shorter spindle length, and delayed microtubule regrowth at the nonpermissive temperature. Overexpression of CDC5 moderately rescued the spc98-2 growth defect. Interestingly, both Cdc28 and Cdc5 were required for the proper modification of the spindle pole body components Nud1, Slk19, and Stu2 in vivo. They also phosphorylated these three proteins in vitro. Taken together, these observations suggest that concerted action of Cdc28 and Cdc5 on Nud1, Slk19, and Stu2 is important for proper spindle functions.

Requirement of hCenexin for proper mitotic functions of polo-like kinase 1 at the centrosomes.

Outer dense fiber 2 (Odf2) was initially identified as a major component of sperm tail cytoskeleton and later was suggested to be a widespread component of centrosomal scaffold that preferentially associates with the appendages of the mother centrioles in somatic cells. Here we report the identification of two Odf2-related centrosomal components, hCenexin1 and hCenexin1 variant 1, that possess a unique C-terminal extension. Our results showed that hCenexin1 is the major isoform expressed in HeLa cells, whereas hOdf2 is not detectably expressed. Mammalian polo-like kinase 1 (Plk1) is critical for proper mitotic progression, and its association with the centrosome is important for microtubule nucleation and function. Interestingly, depletion of hCenexin1 by RNA interference (RNAi) delocalized Plk1 from the centrosomes and the C-terminal extension of hCenexin1 was crucial to recruit Plk1 to the centrosomes through a direct interaction with the polo-box domain of Plk1. Consistent with these findings, the hCenexin1 RNAi cells exhibited weakened gamma-tubulin localization and chromosome segregation defects. We propose that hCenexin1 is a critical centrosomal component whose C-terminal extension is required for proper recruitment of Plk1 and other components crucial for normal mitosis. Our results further suggest that the anti-Odf2 immunoreactive centrosomal antigen previously detected in non-germ line cells is likely hCenexin1.