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The polo-like kinase Plk4 (also called Sak) is required for late mitotic progression, cell survival and postgastrulation embryonic development. Here we identified a phenotype resulting from Plk4 haploinsufficiency in Plk4 heterozygous cells and mice. Plk4+/− embryonic fibroblasts had increased centrosomal amplification, multipolar spindle formation and aneuploidy compared with wild-type cells. The incidence of spontaneous liver and lung cancers was ∼15 times high in elderly Plk4+/− mice than in Plk4+/+ littermates. Using the in vivo model of partial hepatectomy to induce synchronous cell cycle entry, we determined that the precise regulation of cyclins D1, E and B1 and of Cdk1 was impaired in Plk4+/− regenerating liver, and p53 activation and p21 and BubR1 expression were suppressed. These defects were associated with progressive cell cycle delays, increased spindle irregularities and accelerated hepatocellular carcinogenesis in Plk4+/− mice. Loss of heterozygosity occurs frequently (∼60%) at polymorphic markers adjacent to the PLK4 locus in human hepatoma. Reduced Plk4 gene dosage increases the probability of mitotic errors and cancer development. [ABSTRACT FROM AUTHOR]

TMF/ARA160 is a Golgi resident protein whose cellular functions have not been conclusively revealed. Herein we show that TMF/ARA160 can direct the proteasomal degradation of the key cell growth regulator-Stat3. TMF/ARA160 was dispersed in the cytoplasm of myogenic C2C12 cells that were grown under low-serum conditions. The cytoplasmic distribution of TMF/ARA160 was accompanied by its transient association with the tyrosine kinase Fer and with Stat3, which underwent proteasomal degradation under those conditions. Moreover, serum deprivation induced the association of ubiquitinated proteins, with the TMF/ARA160 complex. However, TMF/ARA160 did not bind Stat1, whose cellular levels were increased in serum-starved C2C12 cells. Amino-acid sequence analysis identified a BC-box element in TMF/ARA160 that mediated the binding of this protein to elongin C. Ectopic expression of TMF/ARA160 in serum-starved C2C12 cells drove the ubiquitination and proteasomal degradation of Stat3, an effect that was not caused by TMF/ARA160 devoid of the BC-box motif. Thus, the Golgi apparatus harbors a novel BC-box-containing protein that can direct Stat3 to proteasomal degradation. Interestingly, the level of TMF/ARA160 was significantly decreased in malignant brain tumors, implying a suppressive role of that protein in tumor progression.Oncogene (2004) 23, 8908-8919. doi:10.1038/sj.onc.1208149 Published online 4 October 2004 [ABSTRACT FROM AUTHOR]

The evolutionarily conserved proteins Par-6, atypical protein kinase C (aPKC), Cdc42 and Par-3 associate to regulate cell polarity and asymmetric cell division, but the downstream targets of this complex are largely unknown. Here we identify direct physiological interactions between mammalian aPKC, murine Par-6C (mPar-6C) and Mlgl, the mammalian orthologue of the Drosophila melanogaster tumour suppressor Lethal (2) giant larvae. In cultured cell lines and in mouse brain, aPKC, mPar-6C and Mlgl form a multiprotein complex in which Mlgl is targeted for phosphorylation on conserved serine residues. These phosphorylation sites are important for embryonic fibroblasts to polarize correctly in response to wounding and may regulate the ability of Mlgl to direct protein trafficking. Our data provide a direct physical and regulatory link between proteins of distinct polarity complexes, identify Mlgl as a functional substrate for aPKC in cell polarization and indicate that aPKC is directed to cell polarity substrates through a network of protein-protein interactions. [ABSTRACT FROM AUTHOR]

The tyrosine phosphatase Shp2 is recruited into tyrosine-kinase signalling pathways through binding of its two amino-terminal SH2 domains to specific phosphotyrosine motifs, concurrent with its re-localization and stimulation of phosphatase activity. Shp2 can potentiate signalling through the MAP-kinase pathway and is required during early mouse development for gastrulation. Chimaeric analysis can identify, by study of phenotypically normal embryos, tissues that tolerate mutant cells (and therefore do not require the mutated gene) or lack mutant cells (and presumably require the mutated gene during their developmental history). We therefore generated chimaeric mouse embryos to explore the cellular requirements for Shp2. This analysis revealed an obligatory role for Shp2 during outgrowth of the limb. Shp2 is specifically required in mesenchyme cells of the progress zone (PZ), directly beneath the distal ectoderm of the limb bud. Comparison of Ptpn11 (encoding Shp2)-mutant and Fgfr1 (encoding fibroblast growth factor receptor-1)-mutant chimaeric limbs indicated that in both cases mutant cells fail to contribute to the PZ of phenotypically normal chimaeras, leading to the hypothesis that a signal transduction pathway, initiated by Fgfr1 and acting through Shp2, is essential within PZ cells. Rather than integrating proliferative signals, Shp2 probably exerts its effects on limb development by influencing cell shape, movement or adhesion. Furthermore, the branchial arches, which also use Fgfs during bud outgrowth, similarly require Shp2. Thus, Shp2 regulates phosphotyrosine-signalling events during the complex ectodermal-mesenchymal interactions that regulate mammalian budding morphogenesis. [ABSTRACT FROM AUTHOR]