A novel role for Plk4 in regulating cell spreading and motility

Polo family kinase 4 (Plk4) is required for mitotic progression, and is haploinsufficient for tumor suppression and timely hepatocyte polarization in regenerating liver. At the same time, recent evidence suggests that Plk4 expression may have a role in clinical cancer progression, although the mechanisms are not clear. Here we identify a gene expression pattern predictive of reduced motility in Plk4(+/-) murine embryonic fibroblasts (MEFs) and validate this prediction with functional assays of cell spreading, migration and invasion. Increased Plk4 expression enhances cell spreading in Plk4(+/-) MEFs and migration in human embryonic kidney 293T cells, and increases invasion by DLD-1 colon cancer cells. Plk4 depletion impairs invasion of wild-type MEFs and suppresses invasion by MDA-MB231 breast cancer cells. Cytoskeletal reorganization and development of polarity are impaired in Plk4-deficient cells that have been stimulated to migrate. Endogenous Plk4 phosphorylated at the autophosphorylation site S305 localizes to the protrusions of motile cells, coincident with the RhoA GEF Ect2, GTP-bound RhoA and the RhoA effector mDia. Taken together, our findings reveal an unexpected activity of Plk4 that promotes cell migration and may underlie an association between increased Plk4 expression, cancer progression and death from metastasis in solid tumor patients.

Abstract 2331: Role of MMP expression in the effect of Plk4 on cell migration and invasion

Plk4 is a haploinsufficient tumor suppressor in mice (Ko et al, Nature Genetics, 2005). Plk4+/− mouse embryonic fibroblasts (MEFs) spontaneously immortalize in culture and become tumorigenic by passage 15 (Rosario et al, PNAS, 2010). We investigated the secondary genetic alterations associated with tumorigenicity.

Early passage (P3) Plk4+/+, P3 Plk4+/− and late passage (P15) Plk4+/− MEFs were compared by genome-wide expression array (Illumina). When differentially expressed genes were organized by biological function, increased cell proliferation and death (p=2.13E10-1.31E02, p = 2.52E07-1.14E02 respectively), and decreased cell motility (p = 3.74E07-1.27E02) in the P15 MEFs were predicted. An independent array analysis comparing tumorigenic (T) to nontumorigenic (NT) Plk4+/− MEFs showed a pattern of altered gene expression predictive of decreased motility in the T MEFs. These results suggested the hypothesis that Plk4 modulates cell motility. To directly test this hypothesis in functional assays we measured cell spreading, scratch-wound healing and transwell migration through Matrigel; in all these assays P3 Plk4+/− were inferior to P3 Plk4+/+ MEFs. We hypothesized that altered MMP expression was one mechanism for the difference observed in migration, and tested this by comparing MMP-13,-10 and -3 expression as shown in the Table. Plk4 status was associated with altered expression of MMPs; in particular, MMP-13 expression was confirmed to be significantly reduced in P3 Plk4+/− compared to P3 Plk4+/+ MEFs by RT-qPCR.

Nucleolar release of Hand1 acts as a molecular switch to determine cell fate

The bHLH transcription factor Hand1 is essential for placentation and cardiac morphogenesis in the developing embryo. Here we implicate Hand1 as a molecular switch that determines whether a trophoblast stem cell continues to proliferate or commits to differentiation. We identify a novel interaction of Hand1 with a protein that contains an I-mfa (inhibitor of myogenic factor) domain that anchors Hand1 in the nucleolus where it negatively regulates Hand1 activity. In the trophoblast stem-cell line Rcho-1, nucleolar sequestration of Hand1 accompanies sustained cell proliferation and renewal, whereas release of Hand1 into the nucleus leads to its activation, thus committing cells to a differentiated giant-cell fate. Site-specific phosphorylation is required for nucleolar release of Hand1, for its dimerization and biological function, and this is mediated by the non-canonical polo-like kinase Plk4 (Sak). Sak is co-expressed in Rcho-1 cells, localizes to the nucleolus during G2 and phosphorylates Hand1 as a requirement for trophoblast stem-cell commitment to a giant-cell fate. This study defines a novel cellular mechanism for regulating Hand1 that is a crucial step in the stem-cell differentiation pathway.

Plk4 haploinsufficiency causes mitotic infidelity and carcinogenesis

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 approximately 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 (approximately 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.