Greatwall is a protein kinase involved in the inhibition of protein phosphatase 2 (PP2A)-B55 complexes to maintain the mitotic state. Although its biochemical activity has been deeply characterized in Xenopus, its specific relevance during the progression of mitosis is not fully understood. By using a conditional knockout of the mouse ortholog, Mastl, we show here that mammalian Greatwall is essential for mouse embryonic development and cell cycle progression. Yet, Greatwall-null cells enter into mitosis with normal kinetics. However, these cells display mitotic collapse after nuclear envelope breakdown (NEB) characterized by defective chromosome condensation and prometaphase arrest. Intriguingly, Greatwall is exported from the nucleus to the cytoplasm in a CRM1-dependent manner before NEB. This export occurs after the nuclear import of cyclin B–Cdk1 complexes, requires the kinase activity of Greatwall, and is mediated by Cdk-, but not Polo-like kinase 1-dependent phosphorylation. The mitotic collapse observed in Greatwall-deficient cells is partially rescued after concomitant depletion of B55 regulatory subunits, which are mostly cytoplasmic before NEB. These data suggest that Greatwall is an essential protein in mammals required to prevent mitotic collapse after NEB.

A link between lipid metabolism and epithelial-mesenchymal transition provides a target for colon cancer therapy

Heterodimers of the retinoid X receptor (RXR) with the thyroid hormone receptor (TR) are considered to be nonpermissive. It is believed that within these complexes RXR acts as a “silent partner.” We demonstrate here that a permissive heterodimer mediates stimulation of prolactin expression by the thyroid hormone T3 and by 9-cis retinoic acid (9-cis-RA). A response element located in the prolactin distal enhancer mediates transactivation by both ligands in pituitary cells, and RXR recruits coactivators when bound to this element as a heterodimer with TR. Furthermore, transcription by the RXR agonist can be obtained in CV-1 cells only after overexpression of coactivators, and overexpression of corepressors inhibits the response in pituitary cells. Thus, cell type-specific differences in coregulator recruitment can determine the cellular response to both ligands. Coactivator recruitment by 9-cis-RA requires the ligand-dependent transactivation domains (AF-2) of both heterodimeric partners. Interestingly, the presence of the RXR ligand can overcome the deleterious effect of the AF-2 mutation E401Q on association with coactivators and transactivation. These results demonstrate an unexpected role for RXR in TR signaling and show that in particular cellular environments this receptor can act as a “nonsilent” partner of TR, allowing stimulation by RXR agonists.

Abnormal acyl-CoA synthetase/ stearoyl-CoA desaturase (ACSL/SCD) lipid network fuels colon cancer progression endowing cells with invasive and migratory properties. Therapies against this metabolic network may be useful to improve clinical outcomes. Since miRNAs (miRNAs/miR) are important epigenetic regulators, we investigated novel miRNAs targeting this pro-tumorigenic axis; and hence to be used both as therapeutic or prognostic miRNAs. Thirty-one putative common miRNAs were predicted to target simultaneously the three enzymes comprising ACSL/SCD network. Target validation by RT-QPCR, Western blotting and luciferase assays showed miR-544a, miR-142 and miR-19b-1 as major regulators of the metabolic axis ACSL/SCD. Importantly, lower miR-19b-1 expression associates with a decreased survival rate in CRC patients, accordingly with ACSL/SCD involvement in patient relapse. Finally, miR-19b-1 regulates the pro-tumorigenic axis ACSL/SCD being able to inhibit invasion in colon cancer cells. Since its expression correlates with an increased survival rate in CRC patients, we propose miR-19b-1 as a potential noninvasive biomarker of disease-free survival (DFS) and a promising therapeutic miRNA in CRC.