The phosphatase Pgam5 antagonizes Wnt/ß-Catenin signaling in embryonic anterior-posterior axis patterning.

The scaffold protein Dishevelled is a central intracellular component of Wnt signaling pathways. Various kinases have been described that regulate and modulate Wnt signaling through phosphorylation of Dishevelled. However, besides general protein phosphatases 1 and 2 (PP1 and PP2), no specific protein phosphatases have been identified. Here, we report on the identification and functional characterization of the protein phosphatase Pgam5 in vitro and in vivo in Xenopus Pgam5 is a novel antagonist of Wnt/ß-Catenin signaling in human cells and Xenopus embryogenesis. In early development, Pgam5 is essential for head formation, and for establishing and maintaining the Wnt/ß-Catenin signaling gradient that patterns the anterior-posterior body axis. Inhibition of Wnt/ß-Catenin signaling and developmental function depend on Pgam5 phosphatase activity. We show that Pgam5 interacts with Dishevelled2 and that Dishevelled2 is a substrate of Pgam5. Pgam5 mediates a marked decrease in Dishevelled2 phosphorylation in the cytoplasm and in the nucleus, as well as decreased interaction between Dishevelled2, Tcf1 and ß-Catenin, indicating that Pgam5 regulates Dishevelled function upstream and downstream of ß-Catenin stabilization.

Drug-Drug Combinations Can Enhance Toxicity as Shown by Monocyte-Derived Hepatocyte-like Cells From Patients With Idiosyncratic Drug-Induced Liver Injury.

Drug-induced liver injury (DILI) is a major cause for acute liver failure and regulatory actions on novel drugs. Individual patient characteristics are the main determinant of idiosyncratic DILI, making idiosyncratic DILI (iDILI) one of the most challenging diagnoses in hepatology. Individual drug-drug interactions might play a role in iDILI. However, the current approaches to iDILI diagnosis are focused on single drugs as causative agents. For the present analysis, 48 patients with acute liver injury who took 2 drugs and who were diagnosed as iDILI were investigated. A novel in vitro test was employed using monocyte-derived hepatocyte-like cells (MH cells) generated from these patients. iDILI diagnosis and causality were evaluated using clinical causality assessment supported by Roussel-Uclaf Causality Assessment Method. In 13 of these 48 patients (27%), combinations of drugs increased toxicity in the MH test when compared with the single drugs. Interestingly, whereas in 24 cases (50%) drug-drug combinations did not enhance toxicity, in 11 cases (23%) only the combinations caused toxicity. The incidence of severe cases fulfilling Hy's law was higher in patients with positive interactions (57% vs 43%; p = .04), with acute liver failure occurring in 40% versus 8% (p = .01). The most common drug combinations causing increased toxicity were amoxicillin/clavulanate (8 of 9 cases) and diclofenac in combination with steroid hormones (4 of 9 cases). Drug-drug interactions may influence the incidence and/or the severity of idiosyncratic DILI. MH cell testing can identify relevant drug-drug interactions. The data generated by this approach may improve patient safety. STUDY IDENTIFIER: ClinicalTrials.gov NCT02353455.

Pgam5 released from damaged mitochondria induces mitochondrial biogenesis via Wnt signaling.

Mitochondrial abundance is dynamically regulated and was previously shown to be increased by Wnt/ß-catenin signaling. Pgam5 is a mitochondrial phosphatase which is cleaved by the rhomboid protease presenilin-associated rhomboid-like protein (PARL) and released from membranes after mitochondrial stress. In this study, we show that Pgam5 interacts with the Wnt pathway component axin in the cytosol, blocks axin-mediated ß-catenin degradation, and increases ß-catenin levels and ß-catenin-dependent transcription. Pgam5 stabilized ß-catenin by inducing its dephosphorylation in an axin-dependent manner. Mitochondrial stress triggered by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) treatment led to cytosolic release of endogenous Pgam5 and subsequent dephosphorylation of ß-catenin, which was strongly diminished in Pgam5 and PARL knockout cells. Similarly, hypoxic stress generated cytosolic Pgam5 and led to stabilization of ß-catenin, which was abolished by Pgam5 knockout. Cells stably expressing cytosolic Pgam5 exhibit elevated ß-catenin levels and increased mitochondrial numbers. Our study reveals a novel mechanism by which damaged mitochondria might induce replenishment of the mitochondrial pool by cell-intrinsic activation of Wnt signaling via the Pgam5-ß-catenin axis.