Myotubularin-related protein 7 activates peroxisome proliferator-activated receptor-gamma

Philip Weidner,Florian N Rohrbacher,Michaela Söhn,Tobias Gutting,Veronika Hauber,Laura Helm,Johannes Betge,Rony Seger,Jeffrey W Bode,Matthias P A Ebert,Ariane Nunes-Alves,Rebecca C Wade,Elke Burgermeister,Daniel Saar,Christoph Röcken,Vijaya R Pattabiraman,Torsten Schroeder

doi:10.1038/s41389-020-0238-8

Abstract

Peroxisome proliferator-activated receptor-gamma (PPARγ) is a transcription factor drugable by agonists approved for treatment of type 2 diabetes, but also inhibits carcinogenesis and cell proliferation in vivo. Activating mutations in the Kirsten rat sarcoma viral oncogene homologue (KRAS) gene mitigate these beneficial effects by promoting a negative feedback-loop comprising extracellular signal-regulated kinase 1/2 (ERK1/2) and mitogen-activated kinase kinase 1/2 (MEK1/2)-dependent inactivation of PPARγ. To overcome this inhibitory mechanism, we searched for novel post-translational regulators of PPARγ. Phosphoinositide phosphatase Myotubularin-Related-Protein-7 (MTMR7) was identified as cytosolic interaction partner of PPARγ. Synthetic peptides were designed resembling the regulatory coiled-coil (CC) domain of MTMR7, and their activities studied in human cancer cell lines and C57BL6/J mice. MTMR7 formed a complex with PPARγ and increased its transcriptional activity by inhibiting ERK1/2-dependent phosphorylation of PPARγ. MTMR7-CC peptides mimicked PPARγ-activation in vitro and in vivo due to LXXLL motifs in the CC domain. Molecular dynamics simulations and docking predicted that peptides interact with the steroid receptor coactivator 1 (SRC1)-binding site of PPARγ. Thus, MTMR7 is a positive regulator of PPARγ, and its mimicry by synthetic peptides overcomes inhibitory mechanisms active in cancer cells possibly contributing to the failure of clinical studies targeting PPARγ.

Highlights

Introduction The nuclear transcription factorPPARγ has been established as a target in type 2 diabetes for many years
In contrast to other member of the myotubularin (MTM), characterized as “survival phosphatases”[21,22], we demonstrated that MTMR7 reduces proliferation of colorectal cancer (CRC) cells in vitro, even in the presence of activating mutations of Kirsten rat sarcoma viral oncogene homologue (KRAS) and active insulin signalling, due to inhibition of both RAS-extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphoinositide 3-kinase (PI3K)-AKTmTOR signalling[25]
MTMR7 may enforce the classical nuclear function of PPARγ by inhibiting the RAS-ERK1/2 cascade, which otherwise evokes inactivation of PPARγ by ERK1/2-mediated phosphorylation and mitogen-activated kinase kinase 1/2 (MEK1/2)-dependent sequestration of PPARγ in the cytosol[16], where it interacts with molecules that restrain PPARγ activity including caveolin-126 and heat shock proteins[55]

Summary

Introduction

Introduction The nuclear transcription factorPPARγ has been established as a target in type 2 diabetes for many years. In addition to its lipid lowering and insulin sensitizing properties, pharmacological activation of PPARγ shows benefits in malignant and inflammatory human diseases[1,2,3], and inhibition of RAS-ERK1/2 signalling was observed in (pre)clinical models[4,5,6]. It is the latter characteristic that renders the nuclear receptor a promising target in gastrointestinal tumours with frequent, activating mutations in the RAS-ERK1/2 signalling cascade, e.g. RAS genes are a major obstacle for effective treatment in advanced disease[8], and new drugable targets which inhibit RAS-ERK1/2 signalling are needed[9]. One reason for this discrepancy between preclinical and clinical studies may rely on the complex regulation of PPARγ by the RAS-ERK1/2 signalling cascade, which has not been taken into account in any of the before mentioned trials: we[15,16] and others[17,18] demonstrated that downstream effectors of RAS inhibit PPARγ, e.g. by ERK1/2-dependent phosphorylation as well as by nuclear

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Conclusion