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Oncotarget. 2017 Dec 31;9(5):5716-5735. doi: 10.18632/oncotarget.23788. eCollection 2018 Jan 19.

The protein kinase SIK downregulates the polarity protein Par3.

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Ludwig Institute for Cancer Research, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.
Integrated Cardio Metabolic Center, Novum, Karolinska Institute, Huddinge, Sweden.
Department of Medical Biochemistry and Microbiology, Science for Life Laboratory, Uppsala University, Uppsala, Sweden.
Department of Oncology and Pathology, Karolinska Biomics Center, Karolinska Institute, Stockholm, Sweden.
Department of Obstetrics and Gynaecology, Academic Uppsala Hospital, Uppsala, Sweden.
School of Anatomy, Physiology and Human Biology, The University of Western Australia, Crawley, WA, Australia.
Novo Nordisk Foundation Center for Protein Research, University of Copenhagen, Copenhagen, Denmark.
Cancer Research UK, Cambridge Institute, University of Cambridge, Li Ka Shing Center, Cambridge, UK.
Department of Immunology, Genetics and Pathology, Rudbeck Laboratory, Uppsala University, Uppsala, Sweden.


The multifunctional cytokine transforming growth factor β (TGFβ) controls homeostasis and disease during embryonic and adult life. TGFβ alters epithelial cell differentiation by inducing epithelial-mesenchymal transition (EMT), which involves downregulation of several cell-cell junctional constituents. Little is understood about the mechanism of tight junction disassembly by TGFβ. We found that one of the newly identified gene targets of TGFβ, encoding the serine/threonine kinase salt-inducible kinase 1 (SIK), controls tight junction dynamics. We provide bioinformatic and biochemical evidence that SIK can potentially phosphorylate the polarity complex protein Par3, an established regulator of tight junction assembly. SIK associates with Par3, and induces degradation of Par3 that can be prevented by proteasomal and lysosomal inhibition or by mutation of Ser885, a putative phosphorylation site on Par3. Functionally, this mechanism impacts on tight junction downregulation. Furthermore, SIK contributes to the loss of epithelial polarity and examination of advanced and invasive human cancers of diverse origin displayed high levels of SIK expression and a corresponding low expression of Par3 protein. High SIK mRNA expression also correlates with lower chance for survival in various carcinomas. In specific human breast cancer samples, aneuploidy of tumor cells best correlated with cytoplasmic SIK distribution, and SIK expression correlated with TGFβ/Smad signaling activity and low or undetectable expression of Par3. Our model suggests that SIK can act directly on the polarity protein Par3 to regulate tight junction assembly.


Par3; SIK; TGFβ; cell junctions; signal transduction

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