SGK1 Inhibits Autophagy in Murine Muscle Tissue.

Theresia Zuleger,Catherine Hunter,Zsuzsanna Takacs,Tassula Proikas-Cezanne,Julia Heinzelbecker,Jakob Voelkl,Florian Lang

doi:10.1155/2018/4043726

Theresia Zuleger, Catherine Hunter + Show 5 more

Open Access

https://doi.org/10.1155/2018/4043726

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Abstract

Background/Aims As autophagy is linked to several pathological conditions, like cancer and neurodegenerative diseases, it is crucial to understand its regulatory signaling network. In this study, we investigated the role of the serum- and glucocorticoid-induced protein kinase 1 (SGK1) in the control of autophagy. Methods To measure autophagic activity in vivo, we quantified the abundance of the autophagy conjugates LC3-PE (phosphatidylethanolamine) and ATG12-ATG5 in tissue extracts of SGK1 wild-type (Sgk1 +/+) and knockout (Sgk1 −/−) mice that were either fed or starved for 24 h prior sacrifice. In vitro, we targeted SGK1 by RNAi using GFP-WIPI1 expressing U-2 OS cells to quantify the numbers of cells displaying newly formed autophagosomes. In parallel, these cells were also assessed with regard to LC3 and ULK1 by quantitative Western blotting. Results The abundance of both LC3-PE (LC3-II) and ATG12-ATG5 was significantly increased in red muscle tissues of SGK1 knockout mice. This was found in particular in fed conditions, suggesting that SGK1 may keep basal autophagy under control in red muscle in vivo. Under starved conditions, significant differences were observed in SGK1-deficient white muscle tissue and, under fed conditions, also in the liver. In vitro, we found that SGK1 silencing provoked a significant increase of cells displaying WIPI1-positive autophagosomes and autophagosomal LC3 (LC3-II). Moreover, autophagic flux assessments revealed that autophagic degradation significantly increased in the absence of SGK1, strongly suggesting that SGK1 inhibits both autophagosome formation and autophagic degradation in vitro. In addition, more ULK1 protein lacking the inhibitory, TORC1-specific phosphorylation at serine 758 was detected in the absence of SGK1. Conclusions Combined, our data strongly support the idea that SGK1 inhibits the process of autophagy. Mechanistically, our data suggest that SGK1 should act upstream of ULK1 in regulating autophagy, and we hypothesize that SGK1 contributes to the regulation of ULK1 gene expression.

Highlights

Macroautophagy is a catabolic pathway that degrades cytosolic components, like proteins and damaged organelles
We found that the abundances of both LC3-II (Figure 1(a), upper panel and lower left panel) and ATG12ATG5 (Figure 1(b), upper panel and lower left panel) were significantly increased in red muscle tissue derived from mice deficient for serum- and glucocorticoid-induced protein kinase 1 (SGK1) (Sgk1−/−)
To investigate the role of SGK1 in autophagy, we analysed the abundance of the autophagy markers LC3-II and ATG12-ATG5 in organ tissues derived from SGK1 knockout and wild-type mice

Summary

Introduction

Macroautophagy (referred to as autophagy) is a catabolic pathway that degrades cytosolic components, like proteins and damaged organelles. The degraded monomers are released to the cytoplasm for recycling processes engaging anabolic pathways. Dysregulation of autophagy often results in pathological conditions like cancer and metabolic and neurodegenerative diseases [1, 2]. Autophagosome formation is tightly regulated by ATG (autophagy-related) proteins [2]. Amongst the ATG proteins, the ULK1 protein is crucial for the initiation of autophagosome formation and interacts with both the AMPactivated protein kinase (AMPK) and the mechanistic target of rapamycin complex 1 (mTORC1), the energy and nutrient sensors of the eukaryotic cell. AMPK activates, and under nutrient-rich conditions, mTORC1 inhibits ULK1 and autophagy [3, 4]. The generation of phosphatidylinositol 3-phosphate (PI3P) by the phosphatidylinositol 3-kinase class III (PI3KC3) is essential

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