Selenoprotein S attenuates high glucose and/or ox-LDL-induced endothelium injury by regulating Akt/mTOR signaling and autophagy

Highlights

• Glucolipid metabolism disorder had a synergistic effect on endothelial injury.

• Selenoprotein S could attenuate endothelial injury in HAECs.

• Selenoprotein S inhibited human aortic endothelial cell autophagy.

• Selenoprotein S regulated autophagy through Akt/mTOR signaling.

Abstract

Glucolipid metabolism disorder in diabetes mellitus (DM) causes human endothelial injury and autophagy dysfunction is an important cause of endothelial dysfunction (ED). Selenoprotein S (SelS) could protect endothelium from oxidative stress, inflammatory responses, and apoptosis. This study assessed the effect of SelS on autophagy in glucolipid metabolic disorders and protection of the resulted vascular endothelial injury. The results showed that high glucose (HG), high oxidized low-density lipoprotein (HL), and HG combined with HL (HGL) could reduce viability of human aortic endothelial cells (HAECs), induce HAECs injury and increase SelS expression in a time-dependent manner. HG, HL, and HGL also initially induced autophagy but later reduced it in HAECs, while activity of the Akt/mTOR signaling was inhibited, especially in HGL culture of HAECs. SelS overexpression reduced the endothelial injury and autophagy and activated the Akt/mTOR signaling in HG, HL and HGL-cultured HAECs, compared to the control. Conversely, knockdown of SelS expression had the opposite effects on HAECs. In conclusion, SelS demonstrated a protective effect on endothelial injury induced by high glucose and/or ox-LDL and the underlying molecular events might be related to its regulation of HAECs autophagy by activating the Akt/mTOR signaling. SelS could be a potential intervention target in prevention and treatment of diabetic vascular complications.

Introduction

Diabetes mellitus (DM) is characterized by a continuously high blood sugar level; clinically, if unsuccessfully controlled, DM causes many complications, like cardiovascular disease, stroke, and chronic kidney, nerve, and eyes diseases, due to damages to endothelial cells (Cole and Florez, 2020, Faselis et al., 2020). To date, DM currently affects over 463 million people worldwide and the number of diabetics is expected to increase to 578 million by 2030 according to the latest International Diabetes Federation report (Huang et al., 2018). DM-induced vascular complications are a major cause for reduced people’s life expectancy and induced mortality (Zheng et al., 2018). Histologically, these vascular complications include damages to macrovessels, like atherosclerosis (AS) or endothelial dysfunction (ED) and the latter is a critically initial AS event and leads to defective endothelial repair and inability of angiogenesis (Xu et al., 2021). Diabetic patients usually complicate with dyslipidemia, the risk factors for AS (Poznyak et al., 2020). Previous studies revealed that high glucose and oxidized low-density lipoprotein (ox-LDL) could alter the oxidation-reduction equilibrium in vascular endothelial cells and induce them to undergo apoptosis; thus leading to ED (Lubrano and Balzan, 2014, Volpe et al., 2018). However, the underlying precise molecular mechanisms remain to be fully elucidated. In this regard, further identification and validation of novel approaches are urgently needed to effectively control DM and vascular complications in order to prevent or treat DM-related vascular complications.

DM-related endothelial damages could be due to increases in oxidation, inflammation, endoplasmic reticulum-related stress, apoptosis and autophagy disorders (Demirtas et al., 2016, Yuan et al., 2019). Autophagy is a tightly regulated metabolic process in cells and occurs in both physiologic and pathologic conditions and the latter includes diabetes (Demirtas et al., 2016). During autophagy, a cell digests cell own components via the lysosomal machinery, a survival-oriented function under base or stress conditions (Gross and Graef, 2020), which is resulted from a series of strictly regulated catabolic processes by which cytoplasmic components are transported to lysosomes and degraded and the digested cell components will be used to generate energy and protein synthesis (Cao et al., 2021). Cell components are tagged for degradation through ubiquitination and linked to microtubule associated protein 1 light chain 3 (LC3) through the adapter protein P62 (Wu and Lu, 2019). During autophagy initiation, LC3-I is lipidated to form LC3-II as an activated form of LC3 and then target the damaged cell components to form a double-membrane autophagosome (Hanada et al., 2007). Beclin-1 is a part of the lipid-phosphatidylinositol-3 kinase (PI3K) complex to regulate autophagosome formation and autophagy initiation (Kaur and Changotra, 2020). The contents of the autophagolysosome are degraded by cathepsins and P62 is degraded along with the targeted organelles and proteins (Cao et al., 2021, Kaur and Changotra, 2020). Thus, autophagy plays an important role in cell homeostasis and functions of heart and vessels, while defective or excessive autophagy could lead to diseases, like cardiovascular diseases or even carcinogenesis (Li et al., 2020, Mialet-Perez and Vindis, 2017). In DM a previous study reported that autophagy was able to protect high glucose- induced ED from senescence (Chen et al., 2014), while another previous report showed that autophagy played a detrimental role in hyperglycemia-induced ED (Niu et al., 2019). High glucose-induced autophagy could damage the endothelial progenitor cells, aggravate the mitochondrial oxidative stress, and prevent neovascularization (Kim et al., 2014). Moreover, ox-LDL was able to induce injury of human umbilical vein endothelial cells (HUVECs) via autophagy (Fan et al., 2015). Taken them all together, glycolipid metabolic disorders can cause endothelial injury via autophagy induction.

Selenoprotein S (SelS) is a transmembrane protein with extensive histological distributions, containing eight-serine and one-threonine phosphorylation sites (Gao et al., 2007). SelS is reported to be involved in regulation of the inflammatory response and oxidative and apoptosis during DM-induced macrovascular complications development and progression; therefore, SelS association with DM and macroangiopathy has gradually received attention from scholars in the field (Cui et al., 2018, Yu and Du, 2017, Yu et al., 2018, Zhong et al., 2020). Our group revealed that SelS upregulation was able to attenuate high glucose- and ox-LDL-induced endothelial injury in vitro (Zhong et al., 2020). However, it remains to be determined whether SelS endothelial protective effect is associated with autophagy and protein kinase B (Akt)/ mammalian target of rapamycin (mTOR) signaling.

In this study, we assessed the role of SelS in alleviating glucolipid metabolic disorders-induced ED and changes in autophagy. We cultured human aortic endothelial cells (HAECs) in HG, HL and HGL and detected changes in HAECs damages, autophagy, and expression of the related proteins in vitro. We then manipulated SelS expression in HAECs and further cultured these HAECs in HG, HL, and HGL to further identify the role of SelS in HAECs in vitro. We expected to provide novel information regarding SelS in protection of endothelial cell damages induced by HG, HL, and HGL, which could create an experimental basis for SelS used as a targeted therapeutic strategy for protection of ED caused by glucolipid metabolism disorders in future.