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Nutrients 2016, 8(4), 227; doi:10.3390/nu8040227

Insulin Protects Hepatic Lipotoxicity by Regulating ER Stress through the PI3K/Akt/p53 Involved Pathway Independently of Autophagy Inhibition

1,†
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1,2,3,†
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1,2
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1,4,* and 1,2,*
1
Department of Nutrition and Food Hygiene, Public Health College, Harbin Medical University, Harbin 150086, China
2
Research Institute of Food, Nutrition and Health, Sino-Russian Medical Research Center, Harbin Medical University, Harbin 150086, China
3
Harbin Center for Disease Control and Prevention, Harbin 150086, China
4
Key Laboratory of Cardiovascular Medicine Research (Harbin Medical University), Ministry of Education, Harbin 150086, China
These authors contribute equally to this paper.
*
Authors to whom correspondence should be addressed.
Received: 9 March 2016 / Revised: 30 March 2016 / Accepted: 8 April 2016 / Published: 19 April 2016
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Abstract

The detrimental role of hepatic lipotoxicity has been well-implicated in the pathogenesis of NAFLD. Previously, we reported that inhibiting autophagy aggravated saturated fatty acid (SFA)-induced hepatotoxicity. Insulin, a physiological inhibitor of autophagy, is commonly increased within NAFLD mainly caused by insulin resistance. We therefore hypothesized that insulin augments the sensitivity of hepatocyte to SFA-induced lipotoxicity. The present study was conducted via employing human and mouse hepatocytes, which were exposed to SFAs, insulin, or their combination. Unexpectedly, our results indicated that insulin protected hepatocytes against SFA-induced lipotoxicity, based on the LDH, MTT, and nuclear morphological measurements, and the detection from cleaved-Parp-1 and -caspase-3 expressions. We subsequently clarified that insulin led to a rapid and short-period inhibition of autophagy, which was gradually recovered after 1 h incubation in hepatocytes, and such extent of inhibition was insufficient to aggravate SFA-induced lipotoxicity. The mechanistic study revealed that insulin-induced alleviation of ER stress contributed to its hepatoprotective role. Pre-treating hepatocytes with insulin significantly stimulated phosphorylated-Akt and reversed SFA-induced up-regulation of p53. Chemical inhibition of p53 by pifithrin-α robustly prevented palmitate-induced cell death. The PI3K/Akt pathway blockade by its special antagonist abolished the protective role of insulin against SFA-induced lipotoxicity and p53 up-regulation. Furthermore, we observed that insulin promoted intracellular TG deposits in hepatocytes in the present of palmitate. However, blocking TG accumulation via genetically silencing DGAT-2 did not prevent insulin-protected lipotoxicity. Our study demonstrated that insulin strongly protected against SFA-induced lipotoxicity in hepatocytes mechanistically through alleviating ER stress via a PI3K/Akt/p53 involved pathway but independently from autophagy. View Full-Text
Keywords: insulin; lipotoxicity; hepatocytes; autophagy; NAFLD insulin; lipotoxicity; hepatocytes; autophagy; NAFLD
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Ning, H.; Sun, Z.; Liu, Y.; Liu, L.; Hao, L.; Ye, Y.; Feng, R.; Li, J.; Li, Y.; Chu, X.; Li, S.; Sun, C. Insulin Protects Hepatic Lipotoxicity by Regulating ER Stress through the PI3K/Akt/p53 Involved Pathway Independently of Autophagy Inhibition. Nutrients 2016, 8, 227.

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