Palmitate-induced Slc2a4/GLUT4 downregulation in L6 muscle cells: evidence of inflammatory and endoplasmic reticulum stress involvement

Patrícia Ebersbach-Silva,Ana Cláudia Poletto,Gabriel Forato Anhê,Ubiratan Fabres Machado,Aline David-Silva,Patrícia Monteiro Seraphim,Daniela Tomie Furuya,Marisa Passarelli

doi:10.1186/s12944-018-0714-8

Abstract

BackgroundObesity is strongly associated to insulin resistance, inflammation, and elevated plasma free fatty acids, but the mechanisms behind this association are not fully comprehended. Evidences suggest that endoplasmic reticulum (ER) stress may play a role in this complex pathophysiology. The aim of the present study was to investigate the involvement of inflammation and ER stress in the modulation of glucose transporter GLUT4, encoded by Slc2a4 gene, in L6 skeletal muscle cells.MethodsL6 cells were acutely (2 h) and chronically (6 and 12 h) exposed to palmitate, and the expression of several proteins involved in insulin resistance, ER stress and inflammation were analyzed.ResultsChronic and acute palmitate exposure significantly reduced GLUT4 protein (~ 39%, P < 0.01) and its mRNA (18%, P < 0.01) expression. Only acute palmitate treatment increased GRP78 (28%, P < 0.05), PERK (98%, P < 0.01), eIF-2A (35%, P < 0.01), IRE1a (60%, P < 0.05) and TRAF2 (23%, P < 0.05) protein content, and PERK phosphorylation (106%, P < 0.001), but did not elicit eIF-2A, IKK phosphorylation or increased XBP1 nuclear content. Additionally, acute and chronic palmitate increased NFKB p65 nuclear content (~ 30%, P < 0.05) and NFKB binding activity to Slc2a4 gene promoter (~ 45%, P < 0.05).ConclusionDifferent pathways are activated in acute and chronic palmitate induced-repression of Slc2a4/GLUT4 expression. This regulation involves activation of initial component of ER stress, such as the formation of a IRE1a-TRAF2-IKK complex, and converges to NFKB-induced repression of Slc2a4/GLUT4. These results link ER stress, inflammation and insulin resistance in L6 cells.

Highlights

Obesity is strongly associated to insulin resistance, inflammation, and elevated plasma free fatty acids, but the mechanisms behind this association are not fully comprehended
As it can be observed, the repression effect of palmitate was greater in GLUT4 protein expression than in expression of its respective mRNA, suggesting the presence of a transcriptional and posttranscriptional regulation, such as unfolded protein response (UPR) signaling
We demonstrated that acute palmitate even though was able to increase IRE1a expression, it did not result in its activation since phosphorylation of IRE1a and XBP1 nuclear content was not altered

Summary

Introduction

Obesity is strongly associated to insulin resistance, inflammation, and elevated plasma free fatty acids, but the mechanisms behind this association are not fully comprehended. The aim of the present study was to investigate the involvement of inflammation and ER stress in the modulation of glucose transporter GLUT4, encoded by Slc2a4 gene, in L6 skeletal muscle cells. While the knockout of the Slc2a4 gene, which Insulin resistance states, such as diabetes and obesity, have been associated to low-grade systemic inflammation [3, 8, 9]. In addition to insulin resistance and inflammation, obesity is strongly related to elevated plasma free fatty acids (FFA), but the mechanisms behind this association are not fully comprehended.

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