Role of the O-GlcNAc modification on insulin resistance and endoplasmic reticulum stress in 3T3-L1 cells.

O-linked N-acetyl-glucosamine (O-GlcNAc) is a post-translational protein modification that regulates cell signaling and involves in several pathological conditions. O-GlcNAc transferase (OGT) catalyzes the attachment, while O-GlcNAcase (OGA) splits the GlcNAc molecules from the serine or threonine residues of the nuclear and cellular proteins. The hexosamine biosynthesis pathway (HBP) is a small branch of glycolysis that provides a substrate for the OGT and serves as a nutrient sensor. In this study, we investigated the impact of external O-GlcNAc modification stimulus on the insulin signal transduction, unfolded protein response, and HBP in 3T3-L1 cells. First, we treated cells with glucosamine and PUGNAc to stimulate the O-GlcNAcylation of total proteins. Also, we treated cells with tunicamycin as a positive internal control, which is a widely-used endoplasmic reticulum stressor. We used two in vitro models to understand the impact of the cellular state of insulin sensibility on this hypothesis. So, we employed insulin-sensitive preadipocytes and insulin-resistant adipocytes to answer these questions. Secondly, the OGT-silencing achieved in the insulin-resistant preadipocyte model by using the short-hairpin RNA (shRNA) interference method. Thereafter, the cells treated with the above-mentioned compounds to understand the role of the diminished O-GlcNAc protein modification on the insulin signal transduction, unfolded protein response, and HBP. We found that elevated O-GlcNAcylation of the total proteins displayed a definite correlation in insulin resistance and endoplasmic reticulum stress. Furthermore, we identified that the degree of this correlation depends on the cellular state of insulin sensitivity. Moreover, reduced O-GlcNAcylation of the total proteins by the shRNA-mediated silencing of the OGT gene, which is the only gene to modify proteins with the O-GlcNAc molecule, reversed the insulin resistance and endoplasmic reticulum stress phenotype, even with the externally stimulated O-GlcNAc modification conditions. In conclusion, our results suggest that OGT regulates insulin receptor signaling and unfolded protein response by modulating O-GlcNAc levels of total proteins, in response to insulin resistance. Therefore, it can be a potential therapeutic target to prevent insulin resistance and endoplasmic reticulum stress.

Saturated Fatty Acids Promote GDF15 Expression in Human Macrophages through the PERK/eIF2/CHOP Signaling Pathway.

Growth differentiation factor-15 (GDF-15) and its receptor GFRAL are both involved in the development of obesity and insulin resistance. Plasmatic GDF-15 level increases with obesity and is positively associated with disease progression. Despite macrophages have been recently suggested as a key source of GDF-15 in obesity, little is known about the regulation of GDF-15 in these cells. In the present work, we sought for potential pathophysiological activators of GDF15 expression in human macrophages and identified saturated fatty acids (SFAs) as strong inducers of GDF15 expression and secretion. SFAs increase GDF15 expression through the induction of an ER stress and the activation of the PERK/eIF2/CHOP signaling pathway in both PMA-differentiated THP-1 cells and in primary monocyte-derived macrophages. The transcription factor CHOP directly binds to the GDF15 promoter region and regulates GDF15 expression. Unlike SFAs, unsaturated fatty acids do not promote GDF15 expression and rather inhibit both SFA-induced GDF15 expression and ER stress. These results suggest that free fatty acids may be involved in the control of GDF-15 and provide new molecular insights about how diet and lipid metabolism may regulate the development of obesity and T2D.

Deletion of the nuclear receptor RORα in macrophages does not modify the development of obesity, insulin resistance and NASH.

Retinoic acid receptor-related orphan receptor-alpha (RORα) is a transcription factor from the nuclear receptor family expressed by immune cells and involved in the development of obesity, insulin resistance (IR) and non-alcoholic steatohepatitis (NASH). It was recently reported that mice deficient for RORα in macrophages develop more severe NASH upon high fat diet (HFD) feeding due to altered Kupffer cell function. To better understand the role of RORα in obesity and IR, we independently generated a macrophage RORα-deficient mouse line. We report that RORα deletion in macrophages does not impact on HFD-induced obesity and IR. Surprisingly, we did not confirm an effect on NASH development upon HFD feeding nor in the more severe and obesity-independent choline-deficient, L-amino acid-defined diet model. Our results therefore show that RORα deletion in macrophages does not alter the development of obesity and IR and question its role in NASH.