Background: Adipose tissue (AT) may have a significant role in obesity associated inflammation but, the mechanisms underlying the pathogenesis of obesity induced inflammation remains unclear. Recent murine studies indicate that ER stress is critical to the initiation and integration of inflammation and insulin signalling pathways. The ER stress occurs primarily due to accumulation of unfolded proteins and results in activation of the unfolded protein response to restore ER homeostasis. This up-regulates PERK, IRE1α, ATF6 and other chaperones. Therefore, our aims were to determine the existence and causes of ER stress in human adipocytes along with therapeutic interventions.
Methods: Human abdominal subcutaneous (AbSc) AT was obtained from Caucasian non-diabetic population (BMI: 27.9±7.3 kg/m2: age 3649 years; n=4; female subjects) that underwent elective liposuction. Preadipocytes were isolated from stromal fraction, grown and fully differentiated into adipocytes (n=5). Adipocytes were then treated with LPS (100 ng/ml), tunicamycin (750 ng/ml), high glucose (25 mM, HG) and in combination with 20 mM salicylate, salicylate alone and without (controls). To characterise markers of ER stress total protein and RNA was extracted from adipose tissue and cultured adipocytes. Western blots and qRT-PCR were performed to examine expression levels.
Results: ER stress proteins Calnexin1, BiP1, Ero-1α, PDI, IRE1α and phospho-PERK were significantly up-regulated in obese AbSc AT compared to lean (n=4; P<0.05 to P<0.001). phospho-eIF2α (n=5; P<0.005) and calnexin (n=5; P<0.02) were both significantly induced by LPS, tunicamycin and HG in differentiated human adipocytes and were significantly reduced when treated with salicylate. Similarly, phospho-Akt (S473) (n=5; P<0.002) was activated and GLUT4 up-regulated, when ER stress was down-regulated by salicylate.
Conclusion: Our findings highlight, up-regulation of ER stress in human obese AbSc AT compared to lean subjects. From this study we also conclude that salicylates improve insulin sensitivity and alleviate ER stress induced by LPS and high glucose in cultured primary human adipocytes.