Osteocalcin Reverses Endoplasmic Reticulum Stress and Improves Impaired Insulin Sensitivity Secondary to Diet-Induced Obesity Through Nuclear Factor-κB Signaling Pathway

Abstract

Osteocalcin, a synthetic osteoblast-specific protein, has recently emerged as an important regulator of energy metabolism, but the underlying mechanisms are not fully understood. In the present study, mice fed a high-fat diet and receiving osteocalcin showed reduced body weight gain, less fat pad gain, and improved insulin sensitivity as well as increased energy expenditure compared with mice fed a high-fat diet and receiving vehicle. Meanwhile, increased endoplasmic reticulum (ER) stress, defective insulin signaling, and mitochondrial dysfunction induced by obesity were also effectively alleviated by treatment with osteocalcin. Consistent with these findings, the addition of osteocalcin to the culture medium of 3T3-L1 adipocytes, Fao liver cells, and L6 muscle cells markedly reduced ER stress and restored insulin sensitivity. These effects were nullified by blockade of nuclear factor-κB (NF-κB) or phosphatidylinositol 3-kinase but not by U0126, a mitogen-activated protein kinase inhibitor, indicating the causative role of phosphatidylinositol 3-kinase/NF-κB in action of osteocalcin. In addition, the reversal effects of osteocalcin in cells deficient in X-box-binding protein-1, a transcription factor that modulates ER stress response, further confirmed its protective role against ER stress and insulin resistance. Our findings suggest that osteocalcin attenuates ER stress and rescues impaired insulin sensitivity in insulin resistance via the NF-κB signaling pathway, which may offer novel opportunities for treatment of obesity and diabetes.

Administration of progranulin (PGRN) triggers ER stress and impairs insulin sensitivity via PERK-eIF2α-dependent manner

“…36,44 PERK signaling activates NF-kB, a transcriptional regulator that plays a central role in mediating the responses to inflammatory signaling, while translation of IkBa, the main negative regulator of NF-kB, is known to be inhibited by phosphorylation of eIF2a. 45 Likewise, the 2 other canonical branches of the UPR, IRE1 and ATF6, have been reported to up-regulate the expression of inflammatory cytokines.…”

“…44 200 mg of cytoplasmic lysate were incubated for 2 h at 4 C with the corresponding antibodies coupled to 20 ml of packed protein ACG sepharose beads (Beyotime). Immunocomplexes were resolved by means of SDS-PAGE and immuno-blotted with the indicated antibodies.…”

Intermittent injections of osteocalcin reverse autophagic dysfunction and endoplasmic reticulum stress resulting from diet-induced obesity in the vascular tissue via the NFκB-p65-dependent mechanism

“…Purification of bacterially produced mouse recombinant uncarboxylatedosteocalcin was performed as described. 49 Briefly glutathione S-transferase (GST)-osteocalcin fusion protein was produced and purified on glutathione-sepharose according to the standard procedure. Then osteocalcin was cleaved out from the GST moiety by using thrombin.…”

An Overview of the Endoplasmic Reticulum Stress and Interrelation with Obesity: Review

“…Bu da fosfatatidilinositol 3-kinaz/NF-κB yolaklarının osteokalsinin etkinliğinde rolü olduğunu belirtmektedir. 100 Genç (dört-altı aylık) ve yaşlı (18-22 aylık) C57BL/6 farelerden epididimal dokusu kullanıla-rak elde edilen adipoz doku stromal hücreler üze-rinde yapılan bir çalışmada, hücrelere taşıyıcı (dimetil sülfoksit) veya ER stresini indüklemek için thapsigargin (330 nM) 4-16 saat boyunca uygulanmış ve bu süreler sonunda hücre kültürü medyumu ve sonrasında da hücreler toplanarak ER stres proteinlerine ait mRNA ve protein düzeyleri belirlenmiştir. Çalışma sonunda; GRP78, CHOP, yarılmış-ATF-6, fosfo-IRE1α, PDIA, EDEM1 ve ERdj4 ve XBP-1 mRNA düzeylerinin yaşlı hayvanlardan elde edilen hücrelerde genç hayvanlara göre anlamlı derecede yüksek olduğu bulunmuştur.…”

“…Ayrıca, obezitede osteokalsin uygulamasının ER stresini, hatalı insülin sinyalleşmesini ve obezite ile indüklenen mitokondriyal işlev bozukluğunu etkin bir şekilde azalttığı bulunmuştur. 100 Üç farklı fare modeli (yüksek yağlı diyetle beslenen C57BL6 fareleri; apolipoprotein E, leptin, ve apolipoprotein B-48 eksikliği olan ApoE 3KO fareleri ve düşük dansiteli lipoprotein reseptörü, leptin ve apolipoprotein B-48 eksikliği olan LDLR 3KO fareleri) üzerinde yapılan bir çalışmada, ER stresinin C57BL6 ve ApoE 3KO farelerinde yüksek olduğu; ancak LDLR 3KO farelerinde diğer iki tip fareye oranla daha düşük olduğu belirlenmiştir. Yüksek yağlı diyetle beslenen C57BL6 ve ApoE 3KO farelerde hiperglisemi gözlenirken, LDLR 3KO farelerinde normal glisemi saptanmıştır.…”