Osteocalcin reverses endoplasmic reticulum stress and improves impaired insulin sensitivity secondary to diet-induced obesity through nuclear factor-κB signaling pathway
Authors: Hongzhi Sun
Bo Zhou
Huixia Li
Shufang Wu
Lin Xu
Weijin Zang
Journal: Endocrinology (2013)
Discipline: BIOLOGY & BIOCHEMISTRY
Keywords: Osteocalcin; energy metabolism; high-fat diet; insulin sensitivity; endoplasmic reticulum stress
Affiliations: Key Laboratory of Environment and Genes Related to Diseases, Ministry of Education, Medical School of Xi'an Jiaotong University, Xi'an, Shaanxi, 710061, China.

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. 

Retracted: No

PubPeer comments:
 Figure 5b: These worms are more similar than expected. Also, the same set of images for WT were used on Fig. 5 and Fig. 7e, could authors clarify whether these experiments were perfomed together. ![file] , 
 We thank the anonymous reader for raising the question of the wild type control in the two figure panels. These panels indeed show images from the same concurrent experiment, in which the same control was used for the image and quantification data. In the experiment triplicates of all conditions were made, with worms placed on microscopic slides and then recorded at once ( 100 different images) to ensure the same recording conditions. The original images were quantified and processed by the software for worm flattening in order to be able to create stack images used for data presentation (as described in Materials and Methods). A multipanel figure was made to summarize all microscopy results, but during the manuscript revision the panels were separated. We also thank the reader for noticing the similarity in the two worm images. By reviewing our original confocal images we have determined that these are two independent images of the same worm that had been taken then u201cflattenedu201d and used for the same stack of images. That being said, it is important to note that this is one of multiple worms that were presented in the illustrative panel as being representative, and that this mistake did not impact quantification of the fluorescence intensities (the u201crealu201d data) or data validity. All original unprocessed images will be available at doi:10.17632 nzvxy6pvdj.1 after the dataset has been approved, in accordance with the Open Research Data and Data Management Plans set by European Research Council regulation. We apologize for any confusion that these situations might have created. T. Keith Blackwell, Collin Y. Ewald, and Milos Filipovic , 
 In Figure 3C: The foldchange of ATF-4 in the overexpression strain seems very low. Indeed, when taking a look at the raw data, the way the samples are currently assigned, there is a very large variation in the expression levels of ATF-4 across the different samples: ![file] Here are the RPKM values for atf-4 taken from the supplement excel sheet: ![file] It rather appears that the first 3 columns are in fact wildtype, the next 3 the knockout strain and the last 3 the overexpression strain. Could the authors please check if there indeed a mixup happened? , 
 #3 Thank you very much for bringing this to our attention. We have confirmed that, as you suspected, at some point in the publication process a labeling mixup occurred in the table. This did not affect our analysis of the samples themselves or our conclusions. In the document GSE173799_SupplementaryTable3-ATF4_RNAseq_rpkm.xlsx on tab RawDataATF4_RNAseq_RPKM the column names should be: newCE1.N2.1 newCE4.N2.2 newCE7.N2.3 newCE2.LD.1 newCE8.LD.3 newCE5.LD.2 newCE3.MS.1 newCE6.MS.2 newCE9.MS.3 (identical column order as in the scaled expression tab). We apologize for this mistake. T. Keith Blackwell and Collin Ewald ,