No abstract

Macroautophagy/autophagy is a highly regulated process involved in the turnover of cytosolic components, however its pivotal role in maintenance of bone homeostasis remains elusive. In the present study, we investigated the direct role of ATG7 (autophagy related 7) during developmental and remodeling stages in vivo using osteoblast-specific Atg7 conditional knockout (cKO) mice. Atg7 cKO mice exhibited a reduced bone mass at both developmental and adult age. The trabecular bone volume of Atg7 cKO mice was significantly lower than that of controls at 5 months of age. This phenotype was attributed to decreased osteoblast formation and matrix mineralization, accompanied with an increased osteoclast number and the extent of the bone surface covered by osteoclasts as well as an elevated secretion of TNFSF11/RANKL (tumor necrosis factor [ligand] superfamily, member 11), and a decrease in TNFRSF11B/OPG (tumor necrosis factor receptor superfamily, member 11b [osteoprotegerin]). Remarkably, Atg7 deficiency in osteoblasts triggered endoplasmic reticulum (ER) stress, whereas attenuation of ER stress by administration of phenylbutyric acid in vivo abrogated Atg7 ablation-mediated effects on osteoblast differentiation, mineralization capacity and bone formation. Consistently, Atg7 deficiency impeded osteoblast mineralization and promoted apoptosis partially in DDIT3/CHOP (DNA-damage-inducible transcript 3)- and MAPK8/JNK1 (mitogen-activated protein kinase 8)-SMAD1/5/8-dependent manner in vitro, while reconstitution of Atg7 could improve ER stress and restore skeletal balance. In conclusion, our findings provide direct evidences that autophagy plays crucial roles in regulation of bone homeostasis and suggest an innovative therapeutic strategy against skeletal diseases.

Reduced caloric intake in mammals causes reduced skeletal growth and GH insensitivity. However, the underlying molecular mechanisms are not fully elucidated. The aim of this study was to determine whether the increased activity of fibroblast growth factor 21 (FGF21) during chronic undernutrition in mice causes GH insensitivity and growth failure. After 4 wk of food restriction, fgf21 knockout (KO) mice exhibited greater body and tibial growth than their wild-type (WT) littermates. Daily injections of recombinant human FGF21 in a subgroup of food-restricted fgf21 KO mice prevented these differences in body and tibial growth. At the end of the 4-wk food restriction, GH binding and GH receptor expression were reduced in the liver and in the growth plate of food-restricted WT mice (compared to WT mice fed ad libitum), whereas they were similar between food-restricted and ad libitum KO mice. In addition, a single injection of GH induced greater liver signal transducer and activator of transcription 5 phosphorylation and IGF-I mRNA in food-restricted KO mice than in WT mice. Lastly, in the tibial growth plate of food-restricted WT mice, FGF21 mRNA and protein expression was greater than that of WT mice fed ad libitum. In contrast, the IGF-I mRNA and protein expression was smaller. Our findings support a causative role for FGF21 in the inhibition of skeletal growth during prolonged undernutrition. Such role may be mediated by the antagonistic effect of FGF21 on GH action in the liver and, possibly, in the growth plate.

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.