Adiponectin Inhibits Osteoclastogenesis and Bone Resorption via APPL1-mediated Suppression of Akt1

Abstract

Adiponectin is an adipokine playing an important role in regulating energy homeostasis and insulin sensitivity. However, the effect of adiponectin on bone metabolism shows contradictory results according to different research studies. In this study femurs were isolated from genetically doublelabeled mBSP9.0Luc/␤-ACT-EGFP transgenic mice and were transplanted into adiponectin knock-out mice or wild type mice to investigate the effect of temporary exposure to adiponectin deficiency on bone growth and metabolism. We found that the growth of bone explants in adiponectin knock-out mice was significantly retarded. Histological analysis, microcomputed tomography analysis, and tartrate-resistant acid phosphatase staining revealed reduced trabecular bone volume, decreased cortical bone, and increased osteoclast number in bone explants in adiponectin knock-out mice. We then found that adiponectin inhibits RANKL-induced osteoclastogenesis from RAW264.7 cells and down-regulates RANKL-enhanced expressions of osteoclastogenic regulators including NFAT2, TRAF6, cathepsin K, and tartrate-resistant acid phosphatase. Adiponectin also increases osteoclast apoptosis and decreases survival/proliferation of osteoclast precursor cells. Using siRNA specifically targeting APPL1, the first identified adaptor protein of adiponectin signaling, we found that the inhibitory effect of adiponectin on osteoclasts was induced by APPL1-mediated down-regulation of Akt1 activity. In addition, overexpression of Akt1 successfully reversed adiponectin-induced inhibition in RANKL-stimulated osteoclast differentiation. In conclusion, adiponectin is important in maintaining the balance of energy metabolism, inflammatory responses, and bone formation.Adipose tissue is not just an inert organ for energy storage. It also secretes proinflammatory cytokines and synthesizes a wide range of biologically active molecules known as adipokines (1, 2). Adiponectin, a 30-kDa protein containing a collagen-repeat domain at the N terminus and a globular domain at the C terminus, is among these adipokines (3). It has been reported that adiponectin plays an important role in regulating energy homeostasis and insulin sensitivity, and plasma adiponectin levels correlate positively with insulin sensitivity (4, 5). APPL1 (adaptor protein containing pleckstrin homology domain, phosphotyrosine domain, and leucine zipper motif), is the first identified protein interacting with adiponectin receptors and is suggested to be an adaptor protein responsible for the mediation of adiponectin signal transduction (6). Knockdown of APPL1 expression resulted in a significant reduction in insulin-stimulated Akt phosphorylation (6). In addition to its insulin-sensitizing effect, adiponectin has also been reported to have potent antiinflammatory properties by suppressing the expressions of inflammatory cytokines while inducing production of anti-inflammatory cytokines (7-9). However, unlike other adipose tissue-derived molecules, adiponectin mRNA and plasma protein levels were shown to decreas...

CCAAT/Enhancer-binding Protein α (C/EBPα) Is Important for Osteoclast Differentiation and Activity

“…We thus focused on investigating whether C/EBP␣ could activate the Akt signaling pathway in OC precursors, which has also been reported to be a strong regulator of OC survival (54 -57). We revealed that C/EBP␣ can induce Akt activation to promote OC survival (58,59). Importantly, this finding also suggests that a threshold level of C/EBP␣ may be required for effective Akt activation and that a drastic increase in C/EBP␣ expression may not substantially enhance Akt activation by RANKL.…”

The Activity of Adiponectin in Bone

“…peripheral blood mononuclear cells (PBMC), mouse primary bone marrow cultures and in RAW264.7 cells [38,43,48,60,61]. Only some of these studies mention the use of columns to remove LPS contamination or show that the effects were dependent on AdipoR1 [59,60].…”

“…One of the challenges of knockout animal studies is the adaption and compensation mechanisms that can sometimes mask the initial effect of the deficiency and lead to mistaken conclusions about the physiological role of the absent protein. An interesting approach aimed to overcome these challenges was used by Tu et al [48] who used femur explants from 3-day-old mice that were transplanted into APN-KO and WT controls and studied the effect of a transient exposure to adiponectin deficiency on bone growth and metabolism. The growth of bone explants was significantly retarded in the APN-KO mice in comparison with WT controls.…”

Role of Osteoglycin in the Linkage between Muscle and Bone

“…Adiponectin is one of the adipokines that is produced in adipose tissue and regulates energy homeostasis and insulin sensitivity. Several studies indicate that adiponectin increases osteoclast apoptosis and decreases proliferation of osteoclast precursor cells (29,30). Leptin, a circulating hormone produced by adipose tissue, is a multifunctional hormone that plays important roles in body weight homeostasis, neuroendocrine function, fertility, immune function, and angiogenesis (31).…”