Lowering circulating apolipoprotein E levels improves aged bone fracture healing
Rong Huang, Xiaohua Zong, Puviindran Nadesan, Janet L. Huebner, Virginia B. Kraus, James P. White, Phillip J. White, Gurpreet S. Baht
Rong Huang, Xiaohua Zong, Puviindran Nadesan, Janet L. Huebner, Virginia B. Kraus, James P. White, Phillip J. White, Gurpreet S. Baht
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Research Article Bone biology

Lowering circulating apolipoprotein E levels improves aged bone fracture healing

Abstract

Age is a well-established risk factor for impaired bone fracture healing. Here, we identify a role for apolipoprotein E (ApoE) in age-associated impairment of bone fracture healing and osteoblast differentiation, and we investigate the mechanism by which ApoE alters these processes. We identified that, in both humans and mice, circulating ApoE levels increase with age. We assessed bone healing in WT and ApoE–/– mice after performing tibial fracture surgery: bone deposition was higher within fracture calluses from ApoE–/– mice. In vitro recombinant ApoE (rApoE) treatment of differentiating osteoblasts decreased cellular differentiation and matrix mineralization. Moreover, this rApoE treatment decreased osteoblast glycolytic activity while increasing lipid uptake and fatty acid oxidation. Using parabiosis models, we determined that circulating ApoE plays a strong inhibitory role in bone repair. Using an adeno-associated virus–based siRNA system, we decreased circulating ApoE levels in 24-month-old mice and demonstrated that, as a result, fracture calluses from these aged mice displayed enhanced bone deposition and mechanical strength. Our results demonstrate that circulating ApoE as an aging factor inhibits bone fracture healing by altering osteoblast metabolism, thereby identifying ApoE as a new therapeutic target for improving bone repair in the elderly.

Authors

Rong Huang, Xiaohua Zong, Puviindran Nadesan, Janet L. Huebner, Virginia B. Kraus, James P. White, Phillip J. White, Gurpreet S. Baht

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Figure 3

rApoE treatment decreases osteoblast glycolytic activity.

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rApoE treatment decreases osteoblast glycolytic activity. WT osteoblasts...
WT osteoblasts differentiated in the absence and in the presence of rApoE were assessed for metabolic changes. (A) The Seahorse XF24 extracellular flux metabolic analyzer was used to measure the extracellular acidification rate (ECAR) in response to the addition of glucose, oligomycin (oligo), and 2-deoxyglucose (2DG). These observations were used to determine (B) basal glycolysis and (C) glycolytic capacity. (D) Glucose uptake was measured by pulsing differentiating osteoblasts with 3H-2-deoxyglucose. (E) Transcript levels of key genes involved in glycolytic metabolism were assessed using RT-PCR. (F) Immunohistochemistry was used to investigate the level of Glut1 in healing, 21-day fracture calluses (osteoblasts indicated by arrows). (G) Lipid uptake in cultures was measured in both the absence and presence of rApoE as was (H) fatty acid oxidation (FAO). For metabolic flux experiments, WT + vehicle, n = 5; WT + rApoE, n = 5. For RT-PCR, WT + vehicle, n = 6; WT + rApoE, n = 6. For IHC, WT, n = 4; ApoE–/–, n = 4. For lipid uptake and FAO, WT + vehicle, n = 6, WT + rApoE, n = 6. Data are expressed as mean ± 95% confidence interval. *P < 0.05.