Overexpression of Bcl2 in Osteoblasts Inhibits Osteoblast Differentiation and Induces Osteocyte Apoptosis

Department of Cell Biology, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Japan.
PLoS ONE (Impact Factor: 3.23). 11/2011; 6(11):e27487. DOI: 10.1371/journal.pone.0027487
Source: PubMed


Bcl2 subfamily proteins, including Bcl2 and Bcl-X(L), inhibit apoptosis. As osteoblast apoptosis is in part responsible for osteoporosis in sex steroid deficiency, glucocorticoid excess, and aging, bone loss might be inhibited by the upregulation of Bcl2; however, the effects of Bcl2 overexpression on osteoblast differentiation and bone development and maintenance have not been fully investigated. To investigate these issues, we established two lines of osteoblast-specific BCL2 transgenic mice. In BCL2 transgenic mice, bone volume was increased at 6 weeks of age but not at 10 weeks of age compared with wild-type mice. The numbers of osteoblasts and osteocytes increased, but osteoid thickness and the bone formation rate were reduced in BCL2 transgenic mice with high expression at 10 weeks of age. The number of BrdU-positive cells was increased but that of TUNEL-positive cells was unaltered at 2 and 6 weeks of age. Osteoblast differentiation was inhibited, as shown by reduced Col1a1 and osteocalcin expression. Osteoblast differentiation of calvarial cells from BCL2 transgenic mice also fell in vitro. Overexpression of BCL2 in primary osteoblasts had no effect on osteoclastogenesis in co-culture with bone marrow cells. Unexpectedly, overexpression of BCL2 in osteoblasts eventually caused osteocyte apoptosis. Osteocytes, which had a reduced number of processes, gradually died with apoptotic structural alterations and the expression of apoptosis-related molecules, and dead osteocytes accumulated in cortical bone. These findings indicate that overexpression of BCL2 in osteoblasts inhibits osteoblast differentiation, reduces osteocyte processes, and causes osteocyte apoptosis.

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    • "Another model for the evaluation of osteocyte functions is osteoblast-specific BCL2 transgenic mice under the control of 2.3-kb Col1a1 promoter, which highly induces the transgene expression in osteoblasts but very weakly in osteocytes. In BCL2 transgenic mice, the number of osteocyte processes is severely reduced (Moriishi et al., 2011), probably due to the formation of a complex of BCL2, actin, and gelsolin, which reduces gelsolinsevering activity to increase actin polymerization (Ke et al., 2010). The reduction in the number of processes occurs in osteoblasts in BCL2 transgenic mice because of the high transgene expression, and osteoblasts with reduced processes are embedded into bone matrix, becoming osteocytes. "
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    ABSTRACT: The major types of osteoporosis in humans are postmenopausal osteoporosis, disuse osteoporosis, and glucocorticoid-induced osteoporosis. Animal models for postmenopausal osteoporosis are generated by ovariectomy. Bone loss occurs in estrogen deficiency due to enhanced bone resorption and impaired osteoblast function. Estrogen receptor α induces osteoclast apoptosis, but the mechanism for impaired osteoblast function remains to be clarified. Animal models for unloading are generated by tail suspension or hind limb immobilization by sciatic neurectomy, tenotomy, or using plaster cast. Unloading inhibits bone formation and enhances bone resorption, and the involvement of the sympathetic nervous system in it needs to be further investigated. The osteocyte network regulates bone mass by responding to mechanical stress. Osteoblast-specific BCL2 transgenic mice, in which the osteocyte network is completely disrupted, can be a mouse model for the evaluation of osteocyte functions. Glucocorticoid treatment inhibits bone formation and enhances bone resorption, and markedly reduces cancellous bone in humans and large animals, but not consistently in rodents. Copyright © 2015. Published by Elsevier B.V.
    Preview · Article · Mar 2015 · European journal of pharmacology
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    • "When the osteoblasts with reduced number of processes are embedded into bone matrix and become osteocytes, the osteocytes also have a reduced number of processes, and the number of canaliculi, which the processes pass through, is also reduced. The osteocytes cannot get enough oxygen, nutrient, and survival factors through Gap junction and canaliculi and die by apoptosis.[19] Indeed, secondary necrosis occurs in these osteocytes but inflammatory reaction does not occur, because the number of canaliculi is severely reduced and immunostimulatory molecules cannot be released from lacunae to the bone surface and vascular channels (Fig. 1). "
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    ABSTRACT: Osteocytes establish an extensive intracellular and extracellular communication system via gap junction-coupled cell processes and canaliculi, through which cell processes pass throughout bone, and the communication system is extended to osteoblasts on the bone surface. To examine the osteocyte function, several mouse models were established. To ablate osteocytes, osteocytes death was induced by diphtheria toxin. However, any types of osteocyte death result in necrosis, because dying osteocytes are not phagocytosed by scavengers. After the rupture of cytoplasmic membrane, immunostimulatory molecules are released from lacunae to bone surface through canaliculi, and stimulate macrophages. The stimulated macrophages produce interleukin (IL)-1, IL-6, and tumor necrosis factor-alpha (TNF-α), which are the most important proinflammatory cytokines triggering inflammatory bone loss. Therefore, the osteocyte ablation results in necrosis-induced severe osteoporosis. In conditional knockout mice of gap junction protein alpha-1 (GJA1), which encodes connexin 43 in Gap junction, using dentin matrix protein 1 (DMP1) Cre transgenic mice, osteocyte apoptosis and enhanced bone resorption occur, because extracellular communication is intact. Overexpression of Bcl-2 in osteoblasts using 2.3 kb collagen type I alpha1 (COL1A1) promoter causes osteocyte apoptosis due to the severe reduction in the number of osteocyte processes, resulting in the disruption of both intracellular and extracellular communication systems. This mouse model unraveled osteocyte functions. Osteocytes negatively regulate bone mass by stimulating osteoclastogenesis and inhibiting osteoblast function in physiological condition. Osteocytes are responsible for bone loss in unloaded condition, and osteocytes augment their functions by further stimulating osteoclastogenesis and further inhibiting osteoblast function, at least partly, through the upregulation of receptor activator of nuclear factor-kappa B ligand (RANKL) in osteoblasts and Sost in osteocytes in unloaded condition.
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    • "It is estimated that 60–80% of osteoblasts that originally assembled at the resorption pit die by apoptosis. Further, bone loss caused by sex steroid deficiency, glucocorticoid excess, or aging is caused in part by osteoblast apoptosis [9]. "
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