Abstract
During endochondral ossification, chondrocytes undergo hypertrophic differentiation and die by apoptosis. The level of inorganic phosphate (P(i)) elevates at the site of cartilage mineralization, and when chondrocytes were treated with P(i), they underwent rapid apoptosis. Gene silencing of the proapoptotic Bcl-2 homology 3-only molecule bnip3 significantly suppressed P(i)-induced apoptosis. Conversely, overexpression of Bcl-xL suppressed, and its knockdown promoted, the apoptosis of chondrocytes. Bnip3 was associated with Bcl-xL in chondrocytes stimulated with P(i). Bcl-xL was expressed uniformly in the growth plate chondrocytes, whereas Bnip3 expression was exclusively localized in the hypertrophic chondrocytes. Finally, we generated chondrocyte-specific bcl-x knock-out mice using the Cre-loxP recombination system, and we provided evidence that the hypertrophic chondrocyte layer was shortened in those mice because of an increased apoptosis of prehypertrophic and hypertrophic chondrocytes, with the mice afflicted with dwarfism as a result. These results suggest the pivotal role of Bcl-2 family members in the regulation of chondrocyte apoptosis.
Highlights
During endochondral ossification, chondrocytes undergo hypertrophic differentiation and die by apoptosis
The importance of phosphate ions in chondrocyte apoptosis was confirmed by Demay and co-workers [14], who reported that hypophosphatemia leads to impaired apoptosis of hypertrophic chondrocytes and subsequent expansion of the late hypertrophic chondrocyte layer in vitamin D receptor-null mice, which was reversed by feeding with a high phosphate diet
Disorders in Pi homeostasis result in abnormal endochondral ossification, and hypophosphatemia caused by inefficient phosphate reabsorption by the kidney is associated with defective mineralization of the skeleton, which manifests as rickets or osteomalacia [34, 35]
Summary
There are two distinct signaling pathways of apoptosis in mammals. One is initiated by death receptors (death receptor pathways) [15], and the other is regulated by anti- and pro-apoptotic Bcl-2 family members and involves release of cytochrome c from mitochondria into the cytoplasm (mitochondrial pathways) [16, 17]. The anti-apoptotic Bcl-2 family members include mammalian Bcl-2, Bcl-xL, and Mcl-1, and more than 20 pro-apoptotic Bcl-2 family proteins have been identified to date in mammals, which are divided into two groups as follows: multidomain members (Bax, Bak, Bok/Mtd, etc.) and BH3 domain-only members (Bid, Bad, Bim, Bik, Puma, Noxa, Bmf, Hrk, Bnip, Nix, etc.) [18]. The BH3-only family members display tissue-specific distribution patterns, multidomain pro-apoptotic members are ubiquitously expressed, indicating that BH3-only proteins play a tissue/cellspecific and a stimulus-specific role in apoptosis and that the other members play an essential role further downstream [18]. We report here that the balance between the anti-apoptotic Bcl-2 family member Bcl-xL and the pro-apoptotic family member Bnip critically regulates the apoptosis of terminally differentiated chondrocytes both in vitro and in vivo. We generated mutant mice conditionally deficient in the bcl-x gene using the Cre-loxP recombination system, and we found that the hypertrophic chondrocyte layer was shortened in these mice because of increased apoptosis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
