Regulation of chondrocyte differentiation by IRE1α depends on its enzymatic activity.

Bone morphogenetic protein 2(BMP2) is known to activate unfolded protein response (UPR) signal molecules in chondrogenesis. Inositol-requiring enzyme-1α (IRE1α),as one of three unfolded protein sensors in UPR signaling pathways, can be activated during ER stress. However, the influence on IRE1α in chondrocyte differentiation has not yet been elucidated. Here we present evidence demonstrating that overexpression of IRE1α inhibits chondrocyte differentiation, as revealed by reduced expression of collagen II (ColII), Sox9, collagen X (ColX), matrix metalloproteinase 13 (MMP-13), Indian hedgehog (IHH), Runx2 and enhanced expression of parathyroid hormone-related peptide (PTHrP). Furthermore, IRE1α-mediated inhibition of chondrogenesis depends on its enzymatic activity, since its point mutant lacking enzymatic activity completely loses this activity. The RNase and Kinase domains of IRE1α C-terminal are necessary for its full enzymatic activity and inhibition of chondrocyte differentiation. Mechanism studies demonstrate that granulin-epithelin precursor(GEP), a growth factor known to stimulate chondrogenesis, induced IRE1α expression in chondrogenesis. The expression of IRE1α is depended on GEP signaling, and IRE1α expression is hardly detectable in GEP(-/-) embryos. In addition, IRE1α inhibits GEP-mediated chondrocyte differentiation as a negative regulator. Altered expression of IRE1α in chondrocyte hypertrophy was accompanied by altered levels of IHH and PTHrP. Collectively, IRE1α may be a novel regulator of chondrocyte differentiation by 1) inhibition GEP-mediated chondrocyte differentiation as a negative regulator; 2) promoting IHH/PTHrP signaling.

Guo FJ ，Jiang R ，Li X ，Zhang P ，Han X ，Liu C ... -  《-》

IRE1α dissociates with BiP and inhibits ER stress-mediated apoptosis in cartilage development.

Bone morphogenetic protein 2 is known to activate unfolded protein response signaling molecules, including XBP1S, BiP and IRE1α. Endoplasmic reticulum stress is induced in chondrogenesis and activates IRE1α signal pathway, which is associated with ER stress-mediated apoptosis. However, the influence on IRE1α and BiP in BMP2-induced chondrocyte differentiation has not yet been elucidated; the molecular mechanism remains unexplored. In this study, we demonstrate that IRE1α interacts with BiP in unstressed cells and dissociates from BiP in the course of cartilage development. Induction of ER stress-responsive proteins (XBP1S, IRE1α, BiP) was also observed in differentiating cells. IRE1α inhibition ER stress-mediated apoptosis lies in the process of chondrocyte differentiation. Furthermore, knockdown of IRE1α expression by way of the RNAi approach accelerates ER stress-mediated apoptosis in chondrocyte differentiation induced by BMP2, as revealed by enhanced expressions of cleaved caspase3, CHOP and p-JNK1; and this IRE1α inhibition effect on ER stress-mediated apoptosis is required for BiP in chondrogenesis. Collectively, the ER stress sensors were activated during apoptosis in cartilage development, suggesting that selective activation of ER stress signaling was sufficient for induction of apoptosis. These findings reveal a novel critical role of IRE1α in ER stress-mediated apoptosis and the molecular mechanisms involved. These results suggest that activation of p-JNK1, caspase3 and CHOP was detected in developing chondrocytes and that specific ER stress signaling leads to naturally occurring apoptosis during cartilage development.

Han X ，Zhou J ，Zhang P ，Song F ，Jiang R ，Li M ，Xia F ，Guo FJ ... -  《-》

IRE1a constitutes a negative feedback loop with BMP2 and acts as a novel mediator in modulating osteogenic differentiation.

Guo FJ ，Jiang R ，Xiong Z ，Xia F ，Li M ，Chen L ，Liu CJ ... -  《Cell Death & Disease》

Different Roles of GRP78 on Cell Proliferation and Apoptosis in Cartilage Development.

Xiong Z ，Jiang R ，Li X ，Liu Y ，Guo F ... -  《INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES》

Cbfa1-dependent expression of an interferon-inducible p204 protein is required for chondrocyte differentiation.

Zhang Y ，Kong L ，Carlson CS ，Liu CJ ... -  《CELL DEATH AND DIFFERENTIATION》