ABSTRACT
Mitochondrial dysfunction has emerged as an important factor in wide ranging human pathologies. We have previously defined a retrograde signaling pathway that originates from dysfunctional mitochondria (Mt-RS) and causes a global nuclear transcriptional reprograming as its end point. Mitochondrial dysfunction causing disruption of mitochondrial membrane potential and consequent increase in cytosolic calcium [Ca(2) ](c) activates calcineurin and the transcription factors NF-κB, NFAT, CREB, and C/EBPδ. In macrophages, this signaling complements receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastic differentiation. Here, we show that the Mt-RS activated transcriptional coactivator heterogeneous ribonucleoprotein A2 (hnRNP A2) is induced by hypoxia in murine macrophages. We demonstrate that the cathepsin K gene (Ctsk), one of the key genes upregulated during osteoclast differentiation, is transcriptionally activated by Mt-RS factors. HnRNP A2 acts as a coactivator with nuclear transcription factors, cRel, and C/EBPδ for Ctsk promoter activation under hypoxic conditions. Notably, our study shows that hypoxia-induced activation of the stress target factors mediates effects similar to that of RANKL with regard to Ctsk activation. We therefore suggest that mitochondrial dysfunction and activation of Mt-RS, induced by various pathophysiologic conditions, is a potential risk factor for osteoclastogenesis and bone loss.
Subject(s)
Cathepsin K/metabolism , Mitochondria/metabolism , Osteoclasts/metabolism , Osteogenesis , Promoter Regions, Genetic , Signal Transduction , Animals , CCAAT-Enhancer-Binding Protein-delta/antagonists & inhibitors , CCAAT-Enhancer-Binding Protein-delta/genetics , CCAAT-Enhancer-Binding Protein-delta/metabolism , Cathepsin K/antagonists & inhibitors , Cathepsin K/chemistry , Cathepsin K/genetics , Cell Hypoxia , Cyclic AMP Response Element-Binding Protein/antagonists & inhibitors , Cyclic AMP Response Element-Binding Protein/genetics , Cyclic AMP Response Element-Binding Protein/metabolism , Genes, Reporter , Heterogeneous-Nuclear Ribonucleoprotein Group A-B/antagonists & inhibitors , Heterogeneous-Nuclear Ribonucleoprotein Group A-B/genetics , Heterogeneous-Nuclear Ribonucleoprotein Group A-B/metabolism , Mice , Mitochondria/enzymology , NFATC Transcription Factors/antagonists & inhibitors , NFATC Transcription Factors/genetics , NFATC Transcription Factors/metabolism , Osteoclasts/cytology , Osteoclasts/enzymology , RANK Ligand/metabolism , RAW 264.7 Cells , RNA Interference , RNA, Messenger/metabolism , Recombinant Proteins/chemistry , Recombinant Proteins/metabolism , Transcriptional ActivationABSTRACT
Previously we showed that hypoxia-induced mitochondrial respiratory stress in RAW 264.7 macrophages and other cells caused activation of retrograde signaling (also known as mitochondrial respiratory stress signaling) and the appearance of tartrate-resistant acid phosphatase (TRAP)-positive cells. In the present study, we used N-acetyl cysteine and ascorbate (general antioxidants) and MitoQ, a mitochondria-specific antioxidant, to investigate the role of intracellular reactive oxygen species (ROS) in osteoclast differentiation. Our results show that hypoxia-mediated mitochondrial dysfunction, as tested by disruption of mitochondrial transmembrane potential, was suppressed by MitoQ as well as by the other antioxidants. These agents also suppressed the activation of mitochondrial retrograde signaling. Interestingly, in terms of molar concentrations, MitoQ was more than 1000-fold more effective than general antioxidants in suppressing the receptor activator of nuclear factor-B ligand-induced differentiation of RAW 264.7 cells into multinucleated and TRAP-positive osteoclasts. We propose that mitochondrial function and intramitochondrial ROS play important roles in osteoclastogenesis.
