Akt1/Akt2 and mammalian target of rapamycin/Bim play critical roles in osteoclast differentiation and survival, respectively, whereas Akt is dispensable for cell survival in isolated osteoclast precursors.
Akt, also known as protein kinase B, is a serine/threonine protein kinase with antiapoptotic activities; also, it is a downstream target of phosphatidylinositol 3-kinase. Here we show that Akt1/Akt2 play a critical role in osteoclast differentiation but not cell survival and that mammalian target of rapamycin (mTOR) and Bim, a pro-apoptotic Bcl-2 family member, are required for cell survival in isolated osteoclast precursors. To investigate the function of Akt1, Akt2, mTOR, and Bim, we employed a retroviral system for delivery of small interfering RNA into cells. Loss of Akt1 and/or Akt2 protein inhibited osteoclast differentiation due to down-regulation of IkappaB-kinase (IKK) alpha/beta activity, phosphorylation of IkappaB-alpha, nuclear translocation of nuclear factor-kappaB (NFkappaB) p50, and NFkappaB p50 DNA-binding activity. Surprisingly, deletion of Akt1 and/or Akt2 protein did not stimulate cleaved caspase-3 activity and failed to promote apoptosis. Conversely, loss of mTOR protein induced apoptosis due to up-regulation of cleaved caspase-3 activity. In addition, we found that mTOR is downstream of phosphatidylinositol 3-kinase (but not Akt) and that macrophage colony-stimulating factor regulates Bim expression through mTOR activation for cell survival. These results demonstrate that Akt1/Akt2 are key elements in osteoclast differentiation and that the macrophage colony-stimulating factor stimulation of mTOR leading to Bim inhibition is essential for cell survival in isolated osteoclast precursors.
Sugatani
Toshifumi
T
Department of Pediatrics, Cell and Molecular Biology Unit, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Hruska
Keith A
KA
D000818:Animals; D051017:Apoptosis Regulatory Proteins; D000072224:Bcl-2-Like Protein 11; D001854:Bone Marrow Cells / Q000166:cytology; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002454:Cell Differentiation / Q000502:physiology; D002455:Cell Division / Q000502:physiology; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D008264:Macrophages / Q000166:cytology; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D010010:Osteoclasts / Q000166:cytology*; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D034741:RNA, Small Interfering; D015398:Signal Transduction / Q000502:physiology; D013234:Stem Cells / Q000166:cytology* / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases
D051017:Apoptosis Regulatory Proteins; D000072224:Bcl-2-Like Protein 11; C000606776:Bcl2l11 protein, mouse; D002352:Carrier Proteins; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; D011494:Protein Kinases; C546843:mTOR protein, mouse; C494922:Akt2 protein, mouse; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases