Authors: Aki Kashiwagi Mikaela J Fein Masako Shimada
Publish Date: 2011/05/05
Volume: 89, Issue: 1, Pages: 36-42
Abstract
The ubiquitously expressed calpains1 and 2 belong to a family of calciumdependent intracellular cysteine proteases Both calpains are heterodimers consisting of a large catalytic subunit and a small regulatory subunit encoded by the gene Capn4 Ablation of the calpain small subunit eliminates calpain activity and leads to embryonic lethality We previously created osteoblastspecific Capn4 knockout mice to investigate a physiological role for the calpain small subunit in cells of the osteoblast lineage Deletion of Capn4 reduced trabecular and cortical bone mainly due to impaired proliferation and differentiation of cells of the osteoblast lineage To further investigate an underlining mechanism by which osteoblastspecific Capn4 knockout mice develop an osteoporotic bone phenotype we established osteoblastic cell lines stably expressing either control or Capn4 RNA interference for this study Capn4 knockdown cells showed reduced cell proliferation accumulation of total and phosphorylated cyclindependent kinase inhibitor 1B p27Kip1 on serine 10 and reduced phosphorylation of retinoblastoma protein on threonine 821 Moreover ablation of Capn4 increased 27 Kip1 mRNA levels likely due to stabilized binding of Akt to protein phosphatase 2A which presumably results in reduced phosphorylation of Akt on S473 and forkhead Box O FoxO 3A on T32 Collectively calpain regulates cell proliferative function by modulating both transcription and degradation of p27Kip1 in osteoblasts In conclusion calpain is a critical modulator for regulation of p27Kip1 in cells of the osteoblast lineageWe thank Dr Hanno Hock and Laura B PrickettRice at the Center for Regenerative Medicine Massachusetts General Hospital MGH for consultation and assistance with the flowcytometric analysis This work was partially supported by National Institutes of Health grant R01 DK072102 the William F Milton Fund and the MGH interim support fund to M S
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