A new ubiquitin ligase involved in p57^KIP2 proteolysis regulates osteoblast cell differentiation

Transforming growth factor-β1 (TGF-β1) has many physiological functions and inhibits the differentiation of osteoblasts. Previously, we reported that TGF-β1 stimulation induces the degradation of p57^KIP2 in osteoblasts. p57^KIP2 proteolysis depends on the ubiquitin-proteasome pathway and SMAD-mediated transcription; however, the molecular mechanism underlying p57^KIP2 degradation has been largely unknown. Here, we show that FBL12, a new F-box protein expressed in the limb bud of developing embryos, is involved in TGF-β1-induced degradation of p57^KIP2. FBL12 formed an SCF^FBL12 complex and directly ubiquitinated p57^KIP2 in a phosphorylation-dependent manner. Inhibition of FBL12 by RNA interference suppressed the degradation of p57^KIP2 and a dominant-negative mutant of FBL12 (FBL12ΔF) increased the steady-state level of p57^KIP2. Furthermore, wild-type FBL12 inhibited and FBL12ΔF promoted the differentiation of primary osteoblasts. As overexpression of p57^KIP2 promoted osteoblast differentiation, these results indicate the importance of FBL12 and the degradation of p57^KIP2 in the regulation of osteoblast cell differentiation.