TGF beta 1 Suppressed Matrix Mineralization of Osteoblasts Differentiation by Regulating SMURF1-C/EBP beta-DKK1 Axis

Abstract

Transforming growth factor beta 1 (TGF beta 1) is a major mediator in the modulation of osteoblast differentiation. However, the underlying molecular mechanism is still not fully understood. Here, we show that TGF beta 1 has a dual stage-dependent role in osteoblast differentiation; TGF beta 1 induced matrix maturation but inhibited matrix mineralization. We discovered the underlying mechanism of the TGF beta 1 inhibitory role in mineralization using human osteoprogenitors. In particular, the matrix mineralization-related genes of osteoblasts such as osteocalcin (OCN), Dickkopf 1 (DKK1), and CCAAT/enhancer-binding protein beta (C/EBP beta) were dramatically suppressed by TGF beta 1 treatment. The suppressive effects of TGF beta 1 were reversed with anti-TGF beta 1 treatment. Mechanically, TGF beta 1 decreased protein levels of C/EBP beta without changing mRNA levels and reduced both mRNA and protein levels of DKK1. The degradation of the C/EBP beta protein by TGF beta 1 was dependent on the ubiquitin-proteasome pathway. TGF beta 1 degraded the C/EBP beta protein by inducing the expression of the E3 ubiquitin ligase Smad ubiquitin regulatory factor 1 (SMURF1) at the transcript level, thereby reducing the C/EBP beta-DKK1 regulatory mechanism. Collectively, our findings suggest that TGF beta 1 suppressed the matrix mineralization of osteoblast differentiation by regulating the SMURF1-C/EBP beta-DKK1 axis.