科研之友
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摘要
Background: We have previously reported that repeated treatment of human periodontal ligament cells and murine pre-osteoblast MC3T3-E1 cells with transforming growth factor-beta 1 (TGF-beta 1) inhibited their osteoblastic differentiation because of decreased insulin-like growth factor-1 (IGF-1) secretion. We also found that IGF-1/PI3K signaling plays an important role in osteoblast differentiation induced by TGF-beta 1 treatment; however, the downstream signaling controlling this remains unknown. The aim of this current study is to investigate whether Akt activation is required for osteoblast differentiation. Methodology/Principal Findings: MC3T3-E1 cells were cultured in osteoblast differentiation medium (OBM) with or without 0.1 ng/mL TGF-beta 1. OBM containing TGF-beta 1 was changed every 12 h to provide repeated TGF-beta 1 administration. MC3T3-E1 cells were infected with retroviral vectors expressing constitutively active (CA) or dominant-negative (DN)-Akt. Alkaline phosphatase (ALP) activity and osteoblastic marker mRNA levels were substantially decreased by repeated TGF-beta 1 treatment compared with a single TGF-beta 1 treatment. However, expression of CA-Akt restored ALP activity following TGF-beta 1 treatment. Surprisingly, ALP activity increased following multiple TGF-beta 1 treatments as the number of administrations of TGF-beta 1 increased. Activation of Akt significantly enhanced expression of osteocalcin, but TGF-beta 1 treatment inhibited this. Mineralization of MC3T3-E1 cells was markedly enhanced by CA-Akt expression under all medium conditions. Exogenous IGF-1 restored the down-regulation of osteoblast-related gene expression by repeated TGF-beta 1 administration. However, in cells expressing DN-Akt, these levels remained inhibited regardless of IGF-1 treatment. These findings indicate that Akt activation is required for the early phase of osteoblast differentiation of MC3T3-E1 cells induced by TGF-beta 1. However, Akt activation is insufficient to reverse the inhibitory effects of TGF-beta 1 in the late stages of osteoblast differentiation. Conclusions: TGF-beta 1 could be an inducer or an inhibitor of osteoblastic differentiation of MC3T3-E1 cells depending on the state of Akt phosphorylation. Our results indicate that Akt is the molecular switch for TGF-beta 1-induced osteoblastic differentiation of MC3T3-E1 cells.
- 出版日期2014-12-3
