Department of Medicine and Bioregulatory Sciences, University of Tokushima Graduate School of Medicine, 3-18-15 Kuramoto-cho, Tokushima 770-8503, Japan.
Int J Hematol. 2013 Jul;98(1):66-73. doi: 10.1007/s12185-013-1367-z. Epub 2013 May 25.
Endoplasmic reticulum (ER) stress is induced in matrix-producing osteoblasts and plays an essential role in osteoblastogenesis. Although the bone anabolic activity of proteasome inhibitors has been demonstrated, the roles of ER stress induced by proteasome inhibition in osteoblastogenesis remain largely unknown. Here we show that bortezomib translationally increases protein levels of activating transcription factor 4 (ATF4), a downstream mediator of ER stress, in bone marrow stromal cells and MC3T3-E1 preosteoblastic cells. The suppression of ATF4 expression by siRNA abrogated osteocalcin expression and mineralized nodule formation by MC3T3-E1 cells induced by bortezomib, indicating a critical role for ATF4 in bortezomib-mediated osteoblastogenesis. However, bortezomib at 20 nM or higher abolished the mineralized nodule formation along with reductions in the expression of osteoblastogenesis mediators β-catenin and Osterix. Furthermore, at 50 nM, bortezomib induced the expression of C/EBP homologous protein (CHOP), suggesting activation of the ATF4-CHOP pro-apoptotic pathway. These results suggest that a low dose of bortezomib induces osteogenic activity, but that, in contrast, excessive ER stress caused by bortezomib at higher doses hampers osteoblastogenesis. Therefore, dosing schedules for proteasome inhibitors warrant further study to maximize anabolic actions without compromising anti-MM activity in patients with multiple myeloma (MM).
内质网（ER）应激在产生基质的成骨细胞中被诱导，并在成骨细胞发生中起重要作用。尽管已经证明蛋白酶体抑制剂具有骨合成活性，但蛋白酶体抑制诱导的 ER 应激在成骨细胞发生中的作用在很大程度上仍然未知。在这里，我们表明硼替佐米在骨髓基质细胞和 MC3T3-E1 前成骨细胞中翻译性地上调 ER 应激的下游介质激活转录因子 4（ATF4）的蛋白水平。通过 siRNA 抑制 ATF4 表达可消除硼替佐米诱导的 MC3T3-E1 细胞骨钙素表达和矿化结节形成，表明 ATF4 在硼替佐米介导的成骨细胞发生中起关键作用。然而，20 nM 或更高浓度的硼替佐米会消除矿化结节的形成，并降低成骨细胞发生介质β-连环蛋白和 Osterix 的表达。此外，在 50 nM 时，硼替佐米诱导 C/EBP 同源蛋白（CHOP）的表达，表明 ATF4-CHOP 促凋亡途径的激活。这些结果表明，低剂量的硼替佐米诱导成骨活性，但相反，高剂量的硼替佐米引起的过度 ER 应激会阻碍成骨细胞发生。因此，蛋白酶体抑制剂的剂量方案需要进一步研究，以在不损害多发性骨髓瘤（MM）患者的抗-MM 活性的情况下最大程度地发挥合成作用。
