Smith Elisheva, Frenkel Baruch
Department of Orthopedic Surgery and Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California 90033, USA.
J Biol Chem. 2005 Jan 21;280(3):2388-94. doi: 10.1074/jbc.M406294200. Epub 2004 Nov 10.
Glucocorticoids, widely used as immune suppressors, cause osteoporosis by inhibiting bone formation. In MC3T3-E1 osteoblast-like cultures, dexamethasone (DEX) activates glycogen synthase kinase-3beta (GSK3beta) and inhibits a differentiation-related cell cycle that occurs at a commitment stage immediately after confluence. Here we show that DEX inhibition of the differentiation-related cell cycle is associated with a decrease in beta-catenin levels and inhibition of LEF/TCF-mediated transcription. These inhibitory activities are no longer observed in the presence of lithium, a GSK3beta inhibitor. DEX decreased the serum-responsive phosphorylation of protein kinase B/Akt-Ser(473) within minutes, and this inhibition was also observed after 12 h. When the phosphatidylinositol 3-kinase (PI3K)/Akt pathway was inhibited by wortmannin, DEX no longer inhibited beta-catenin levels. Furthermore, DEX-mediated inhibition of LEF/TCF transcriptional activity was attenuated in the presence of dominant negative forms of either PI3K or protein kinase B/Akt. These results suggest cross-talk between the PI3K/Akt and Wnt signaling pathways. Consistent with a role for Wnt signaling in the osteoblast differentiation-related cell cycle, wortmannin partially negated the DEX inhibition of this cell cycle. DEX also induced histone deacetylase (HDAC) 1, which is known to inhibit LEF/TCF transcriptional activity. Overexpression of HDAC1 negated the inhibitory effect of DEX on LEF/TCF transcriptional activity. In the presence of trichostatin A, a deacetylase inhibitor, DEX-mediated inhibition of the differentiation-related cell cycle was partially negated. When administered together, wortmannin and trichostatin A completely negated the inhibitory effect of DEX on the differentiation-related cell cycle. These results suggest that inhibition of a PI3K/Akt/GSK3beta/beta-catenin/LEF axis and stimulation of HDAC1 cooperate to mediate the inhibitory effect of DEX on Wnt signaling and the osteoblast differentiation-related cell cycle.
糖皮质激素作为广泛使用的免疫抑制剂，通过抑制骨形成导致骨质疏松。在MC3T3-E1成骨细胞样培养物中，地塞米松（DEX）激活糖原合酶激酶-3β（GSK3β）并抑制汇合后立即进入的一个与分化相关的细胞周期，该细胞周期发生在一个决定性阶段。在此我们表明，DEX对与分化相关的细胞周期的抑制与β-连环蛋白水平的降低以及LEF/TCF介导的转录的抑制相关。在存在GSK3β抑制剂锂的情况下，这些抑制活性不再被观察到。DEX在数分钟内降低了蛋白激酶B/Akt-Ser(473)的血清反应性磷酸化，并且在12小时后也观察到了这种抑制。当磷脂酰肌醇3-激酶（PI3K）/Akt途径被渥曼青霉素抑制时，DEX不再抑制β-连环蛋白水平。此外，在存在PI3K或蛋白激酶B/Akt的显性负性形式的情况下，DEX介导的对LEF/TCF转录活性的抑制作用减弱。这些结果提示PI3K/Akt和Wnt信号通路之间存在相互作用。与Wnt信号在成骨细胞分化相关细胞周期中的作用一致，渥曼青霉素部分抵消了DEX对该细胞周期的抑制。DEX还诱导了组蛋白脱乙酰酶（HDAC）1，已知其可抑制LEF/TCF转录活性。HDAC1的过表达抵消了DEX对LEF/TCF转录活性的抑制作用。在存在去乙酰化酶抑制剂曲古抑菌素A的情况下，DEX介导的对与分化相关的细胞周期的抑制作用部分被抵消。当一起给予时，渥曼青霉素和曲古抑菌素A完全抵消了DEX对与分化相关的细胞周期的抑制作用。这些结果表明，对PI3K/Akt/GSK3β/β-连环蛋白/LEF轴的抑制以及对HDAC1的刺激共同介导了DEX对Wnt信号和与成骨细胞分化相关的细胞周期的抑制作用。
