Role of a novel Zn finger transcriptional regulator involved in osteoclastogenesis

破骨细胞和巨噬细胞是密切相关的细胞，被认为来自共同的祖细胞。作为一种锌指转录因子，Egr-1被发现是巨噬细胞分化的正向调节因子。我们之前已经证明，用反义寡核苷酸阻断Egr-1的表达可以刺激骨髓培养中破骨细胞的分化。这些数据表明，抑制Egr-1功能的因子参与了破骨细胞的分化。在这项研究中，我们发现了一个BTB/POZ锌指基因，即从破骨细胞cDNA文库中分离出来的破骨细胞衍生的锌指蛋白OCZF，它能有效抑制Egr-1依赖的转录活性。对缺失突变的OCZF蛋白的活性分析表明，POZ和富含Pro的结构域参与了转录抑制活性。组蛋白脱乙酰酶(HDAC)的特异性抑制剂曲古抑菌素A(TSA)显著降低OCZF的抑制作用。有趣的是，TSA的加入严重降低了骨髓培养中破骨细胞的分化。TSA还强烈抑制巨噬细胞系RAW 264的破骨细胞分化，RAW 264可在RANKL刺激下分化为破骨细胞。我们还发现OCZF在该细胞系RAW 264中的表达随着破骨细胞的分化而增加。OCZF在RAW 264细胞中的共性表达抑制了细胞的生长。这些结果表明，OCZF是一种独特的转录调节因子，它选择性地抑制Egr-1反应基因的转录，并由RANKL诱导，参与巨噬细胞的生长调节。此外，我们发现HDAC活性是破骨细胞分化所必需的，而OCZF可能与破骨细胞分化所必需的HDAC部分活性有关。

Osteoclasts and macrophage are closely related cells and are thought to be derived from common progenitor cells. A zinc finger transcription factor, Egr-1 was found to be a positive modulator of macrophage differentiation. We previously have shown that blocking Egr-1 expression by antisense oligonucleotides results in the stimulation of osteoclast differentiation in bone marrow culture. These data suggested that factor which inhibits Egr-1 function is involved in osteoclast differentiation. In this study, we show a BTB/POZ Zn finger gene, OCZF (osteoclast-derived zinc finger protein) isolated from osteoclast cDNA library efficiently suppresses Egr-1-dependent transcriptional activity. Analysis of the activity of deletion mutant OCZF proteins showed that the POZ and proline-rich domains contribute the activity of transcription repression. The trichostatin A (TSA), a specific inhibitor of histone deacetylases (HDAC) significantly reduced the repression effect of OCZF.Interestingly, the addition of TSA severely reduced osteoclast differentiation in bone marrow culture. TSA also strongly inhibited osteoclast differentiation from a macrophage cell line, RAW 264 which can differentiate into osteoclasts by the stimulation of RANKL.We also found that the expression of OCZF was increased along the osteoclast differentiation in this cell line, RAW 264. Ecotropic expression of OCZF in RAW 264 cells resulted in growth inhibition. These results suggest that OCZF is a unique transcriptional regulator which selectively represses the transcription of Egr-1-responsive gene and is induced by RANKL and participates in the regulation of cell growth in macrophage cell. In addition, we show that HDAC activity is required for osteoclast differentiation and OCZF may be contributed to the partial activity of HDAC necessary for osteoclast differentiation.