Biol. Pharm. Bull. 30(11) 2018—2026 (2007)

genated aromatic hydrocarbon that exerts reproductive toxicity, developmental teratogenic effects, and carcinogenicity. Generally, most of the toxic effects of TCDD are mediated through specific binding to the cytosolic aromatic hydrocarbon receptor (AhR). The AhR is a ligand-activated basic region-helix–loop–helix transcription factor that forms a heterodimeric complex with the AhR nuclear translocator. This complex binds to aromatic hydrocarbon-responsive elements (AhREs, also called DREs) in the 5 -flanking region of target genes, and acts as a transcriptional activator. However, some of the functions of TCDD work independently of AhR. Therefore, the molecular mechanisms underlying the toxicity of TCDD have not yet been fully illustrated. TCDD is regarded as an endocrine-disrupting chemical. It has been reported to exert both estrogenic and antiestrogenic effects. Several studies have shown that TCDD reduced both cellular estradiol secretion and estradiol-mediated biologic effects. As an important endocrine-hormone, estrogen is essential for the regulation of the growth, differentiation, and function of target cells. The estrogen receptor (ER), a member of the steroid-thyroid hormone receptor superfamily, mediates its action by binding ligand dependently to the estrogen-responsive element (ERE) in the target gene promoter, regulating their transcription directly. ER has been found in female organs and nonreproductive systems, such as the central nervous system, cardiovascular system, and skeletal system. However, despite the wide variety of estrogen actions, relatively few genes that are directly responsive to this hormone have been identified. Insulin-like growth factors (IGFs) are peptides displaying important functions in regulating cell proliferation, differentiation and metabolism. IGFs are modulated by a family of seven high-affinity IGF binding proteins (IGFBP 1—7). Among them, IGFBP-6 has a 20—100 fold higher binding affinity for IGF-II over IGF-I. The overexpression of IGFBP6 inhibited neuroblastoma growth in vivo and proliferation of human bronchial epithelial cells. Furthermore, IGFBP-6 activated programmed cell death in non-small cell lung cancer cells. In bone, both IGF-II and IGFBP-6 are potent mitogens of osteoblasts, in which the IGF system may play an integral role in skeletal development. In an attempt to understand the underlying mechanisms of the toxic effects of TCDD on osteogenesis, we have previously investigated the regulation of TCDD on IGFBP-6 gene in vivo. Here, we present evidence that IGFBP-6, as a crucial modulator of IGF bioavailability, is expressed in the rat fetal calvaria and osteoblastic osteosarcoma cell line SaOS-2, in which TCDD increased the abundance of IGFBP-6 mRNA and exerts growth-inhibitory effects in the presence of 17-bestradiol (E2). In this study, we first examined the effects of TCDD on estrogen-mediated osteogenesis through a functional ERE on the IGFBP-6 gene promoter. These results should contribute to a better understanding of the molecular mechanisms of the osteogenetic toxicity of TCDD.