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Thyroid Hormone Receptors Suppress Pituitary Tumor Transforming Gene 1 Activity in Hepatoma

Departments of ¹ Biochemistry and ² Medicine, School of Medicine, Chang-Gung University, Taoyuan, Taiwan, and ³ Liver Research Unit, Chang-Gung Medical Center, Taipei, Taiwan, Republic of China

Requests for reprints: Kwang-Huei Lin, Department of Biochemistry, Chang-Gung University, 259 Wen-hwa 1 Road, Taoyuan, Taiwan, Republic of China. Phone/Fax: 886-3-2118263; E-mail: khlin{at}mail.cgu.edu.tw.

Key Words: PTTG1 • thyroid hormone • receptor

Pituitary tumor transforming gene 1 (PTTG1) is expressed in most tumors. However, whether thyroid hormone (T₃) and its receptors (TR) regulate PTTG1 in human hepatocellular carcinomas (HCC) remains unclear. Previous cDNA microarrays revealed PTTG1 is down-regulated by T₃/TR. This study investigated the significance of PTTG1 regulation by T₃ in HCC cells. The PTTG1 mRNA and protein expression were repressed by T₃ in HCC cell lines overexpressing TR. However, after knockdown of TRs expression by RNA interference, PTTG1 repression by T₃ was abolished. Similar results were observed in thyroidectomized rats. To localize the regulatory region in the PTTG1 promoter, serial deletions within the PTTG1 promoter region were constructed. The promoter activity of the PTTG1 gene was repressed (25–51%) by T₃. Additionally, these findings indicate that PTTG1 may be regulated by Sp1. The critical role of the –594 and –520 Sp1 binding sites was confirmed by electrophoretic mobility shift assay. Transfection with Sp1 expression vector enhanced the activity of the PTTG1 promoter fragment reporter. Also, Sp1 was down-regulated in HCC cells and in thyroidectomized rat after T₃ treatment. Additionally, ectopic expression of PTTG1 promotes cell proliferation in Hep3B hepatoma cells. Conversely, knockdown of PTTG1 or Sp1 expression reduced cell proliferation in HepG2 cells. Notably, the expression of PTTG1 and Sp1 was inversely correlated with the expression of TR proteins in HCC. Together, these findings indicate that PTTG1 gene expression is mediated by Sp1 and is indirectly down-regulated by T₃. Finally, overexpression of PTTG1 or SP1 in HCCs is TR-dependent and crucial in the development of HCC. [Cancer Res 2008;68(6):1697–1706]