Mediation of the inhibitory effect of thyroid hormone on the proliferation of hepatoma cells by transforming growth factor-beta

• Main Authors: Yen, Chun-Che, 顏君哲
• Other Authors: Lin, Kwang-Huei
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/96473357064571293532

博士 === 長庚大學 === 基礎醫學研究所 === 94 === Thyroid hormone (T3) is essential for growth, development and differentiation. These biological activities are mediated by its interaction with the nuclear thyroid hormone receptors (TRs), which belong to the steroid/thyroid hormone receptor superfamily of ligand-dependent transcription factors. It has been long recognized that liver is a target organ for TRs. HepG2, a well-differentiated hepatocellular carcinoma cell-line, secretes all 15 plasma proteins. It preserves many liver-specific functions that can be served as an in vitro model. To study the regulation of T3 on the growth of hepatoma cell, TR over-expression hepatoma cell lines (HepG2-TR1 or HepG2-TR) were used. The HepG2-TR1#1 cell growth was inhibited ~55% after 100nM T3 treatment for 72 hr. The colony formation was also inhibited ~80% in soft agar after 100nM T3 treatment for 4 weeks. In an effort to study the mechanism of cell proliferation inhibition after T3 treatment in the HepG2-TR cell lines, we examined the expression of a number of factors that are known to be widely involved in cell cycle. Flow cytometric analysis indicated that the growth inhibitory effect was mainly arrested in G1 into S phase of the cell cycle. The mRNA or protein level of major cell cycle regulators (cyclins, cdks, cdk inhibitors- p21, and Rb) were used to provide more evidences for the growth inhibition of HepG2-TR cell lines. Moreover, p21 was up-regulated 5-fold or 7.3-fold following T3 treatment at protein or mRNA levels, respectively. Cyclin E and cdk2 were down-regulated 27~52% by T3 in a time dependant manner. Phospho-retinoblastoma protein was down-regulated by T3. The expression of transforming growth factor- (TGF- was known to delineate the cell proliferation repression mechanism. TGF- is stimulated by T3 at protein, and RNA levels and its promoter activity was also enhanced 6-8-fold. Furthermore, both T3 and TGF- repressed the expression of cdk2, cyclin E and ppRb and increased p21 protein level. On the other hand, the repression of cdk2, cyclin E and ppRd by T3 was blocked by the TGF- neutralizing antibody but not the control antibody. Interestingly, our lab previous cDNA microarray data found a gene related to cell cycle, prothymosin  (ProT), was down-regulated by T3 in HepG2-TR cell line. The function of ProT has been implicated in increasing cell proliferation mainly when cells enter the S phase. In mRNA and protein level, ProT was repressed by T3 in time and dose manner. These results indicated that T3 might play an important role in the process of liver tumor cell proliferation.