| Summary: | 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-TR1 or HepG2-TR) were used. The HepG2-TR1#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.
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