| Summary: | 博士 === 高雄醫學大學 === 醫學研究所 === 90 === Tamoxifen, originally synthesized as an anti-estrogen drug, was found to be able to inhibit proliferation of estrogen receptor (ER)-negative cancer cells in vitro. However, the molecular basis of such ER-independent growth inhibition is unknown. In this study, we used ER-negative lung cancer cells to clarify the inhibitory effect of Tamoxifen.
Lung carcinoma is one of the most common and devastating human tumors. The regulation of cell growth depends upon the balance between stimulatory and inhibitory growth factors. If a cell gets the ability to grow unlimitedly, it may become a tumor. Recently, study of cell apoptosis has become an important field on tumor treatment. Apoptosis can be induced by extra-cellular signals to make cancer cells die as wished.
According to our study, we found that Tamoxifen induced G1 growth arrest in these cells. However, our results indicated that the expression of G1 cyclins was not affected by Tamoxifen in these lung cancer cells. Additionally, the protein level of G1 acting cyclin-dependent kinases was also unaltered in Tamoxifen-treated lung cancer cells. Subsequently, We examined the effect of Tamoxifen on the expression of cyclin-dependent kinase inhibitors. Our results demonstrated that the expression of p21Waf1 and p27KIP1, but not p57Kip2, was highly elevated by Tamoxifen. Up-regulation of p21Waf1 and p27KIP1also increases the binding with cyclin E /CDK2 and results in Rb hypo-phosphorylation.
In order to determine the molecular mechanism responsible for p21Waf1 and p27kip1 induction by Tamoxifen, we performed a deletion analysis on the p21Waf1 and p27kip1 promoter. The minimal region in the p21Waf1 promoter required for Tamoxifen-activated induction was mapped to a contiguous stretch of 10bp located 83 bases upstream of the transcription initiation site. Besides, the response region in the p27Kip1 promoter required for Tamoxifen-activated induction was located on —545~-532bp upstream of the transcription initiation site. We also try to elucidate the signaling pathway that mediated the activation of p21Waf1 and p27kip1 by Tamoxifen. Tamoxifen does not modulate MAPK and PKC kinase pathway in the activation of p21Waf1 and p27kip1. Conversely, Tamoxifen induced p21Waf1 and p27kip1 activation is regulated by pre-treatment of PKA kinase inhibitor or activator.
Taken together, these results demonstrate that Tamoxifen inhibits cell growth by activate p21Waf1 and p27kip1 expression in ER-negative lung cancer cells and activates the p21Waf1 and p27kip1 promoter via Sp1-binding sites and suggest that PKA may be involved in the induction of p21Waf1 and p27kip1 by Tamoxifen in ER-negative lung cancer cells.
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