| 摘要: | 腫瘤相關的脂肪酸合成（Tumor-associated fatty acid synthase, FASN）的表現在乳癌發生的狀況下，往往與生長因子接受器（Growth factor receptor）的高量表現有關，例如：EGFR（Epidermal growth factor receptor）或是erbB2接受器；而有30%的乳癌患者有erbB2接受器或是它的Trans-ligand，Heregulin（HRG）過量表現的情況；而由上述可以推測：erbB2接受器引發FASN的表現對乳癌發生的進程而言是必需的。在之前的研究中，在人類乳癌細胞株MCF-7，用綠茶多酚 (－)-Epigallocatechin-3-gallate（EGCG），透過抑制PI3K/Akt路徑，造成由EGF引發FASN的表現量下降；這也許提示了，EGCG可以降低由erbB2接受器引發FASN的可能性。在本文中，我們假設EGCG可以抑制由erbB2接受器所引發FASN的表量，且具有降低乳癌細胞增生的能力；所以我們以erbB3接受器的Natural ligand，HRG-β1，刺激MCF-7細胞株，來引發erbB2接受器的活化；我們觀察到HRG-β1可以引發FASN的表現；以加入轉錄抑制劑，Antinomycin D（Act D）與轉譯抑制劑，Cycloheximide（CHX）進行研究，證實了因HRG-β1的影響而引發FASN的表現，其影響點在於轉錄階段（Transcriptional level）。之後為解HRG-β1引發的訊息傳遞路徑是如何提高FASN的表現，所以用專一性的蛋白激抑制劑與Dominant-negative的方法，證實HRG-β1是透過PI3K/Akt路徑與MEK/ERK1/2路徑，引發FASN的表現。我們也發現HRG-β1可以引發erbB2接受器與erbB3接受器可以形成Heterodimer，之後促使FASN的表現。另一方面，EGCG具有降低由HRG-β1所引發FASN表現量的能力，是透過降低Akt如ERK1/2的活性，干擾erbB2接受器與erbB3接受器的偶合與活化，達到效果；這說明了EGCG在乳癌的癌症化學預防上的角色。總而言之，我們的研究證實了在MCF-7乳癌細胞株中，EGCG抑制了由HRG-β1所引發的erbB2與erbB3接受器的Heterodimer，結果降低了MEK/ERK1/2路徑與PI3K/Akt路徑的功能，包括了細胞的生長與存活。
Tumor-associated fatty acid synthase (FASN) has been linked to breast cancer carcinogenesis and can be up-regulated by growth factor receptor such as epidermal growth factor receptor (EGFR) and erbB2. About 30% of breast cancer patients bearing overexpressed erbB2 receptor or its trans-ligand heregulin (HRG) are poor prognosis, suggesting that erbB2-mediated FAS induction may be essential for breast cancer progression. In fact, our previous study shows that green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) could suppress EGF-induced FASN expression in human breast cancer cell line MCF-7 via inhibiting PI3K/Akt pathway, raising the possibility whether EGCG also reduces erb2-mediated FASN expression similar to our previous data as described above.In this study, we hypothesize that EGCG could inhibit erb2-mediated FASN expression and then down-regulate breast cancer cell proliferation. To this end, we used the HRG-β1, a natural ligand of erbB3 which can transactivate erbB2, to stimualte MCF-7 breast cancer cells. We found that HRG-β1 increased FASN expression and this increment of FASN expression could be reduced by the addition of transcriptional inhibitor, antinomycin D (Act D) and translational inhibitor, cycloheximide (CHX), respectively, suggesting that HRG-β1 induces FASN expression at a transcriptional level. Exploring the HRG-β1-mediated signaling pathways involving in FASN induction by specific protein kinase inhibitors and dominant-negative method, we found that PI3K/Akt pathway and MEK/ERK1/2 pathway are essential for the expression of FASN regulated by HRG-β1. We also found that erbB2-erbB3 heterodimerization by HRG-β1 can regulates FASN up-regulation.On the other hand, tea polypolyphenol EGCG strongly inhibited HRG-β1-triggered FASN expression and proliferation by down-regulating ERK1/2 activation, Akt activation and coordination of erbB2 and erbB3, respectively, exhibiting an vital role of EGCG on breast cancer chemoprevention.To sum up, our current study demonstrated that EGCG inhibited HRG-β1-stimulated heterodimerization of erbB2 and erbB3 in MCF-7 cells, and consequently down-regulated the downstream signals including MEK/ERK1/2 pathway and PI3K/Akt pathway whose functions are essential for cell growth and survival. |
