探討女性肝細胞癌中雌激素路徑活性降低之機制研究

Abstract

肝細胞癌(HCC)為國內十大癌症死因之一，目前已發現肝細胞癌發生率在男女間有明顯差異：男性高於女性，其中又以HBV相關的肝細胞癌更為明顯，男女比可達約7:1。性荷爾蒙被推測與性別差異相關。雌激素受體α(Estrogen receptor α, ERα)與雌激素已被發現是一種肝細胞癌之保護因子，可使肝細胞癌在女性病患中有較低發生率。 先前實驗室研究發現，在約60%女性肝癌病患中，相較於非腫瘤組織，腫瘤組織的雌激素受體α蛋白質表現量明顯下降，且此下降主要受微型核醣核酸-18a (miR-18a)之調控。而本研究嘗試探討另外40%有相對正常雌激素受體α表現量的患者，其雌激素通路的活性降低與否，及是否有其他機制使雌激素通路的對HCC的保護功能弱化。 本研究選定的是具有相對較多的雌激素受體α表現(T/N≥0.5)、更年期前的女性肝癌病患(≤50歲，以排除雌激素的變因)，測定雌激素受體α的目標基因表現量。結果顯示此群病患的雌激素受體α通路的功能也降低了。為了瞭解潛在的調控機制，我們首先檢驗在肝腫瘤區是否有雌激素受體α的體細胞突變或是負向抑制變體(dominant negative variants)的增加。但此兩種機制都不是在這群病人中使雌激素通路功能下降的原因。之後，我們檢查兩個雌激素受體α重要的共同因子－Foxa1/2及HNF4α在此群病人檢體中的表現情形。我們發現此二種蛋白質在大部分的病人的腫瘤區都有增加。接著，用細胞實驗證明活化的HNF4α可以抑制雌激素受體α的轉錄因子的能力，為雌激素受體α較多的女性肝細胞癌，提供了可能的機制，而Foxa1/2在此機制中的作用仍需進一步探詢。

Hepatocellular carcinoma (HCC) occurs mainly in men, which is even more evident in HBV-related HCC (with the male to female ratio up to 7 to 1). The sex hormones have long been implicated involved in regulating such gender difference. The estrogen and estrogen receptor α (ERα) were identified as protective factors for HCC and contributing for the lower tumor incidence of HCC in female patients. . Our previous studies pointed out that the expression of ERα protein is decreased in about 60% of female HCC. This could be attributed by the elevation of miR-18a, which targets and decrease the expression of ERα protein. The current study is focused on the rest 40% HCC that have rather normal ERα protein level, aiming to investigate if the activity of estrogen pathway is also decreased in these patients, and moreover if there is any other mechanisms contributing to weaken the protective ability of estrogen pathway in these female HCC. The female patients (age≤50) with high ERα in the HCC tissues were selected for assaying the expression level of specific target gene of ERα. The results indicated that the function of ERα pathway is also decreased in this subgroup of patients. To delineate the underlying mechanisms, we first checked if the somatic mutations or the increase of the dominant negative splicing variants of ERα occurs in the tumorous liver tissues. However, these two mechanisms might not contribute to the decrease of estrogen pathway activity in these HCC. We next examined any aberrations of two critical regulators for ERα function in liver, Foxa1/2 and HNF4α, for their expression pattern in these HCC. Interestingly, the protein level of these two proteins was increased in many HCC. By cell culture based assay, we found that active HNF4α can inhibit the transcriptional activity of ERα. It thus provides one possible mechanism for decreasing the function of ERα in female HCC with high ERα. The involvement of Foxa1/2 in this novel regulatory mechanism awaits further investigation.