BCAS2調控p53與雄性素受體蛋白質功能之研究

Abstract

BCAS2是一個小分子量的核蛋白質(26kDa)。在前人的研究中發現其為雌性素受體的一個轉錄共同活化因子。本研究中我們發現BCAS2可以直接與p53蛋白質結合。在一般狀況下BCAS2會輕微且持續性地減低p53蛋白質進而降緩了p53蛋白質的轉錄活性。然而當DNA發生損傷時, BCAS2則顯著的減低p53蛋白質，藉由這樣的作用方式來抵抗doxorubicin這類化療藥物對細胞所造成的傷害。在p53野生型的細胞中，抑制BCAS2的表現會誘發細胞自裁作用，但是在不俱有p53或是p53突變型的細胞中，抑制BCAS2則會使細胞生長週期停滯在G2/M時期。在抑制p53野生型細胞中BCAS2的表現時，至少有兩個導制細胞自裁的作用機制被啟動，其一為增加了細胞核中p53蛋白質的滯留，進而啟動了細胞自裁相關基因的表現；其二為增加p53蛋白質上Ser46氨基酸磷酸化程度，藉此提高了p53蛋白質的轉錄活性，以及減少p53蛋白質上Ser315氨基酸磷酸化程度，從而降低了p53被降解的作用。 此外，BCAS2也是一個與雄性素受體結合的蛋白質，在雄性素受體所調控的基因表現中，其俱有轉錄共同因子的功能。在臨床攝護腺癌樣本的分析中，我們發現於攝護腺癌樣本中BCAS2蛋白質的表現俱有顯著性的上升趨勢，並且與雄性素受體的表現情形呈現正相關性。而進一步對於BCAS2導致雄性素受體蛋白質增加的機制的研究時，我們發現BCAS2增強雄性素受體蛋白質半衰期的原因，一部分是由於BCAS2抑制了p53蛋白質的表現進而提高了AR的轉錄活性，此為一個p53相關的轉錄調控途徑。而在不俱有p53的細胞中，BCAS2也可以增加雄性素受體蛋白質，顯示了還有另一個與p53非相關的蛋白酶體調控途徑的存在。此外，降低BCAS2表現時，可以增加攝護腺癌細胞對於抗癌藥物17-AAG的敏感性，這顯示利用BCAS2的干擾RNA與已知的抗癌藥物進行合併治療時，將有可能達到更有效力的治療結果。 總結以上的結果，我們發現BCAS2俱有雙重的角色，其一為p53蛋白質的負調控因子，另一則是雄性素受體的保護因子。這些結果顯示BCAS2 將來可以作為攝護腺癌的一個臨床診斷標記，並且可作為一個發展癌症治療藥物的重要標靶。

BCAS2 is a small nuclear protein (26 kDa). It was reported previously as a transcriptional coactivator of estrogen receptor. Here, we report that BCAS2 directly interacts with p53 to reduce p53 transcriptional activity by mildly but consistently decreasing p53 protein in the normal condition. However, in the presence of DNA damage, BCAS2 prominently reduces p53 protein and provides protection against chemotherapeutic agent such as doxorubicin. Deprivation of BCAS2 induces apoptosis in p53 wild-type cells but causes G2/M arrest in p53-null or p53 mutant cells. There are at least two apoptosis mechanisms induced by silencing BCAS2 in wild-type p53-containing cells. Firstly, it increases p53 retention in nucleus that triggers the expression of apoptosis-related genes. Secondly, it increases p53 transcriptional activity by raising p53 phosphorylation at Ser46 and decreases p53 protein degradation by reducing p53 phosphorylation at Ser315. Additionally, BCAS2 is AR interacting protein and functions as a transcription cofactor for AR-mediated gene expression. In clinical prostate cancer specimen analysis, we found that BCAS2 protein is significantly increased and positively correlates with AR level. To further study the mechanisms of BCAS2-increasing AR protein, we discovered that BCAS2 enhances AR protein half life partly from a p53-dependent transcriptional pathway, due to BCAS2 decreases p53 protein resulting in the raise of AR transcription activity. The other reason comes from p53-independent proteasomal pathway; due to BCAS2 can increase AR protein in p53 null cells. Furthermore the diminution of BCAS2 could enhance the sensitivity of prostate cancer cells to anticancer drug 17-AAG; suggesting RNAi targeting BCAS2 can be used in combination with known anti-cancer drugs for effective cancer treatment. In sum, we show that BCAS2 has a dual role one for a negative regulator of p53 and the other for a protector of AR. These results suggest that BCAS2 can be a diagnosis marker of prostate cancer that may be a critical target for developing therapeutic agents against cancer.