北京大學和海南醫學院聯合對肝癌細胞特異性標志物甲胎蛋白（AFP）進行長時間的研究，發現AFP具有抑制PTEN的生物學功能，導致肝癌細胞耐受全反式維甲酸誘導的凋亡，這是AFP新功能的發現。近日以題為“Alpha-fetoprotein: a new member of intracellular signal molecules in regulation of the PI3K/AKT signaling in human hepatoma cell lines”在癌癥雜志《International Journal of Cancer》再次發表封面論文。

甲胎蛋白（α-fetoprotein, AFP）是肝癌細胞表達的高特異性蛋白質，很多肝癌病人（70-80%）在發病期間都有AFP基因高表達的特征，AFP是一種胚原蛋白，其基因在胎兒發育過程開放并表達，而在人出生兩年后基本處于關閉狀態，但是成人發生肝癌或肝臟良性再生時，AFP的基因重新被激活而大量表達，在臨床上被認為是肝癌的經典腫瘤標記物，因而被當作診斷肝癌的金標準。北京大學博士李孟森課題組成員在國家自然科學基金的連續資助下，經過多年的潛心研究，逐步揭示了AFP所隱藏的生物學功能。該課題組研究發現AFP與腫瘤細胞的惡性生長、轉移和侵襲密切相關，并且認為AFP是肝癌細胞耐藥的關鍵性細胞因子，研究結果顯示AFP具有潛在的抗凋亡誘導作用的生物學性質。

全反式維甲酸（ATRA）在臨床上是用于治療白血病的一線藥物，其主要通過與胞內受體結合誘導惡性細胞“改邪歸正”，由于ATRA與受體結合后改變后者的空間結構，導致受體進入細胞核內調節靶基因的轉錄，發揮其抑癌作用。盡管ATRA能誘導惡性細胞分化和抑制癌細胞生長，但是肝癌細胞對ATRA失去敏感性。ATRA能誘導PTEN （Phosphatase and tension homolog deleted on chromosome ten）的表達，而PTEN是水解3-磷酸肌醇（PIP3）為2-磷酸肌醇（PIP2）的磷酸酶，其能阻止3-磷酸肌醇激酶（PI3K）磷酸化蛋白激酶B（Protein kinase B, PKB/AKT），阻斷PI3K/AKT的信號傳遞，因而被認為是一個重要的抑癌基因，其表達下降或功能的喪失是引起生長信號錯誤傳遞的分子基礎，后果是導致癌細胞的過度分裂。約有12.5%的肝癌病人的癌細胞里PTEN基因關閉，而70%的病人PTEN基因開放并高表達，PTEN蛋白合成量與正常組織沒有顯著性差別，因而PTEN在肝癌內喪失發揮抑癌作用可能是由于其功能失活導致的。由于PTEN表達缺失或下降與AFP高表達密切相關，AFP是肝癌細胞高表達的特異性蛋白質，課題組基于AFP與PTEN在肝癌細胞內表達相關性的設想，深入研究肝癌細胞內高表達的AFP是否存在曾未發現的生物學功能。

北京大學博士李孟森研究員課題組聯合北京大學李剛教授實驗室，采用RNA干擾、免疫共沉淀（Co-IP）、熒光共振能量轉移（FRET）和染色體免疫共沉淀（ChIP）等技術研究肝癌細胞內AFP新功能，發現在肝癌細胞內的AFP能與PTEN結合，并抑制PTEN 的生物學活性，促進PI3K/AKT信號的傳遞，而且發現AFP也能特異性與caspase-3結合，抑制caspase-3的活性，阻斷 caspase信號的級聯反應，導致肝癌細胞耐受腫瘤壞死因子相關凋亡誘導配體（TRAIL）的作用。傳統的醫學理論認為AFP基因的高表達是肝癌發生過程中的一個伴隨現象，是肝癌發生的特異性腫瘤標志物。李孟森博士的研究結果表明細胞外的AFP能與其受體結合，介導生長信息的傳遞；細胞內的AFP不僅能抑制凋亡信號的轉導，而且也能促進生長信號的傳遞，這是AFP具有抗凋亡和促進增殖功能的新發現，該研究結果賦予AFP新功能的認識和尋找到治療肝癌的新靶點。研究結果在2009年6月15日出版的《International Journal of Cancer》上以封面介紹論文發表，2011年2月1日出版的《International Journal of Cancer》再次以封面介紹形式公布了AFP新功能的研究成果。

原文檢索：
1. Alpha fetoprotein is a novel protein-binding partner for caspase-3 and blocks the apoptotic signaling pathway in human hepatoma cells. International Journal of Cancer, 2009; 124（12）: 2845-2854.
Although there is increasing evidence that alpha fetoprotein （AFP） may function as regulatory factor in the growth of tumor cells, the precise mechanism is still unclear. In the current study, we investigated the role of the cytoplasmic AFP in caspase-3-mediated signaling of apoptosis. Our results showed that low doses of TNF-related apoptosis-inducing ligand （TRAIL） elevated the activity of caspase-8, but not caspase-3. Caspase-3 colocalized and interacted with AFP in the cytoplasm of Bel 7402 cells, and translocated into nuclei in association with the occurrence of apoptosis while cells were under cotreatment with all-trans retinoic acid （ATRA） or TRAIL. AFP was able to form complexes with caspase-3 and block onward transmission of signaling from caspase-8. Knockdown of AFP increased the sensitivity of Bel 7402 cells to TRAIL, and thereby, triggered caspase-3 signaling. No intermolecule interaction occurred between AFP and caspase-8, nor was caspase-8 activity altered after AFP knockdown, demonstrating the selectivity of AFP in interfering with the apoptotic signaling pathway. The effect of AFP on caspase-3 was further confirmed by transfection of the AFP gene into HLE cells （AFP negative）. We conclude that ATRA or TRAIL resistance in AFP producing hepatoma is at least, in part, attributable to the high level of the cytoplasmic AFP. Therefore, it is possible that the combination of AFP gene silencing together with ATRA/TRAIL cotreatment will benefit the enhancement of the chemotherapeutic efficiency of these agents on tumors.
2. Alpha-fetoprotein: a new member of intracellular signal molecules in regulation of the PI3K/AKT signaling in human hepatoma cell lines. International Journal of Cancer, 2011; 128（3）: 524-532.
Despite its well-defined role as a serum growth factor during fetal liver development and hepatic oncogenesis, the biological significance of cytoplasmic alpha-fetoprotein （AFP） remains incompley understood. Here, we provide evidence to illustrate that cytoplasmic AFP may function as a regulator in the phosphatidylinositol 3-kinase （PI3K）/AKT pathway in human hepatocellular carcinoma cells. The results demonstrated colocalization and interaction of AFP and phosphatase and tensin homolog deleted on chromosome 10 （PTEN） in the cytoplasm of AFP-producing Bel 7402 and HepG2 cells, with an interaction distance of 12.6?2.7 A? as determined with the fluorescence resonance energy transfer technique. Knockdown of AFP mRNA or inhibition of AFP expression by all trans-retinoic acid resulted in enhancement of the PTEN level with a synchronous decrease in phosphorylated AKT. Transfection of the afp gene into HLE cells （originally AFP negative） led to a significant activation of AKT signaling. The inhibition of PI3K signaling by LY294002 was simultaneously reversed by transfection, accompanied by diminution of all trans-retinoic acid-induced upregulation of PTEN and enhancement of cell growth. In conclusion, these results demonstrate that cytoplasmic AFP is involved in regulation of hepatocellular growth and tumorigenesis.

來源：生物通