針對腎細胞癌表現上升之CD74基因的功能性研究

Abstract

腎細胞癌被認為是人類腎臟癌的主要型，它的最大特徵是具高度的致死率以及對化學性及放射性治療有高度的抗拒性。據估計，轉移性腎細胞癌病患可以存活五年以上者的機率乃低於百分之十。由於腎細胞癌發生早期缺乏明顯的病徵，以至於當患者發現致病時，通常癌細胞已惡化或轉移。因此癌症學者正積極致力於尋找其分子標誌及其探討治病機制。 CD74 一般會表現在抗原呈現細胞中，諸如: 巨噬細胞、B 細胞 以及 樹突細胞。一旦CD74 表現後，它會與第二型主要組織相容複合體 (MHC class II) 結合在一起並協助複合體在細胞內的運送、摺疊以及防止複合體與外來抗原不正常的結合。此外，研究發現CD74也會表現在MHC class II缺乏的細胞體及許多的肉瘤組織內，根據這些現象推論除了它的已知活性外CD74很有可能還具有未被發現的蛋白質功能。 巨噬細胞遷移抑制因子 (MIF) 是一種原發炎反應細胞素，其具有調控發炎及免疫反應的作用。最近的研究顯示，在纖維母細胞中的巨噬細胞遷移抑制因子會促進ERK1/2 MAP kinase的訊息活化、細胞增生及PGE2的產生，而巨噬細胞遷移抑制因子所造成的細胞活化乃是透過巨噬細胞遷移抑制因子與CD74鍵結而起始的。最近巨噬細胞遷移抑制因子被認為是一個會促進腫瘤生長的蛋白，然而，是否CD74也參與了巨噬細胞遷移抑制因子在促進腫瘤生長上的調控作用目

前仍然不清楚。 在唐賽文先生之前構築的腎細胞癌全長cDNA庫中發現，CD74基因的轉錄調控呈現過度表現之狀況。本研究發現，在許多臨床的配對採集樣本中，CD74基因之過度轉錄及轉譯狀況是普遍發生於這些腎細胞癌組織中的。針對CD74的功能性研究上，過度表現CD74會刺激ERK1/2 及 p38 訊息的活化，這可能是因為內生性分泌的巨噬細胞遷移抑制因子以自我活化的方式鍵結到細胞膜上的CD74所造成的結果。值得一提的是，腫瘤抑制基因p53之表現量會因為CD74的過度表現而被壓制，然而CD74過度表現卻會促使Hmd2表現量上升，這表示CD74調控可能是透過調控Hmd2表現量上升而促進p53降解最後造成細胞內p53蛋白量下降的結果，另外在HeLa 細胞中我們亦發現，CD74蛋白的胺端會被蛋白質水解脢斷裂並釋放出其N端片段 (NTF) 而移行入細胞核內。綜合實驗結果，我們初步認為過度表現CD74在腫瘤生成上的機制可能是透過MIF的合作性調節而活化ERK1/2 MAPK 及p38訊息，並造成Hmd2表現量的上升及p53的降解，然而CD74經水解後釋放出的N端片段 (NTF) 可能是CD74下游的作用子，而調控腫瘤生長相關的基因表現。

Renal cell carcinoma (RCC) was thought to be one of major types of human kidney cancers, and it is characterized by a high mortality rate and a high resistance to chemotherapeutic treatment as well as radiations. However, high occurrence of advanced and metstatic RCC often attributed to the absence of early warning signs and thus, the oncogenists have taken a lot of efforts to identify the molecular marker and investigate the mechanism of tumorigenesis of RCC. CD74 is expressed in antigen-presenting cells (APCs) such as macrophages, B-cells and dendritic cells. Once synthesized, CD74 is associated with major histocompatibility complex (MHC) class II and assists class II trafficking, class II folding and avoidance of abnormal-antigen binding. In addition, CD74 was found to be expressed in several types of carcinoma and class II negative cells inferring that CD74 may has a novel function besides to its class II chaperon activity. By binding to CD74, MIF was demonstrated to elicit ERK1/2 MAP Kinase signaling, cell proliferation and PGE2 production. However, MIF has been described to be a tumor-promoting protein, and whether CD74 involves in MIF for promoting tumor growth is still needed to be studied Transcriptional up-regulation of CD74 was found previously in full-length cDNA library of a RCC tissue by Mr. S. W. Tang in this laboratory. In this study, up-regulation of CD74 was generally found in mRNA level and protein level of several RCC tissue pairs. In functional study, overexpression of CD74 stimulated ERK1/2 and p38 signaling, which could be initiated by binding of MIFs to CD74 on the cell surface in an autocrine fashion. Surprisely, it was found that tumor suppressor p53 protein was decreased by expression of CD74, and it was accompanied by up-regulation of Hmd2. Also, the N-terminal proteolytically cleavage of CD74 carried out in HeLa cells, and the cleavage fragment (1a.a-58a.a, NTF) could translocate to nucleus. Taken together, we suggested that overexpression of CD74 led to the binding of MIF to CD74 on the cell surface in an autocrine manner, resulting in the activation of ERK1/2 and p38 signaling. In addition, overexpression of CD74 could also up-regulate Hmd2 and the down-regulate p53.