CD24是肝細胞癌的可能標的: 在腫廇細胞生長、分化與侵犯的角色

Abstract

CD24是一個透過GPI anchor，連接到細胞膜表面的一個短小和高度醣化的膜蛋白，是血液系統的分化群組cluster of differentiation (CD) ，主要表現在先驅B淋巴球，中性球，神經組織和再生中的肌肉。因此，CD24在先驅B淋巴球的發育和神經生成中扮演重要角色，也有作為P-selectin 和 L1的粘合分子的作用。目前諸多證據指出CD24跟腫瘤和轉移有關。儘管已有許多人類組織研究認為CD24的表現與腫瘤生長有關連，但在這些過程中CD24所牽動的機制或是治療潛能尚未釐清，尤其是對肝細胞癌(HCC) 。之前，我們發現CD24 mRNA在肝細胞癌會有過度表現，與腫瘤不良分化和p53突變有關。 在本篇研究中，我們利用螢光免疫染色確認了CD24位在於細胞膜上。利用反式retinoic acid處理造成分化的NT2細胞與不繼代處理造成分化的HepG2細胞中，CD24的mRNA和蛋白表現量都會隨之減少。這些結果顯示CD24跟細胞分化有著相關性。我們建立了三種減少CD24表現的方法去證明CD24在生長，侵犯以及它的治療潛能:分別是利用反義RNA有效地持續抑制的方法，短暫抑制的 RNAi oligos方法和專一抗CD24的抗體中和作用。藉由RNAi系統，發現經由反義RNA有效地持續抑制和短暫抑制的RNAi oligos方法都可以達到減緩細胞的生長速率，降低細胞在soft agar的非附著性生長能力，且在含有第一型膠原蛋白的3D膠或Matrigel中，細胞的侵犯能力都有減低。在腫瘤生長的角色，我們把有降低CD24表現的細胞株和對照組注射到NOD-SCID小鼠，觀察小鼠的腫瘤生長速率。我們發現有降低CD24表現的細胞株的生長速率都有明顯下降。除此之外，有短暫抑制CD24表現的細胞，它們附著到不同ECM，包括vitronectin (玻連蛋白)、collagen IV (第四型膠原蛋白)、laminin（層粘連蛋白）或fibronectin（纖維粘連蛋白）的能力都會降低，FAK相關分子也會有減少的表現，包括p-FAK，p-Akt，MMP2 或MMP-9。在Ha22T 細胞，附著到這四種ECM都會減少; Huh-7 細胞附著到laminin（層粘連蛋白）或HeLa 細胞附著到vitronectin (玻連蛋白) 的能力都有下降。有持續抑制CD24表現的Huh-7細胞在第一型膠原蛋白的3D膠的侵犯能力下降，利用反義RNA短暫抑制的HeLa和 RNAi oligos 短暫抑制的 HeLa 和Ha22T細胞侵犯Matrigel gel 的細胞數目也會減少。這些結果證明了CD24的表現改變可以透過 Akt的下降，使得Ha22T和Huh-7的生長速度都減慢，而且侵犯能力的下降有部分會是因為Huh-7 和Ha22T細胞的MMP9表現下降。除了利用RNAi系統外，我們也證明了利用CD24的單株抗體也會達到抑制腫瘤細胞的侵犯能力。 總而言之，我們的研究結果顯示了減少CD24的表現可以抑制腫瘤生長和侵犯能力，因此CD24可以作為肝細胞癌的治療標的，特別是針對膜蛋白CD24的特殊抗體治療法。

CD24 is a surface protein consisting of a short, heavily-glycosylated protein core linked to plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. CD24 is a cluster of differentiation (CD) marker of hematopoietic lineages and plays a role in B-cell development and neurogenesis. CD24 functions as an adhesion molecule and interacts with P-selectin and L1. Cumulative evidence has demonstrated CD24 expression in many common malignancies and association with metastasis. Despite the multitude of human tissue studies correlating CD24 expression with tumorigenesis, the molecular mechanisms which define the role of CD24 in tumorgenesis, and its potential as therapeutic target are not fully understood, especially in hepatocellular carcinoma (HCC), in which CD24 mRNA is overexpressed and correlated with poor tumor differentiation and p53 mutation. In this study, we confirmed the membrane localization of CD24 by immunofluorescence staining. The mRNA and protein levels of CD24 decreased after the differentiation of NT2 cells induced by prolonged trans-retinoic acid treatment and HepG2 cells induced by prolonged cultivation without passage. These findings suggest that CD24 gene is involved in cell differentiation. We have established 3 approaches to knockdown the expression of CD24 and to clarify the role of CD24 in tumorigenesis, invasion and its therapeutic potential: stable knockdown by antisense strand RNA, transient knockdown using RNAi oligos, and neutralization with specific anti-CD24 antibody. With the RNAi system, CD24 stable knockdown and RNAi oligos transient transfected cells showed significant reduction of cell proliferation rate, anchorage-independent colony formation in soft agar assay and invasion ability in 3-dimensional gel containing collagen type I or Matrigel assay. In 3-dimensional collagen type Ι gel, the Huh-7 stable knockdown clones showed less invasive capability whereas the transient antisense transfected HeLa and the RNAi oligos transient transfected HeLa and Ha22T cells showed reduced number of cells invading through the Matrigel. To elucidate the role in tumorigenesis, we inoculated CD24 downregulated clones and control cells into NOD-SCID mice. We found that stable knockdown clones had a significantly slower tumor growth rate. Moreover, the CD24 transient knockdown cells showed altered adhesion to extracellular matrix (ECM) including vitronectin, collagen IV, laminin and fibronectin. There was reduced binding ability to all four types of ECM in Ha22T cells; to laminin in Huh-7 and to vitronectin in HeLa cells. Furthermore, the decreased expression of FAK-mediated pathway molecules, including p-FAK, p-Akt, MMP2 or MMP-9 suggested that altered expression of CD24 changed the proliferation of Ha22T and Huh-7 cells through the downregulation of Akt, and the reduced invasive capability, at least in part through the downregulation of MMP-9. In addition to RNAi system, we also showed inhibition of in vitro tumor cell invasion activity by antibody neutralization using monoclonal anti-CD24 antibody. In conclusion, our study demonstrated that CD24 may serve as a therapeutic target in HCC since RNAi knockdown of CD24 suppresses the in vivo tumorigenesis and in vitro invasion. Besides, CD24, a membrane protein, may serve as an excellent target for immuno-therapy of HCC.