探討Podocalyxin-like 1於癌細胞幹細胞特性以及轉移中扮演之功能性角色

Abstract

癌症的轉移是造成癌症死亡的主要原因，轉移癌主要是因為有一小族群的細胞離開原位癌，散佈到其他器官造成。有之前的假說造成癌症轉移的細胞可以稱之為癌症幹細胞。癌症幹細胞的特性類似於胚胎幹細胞，但癌症幹細胞主要會造成腫瘤的起始，細胞自我更新以及癌症轉移。但在目前還沒有很明確的生物標記可以去分離出這些癌症幹細胞。除此之外，癌症幹細胞是如何造成癌症轉移，其作用的機制目前還未釐清。在我們先前的研究中，我們利用融合瘤技術生產出可以專一性標的到胚胎幹細胞以及癌細胞的抗體。我們發現PODXL 會大量表現在胚胎幹細胞以及一些癌細胞當中。在我們的研究中發現，Podocalyxin (PODXL)可能可以作為一個癌症幹細胞的生物標記，因為其表現會調控癌症腫瘤球的形成。另外我們分析各種臨床檢體以及細胞株之後發現，PODXL的表現量與癌症淋巴轉移以及惡性程度呈現正相關。並且在較惡性的癌症中，PODXL的甲基化程度會有較低的情形產生。而大量表現的PODXL會控制著細胞的移動以及腫瘤自我更新能力。接著我們利用短夾型RNA(shRNA)抑制PODXL在細胞的表現，發現其生長、移動、F肌動蛋白(F-actin)的分佈及 FAK或Paxillin的活化均有明顯受到抑制的現象。在癌症轉移老鼠模型當中，發現抑制PODXL表現後會降低癌細胞轉移的能力。此外PODXL表現降低會影響皮層肌動蛋白(Cortactin)的磷酸化，並且降低invadopodia的形成以及gelatin的降解。綜合以上結果顯示，PODXL可能在細胞移動、癌症轉移以及腫瘤幹細胞特性當中扮演著重要的角色。因此PODXL未來將有潛力成為新的生物標的分子去辨識癌症幹細胞以及治療癌症轉移。

Cancer stem cells (CSCs) are involved in tumor-initiation, self-renewal, and metastasis. CSCs share many similar biological properties as the embryonic stem cells (ESCs). However, currently there still lacks biomarkers that can specifically identify CSCs. More importantly, the underlying signaling mechanism of CSCs leading to metastasis is still unclear. In the present study, we used hybridoma to identify cell surface biomarkers that are expressed on both ESCs and cancer cells. Podocalyxin-like 1 (PODXL) is a CD34-related cell surface molecule with anti-adhesive property. We found that PODXL to be abundantly expressed on both ESCs and cancer cells. The expression of PODXL was upregulated in tumorsphere, and was found to be associated with stem/progenitor markers. Epigenetic regulation through demethylation on PODXL gene was correlated with both PODXL expression and tumor malignancy. PODXL was correlated with lymph node metastasis and cancer malignancy. Furthermore, overexpression of PODXL was associated with cancer cell mobility and tumor self-renewal. Conversely, knockdown of PODXL caused F-actin depolarization, suppressed FAK and Paxillin activation, and inhibited tumor migration and invasion. Suppression of PODXL not only reduced tumor cell proliferation and colony formation, but it also inhibited tumor dissemination in the metastasis mouse model. Moreover, suppression of PODXL downregulated cortactin phosphorylation, and reduced invadopodia formation and gelatin degradation. Taken together, our data demonstrate that PODXL plays an important role in cell mobility, tumor metastasis and tumor stemness, and indicate that PODXL might be served as a specific biomarker for cancer stem cells, and a potential therapeutic target for cancer metastasis.