口腔癌位在世界癌症死亡的前十大死因之中，而口腔鱗狀細胞癌屬於口腔癌的一種，其中以舌頭部位的腫瘤最為好發。隨著病程的進展，90%惡化的口腔癌通常會有淋巴或是其他器官的轉移，到現今無有效的治療方式或是明確的分子機制可以解釋或治療其轉移。L1 細胞黏附分子 (L1CAM) 參與在許多類型癌症的腫瘤發展之中。在我們先前的研究中已經得知，口腔鱗狀細胞癌較惡化的癌細胞相較於正常的口腔角質細胞，L1CAM是高度表達的，而這樣的高度表現與癌細胞的分化和較高的侵襲能力是有相關性的，這也表示著L1CAM在人類口腔鱗狀細胞癌腫瘤進展扮演著重要角色。然而，L1CAM影響口腔鱗狀細胞癌的機制作用我們尚未研究清楚。就以往的研究證明，L1CAM的細胞內結構區域和訊號傳遞過程有關，進而影響癌細胞癌化的發展。所以我們推論L1CAM的細胞內結構區域可能是L1CAM去影響口腔癌細胞癌化的主因。所以在我們這次的研究目標是要去探討L1CAM的細胞內區域是否參與在人類口腔鱗狀細胞癌的腫瘤進展之中，並希望從中去了解其中所調控的分子機制為何。由於別人的研究和我們的初步實驗結果發現L1CAM是個磷酸化分子，因此我們將L1CAM細胞內結構區域可能磷酸化的位點Y1211，Y1229，Y1176和S1248利用定點突變技術進行改造成結構相似但無法磷酸化的酪氨酸和丙胺酸。然後送入口腔鱗狀細胞癌細胞株SCC4，SCC25和SAS細胞中表現，藉此去觀察這樣的磷酸化位點突變是否會去影響口腔鱗狀細胞癌的癌細胞的行為。我們發現到，過度表現完整的L1CAM的確會增加癌細胞的移動、侵襲和生長能力。然而缺乏細胞內結構區域的L1CAM則完全失去此促癌功能。由此證明L1CAM細胞內結構區域的確是L1CAM造成癌細胞惡化的決定部位。我們進一步鑑定出Y1211和Y1229是調控口腔鱗狀細胞癌癌細胞的侵襲、移動和生長能力的主要位點。另外我們也發現L1CAM的細胞內結構區域會去抑制AKT的活化和促進ERK的活化，然而這些影響卻和EMT的過程沒有直接的關係。由此可推測L1CAM可能藉由別的作用機制去影響口腔鱗狀細胞癌的癌化。我們在293FT細胞中證明Y1229有被磷酸化，因此未來將進一步探討Y1229的磷酸化及其相關的訊息傳導途徑是否是調控口腔鱗狀細胞癌的主要分子機制。
Oral cancer is one of the top ten cancers worldwide. Oral squamous cell carcinoma (OSCC) represents 90% of oral cavity tumors and often spread to submandibular and cervical lymph nodes. The neural cell adhesion molecule L1 (L1CAM) has been implicated in tumor progression of many cancer types. We previously demonstrated overexpression of L1CAM in clinical specimens and cell lines of OSCC but not in normal keratinocytes, supporting the significance of L1CAM in human OSCC tumor progression. However, the role of L1CAM in OSCC has not been investigated clearly. Moreover, phosphorylation of L1CAM intracellular domain has shown to be part of signal transduction processes of L1CAM in regulating mobility of cancer cells. To investigate whether intracellular domain of L1CAM is involved in the progression of human OSCC, we constructed several intracellular domain mutates of L1CAM by deleting the entire intracellular domain or replaying putative phosphorylated Y1211, Y1229, Y1176 and S1248 residues to phenylalanine and alanine, respectively using site directed mutagenesis technology. The influence of cell migration, invasion and proliferation of these mutates was determined in both L1CAM high-expressing SCC4 and SAS and low-expressing SCC25 OSCC cell lines. We found that overexpression of full length but not intracellular domain deleted L1CAM promoted ability of cell migration, invasion and proliferation in OSCC cells. In addition, either Y1211F or Y1229F mutant has no effects on promoting the aggressiveness of OSCC cells, indicating that Y1211 and Y1229 are the determining residues at intracellular domain for L1CAM-mediated oncogenic function. We also found that L1CAM intracellular domain downregulated activity of AKT and upregulated ERK activation, but it was not associated with the expression of EMT markers. Since we demonstrated tyrosine phosphorylation at the site of 1229 in 293FT cells, we are currently investigating whether phosphorylated Y1229 and its related signaling transduction pathway is the key molecular mechanism by which L1CAM regulates tumor progression of OSCC.
