CTEN 受 EGFR 訊息傳導活化而磷酸化的功能

Abstract

CTEN 是 tensin 蛋白質家族中的一員，主要座落在細胞膜內側的蛋白質複合體 focal adhesion 中，CTEN 參與調控細胞的貼附、增生、遷移、侵襲與機械力感應等生理功能。CTEN 也大量表現於癌細胞的細胞核中，可以做為轉錄輔因子調控 CDC27 基因的轉錄。過去研究發現，EGFR 的活化會促進 CTEN 的表現，並透過 MEK/ ERK 與 PI3K/ AKT 路徑磷酸化 CTEN 的 T347、S350 及 S386 位點。但目前我們仍未了解 CTEN 磷酸化後的功能，在本篇論文中，我們主要藉由在細胞中表現野生型 CTEN (CTEN-WT) 或三個磷酸化位點突變的 CTEN-M3，研究磷酸化 CTEN 在細胞中參與的分子調控機制。透過監測細胞進行糖解反應時胞外環境酸化的速率，我們確認 CTEN 的磷酸化與否不影響糖解反應進行的速率，顯示 CTEN 的磷酸化不會調控細胞的糖解作用。在大腸癌細胞株 HCT116 中，CTEN-WT 會促進 CDC27 啟動子活性，而 CTEN-M3 則沒有此效果，說明磷酸化可能影響 CTEN 的轉錄調控功能。另外，我們也發現在 EGF 刺激下，相比於表現 CTEN-M3 的細胞，表現 CTEN-WT 細胞的 EGFR 蛋白質穩定性較高，且 EGFR 泛素化的程度較低，顯示磷酸化 CTEN 可能透過抑制 EGFR 的泛素化，進而抑制 EGFR 的降解。

CTEN is a member of tensin protein family. Mostly, CTEN is located in focal adhesion, which is the protein complex associated with the cytoplasmic side of the plasma membrane. CTEN participates in various physiological processes such as cell attachment, proliferation, migration, invasion or mechanical sensing. In cancer cell, CTEN is also found abundant in the nucleus, where CTEN might act as a transcriptional coactivator to regulate CDC27 gene transcription. CTEN expression is upregulated upon EGFR activation and residues T347, S350, and S386 of CTEN are phosphorylated through MEK/ ERK and PI3K/ AKT pathways. However, the cellular functions of phosphorylated CTEN are still unknown. In this study, we investigate what molecular regulations that phosphorylated CTEN takes part in using cells expressing wild type CTEN (CTEN-WT) or CTEN with mutations on the three phosphorylation sites (CTEN-M3). Through monitoring extracellular acidification rate (ECAR) of cells under glycolysis, we found that CTEN phosphorylation does not affect glycolytic rate, suggesting that CTEN phosphorylation does not regulate glycolysis. Moreover, CTEN-WT but not CTEN-M3 promotes the activity of CDC27 promoter in HCT116 colon cancer cell line, suggesting phosphorylation might affect the transcriptional regulatory ability of CTEN. Furthermore, EGFR stability is higher and less EGFR is ubiquitinated in the cells expressing CTEN-WT than in those expressing CTEN-M3 after EGF stimulation. These data suggest that phosphorylated CTEN suppresses ubiquitination of EGFR, thus inhibits the degradation of EGFR.