探討EBP50藉由 Ras-RSK1訊息傳遞輸送至細胞核的機制與功能

Abstract

EBP50（ezrin-radixin-moesin (ERM)-binding phosphoprotein of 50 kDa）在癌症生物學具有爭議性的功能是和此蛋白在各種細胞器的定位息息相關的。位于細胞膜邊緣質體的EBP50是抑制癌症的分子，在細胞核内的EBP50卻是促進癌症的分子。目前的研究對於EBP50進入細胞核的機制並沒有明確的解説。藉由RNA干擾(RNA interference）技術,Ras-ERK訊息傳遞途徑下游的RSK1 (p90 ribosomal S6 kinase alpha 1）獲篩選爲可調控EBP50輸送至細胞核的關鍵分子。在EGF (epidermal growth factor) 的調控下，RSK1與EBP50結合並且磷酸化EBP50的酥胺酸156 (T156) 。氨基酸位點突變 (mutagenesis) 實驗進一步確定T156在EBP50被輸送至細胞核途徑里的關鍵角色。由RSK1調控並且會在分裂中的細胞被催化的EBP50磷酸化亦會促進EBP50和14-3-3蛋白的結合，增加細胞分裂和惡性轉化。因此，我們總結由Ras-RSK1所催化的T156位點磷酸化是EBP50被輸送至細胞核的機轉。 我們的研究有助於只針對EBP50在細胞核的異常表現而不影響此蛋白的正常生理功能在癌症治療策略的可能性。

Differential subcellular localization of Ezrin-radixin-moesin (ERM)-binding phosphoprotein of 50 kDa (EBP50) leads to its controversial role in cancer biology either as a tumor suppressor when it resides at the membrane periphery, or a tumor facilitator at the nucleus. However, the molecular mechanism that regulates nuclear localization of EBP50 remains unclear. A RNA interference screening identified the downstream effector of the Ras-extracellular signal-regulated kinase signaling pathway, p90 ribosomal S6 kinase alpha 1 (RSK1) as the molecule unique for nuclear transport of EBP50. RSK1 binds to EBP50 and phosphorylates it at a conserved threonine residue at position 156 (T156) under regulation of epidermal growth factor. Mutagenesis experiments confirmed the significance of T156 residue in nuclear localization of EBP50. EBP50 phosphorylation by RSK1, which is enhanced in mitotic cells, is also required for recruitment of 14-3-3, cellular proliferation, and oncogenic transformation. Collectively, we discovered the Ras-RSK1-mediated phosphorylation at T156 as the signal underlying nuclear transport of EBP50. Our study sheds light on a possible therapeutic strategy targeting at this aberrant nuclear expression of EBP50 without affecting the normal physiological function of EBP50 at other subcellular localization.