Eps8( EGFR pathway substrate no.8)是1993年由Dr. Di Fiore 所發現的蛋白質，具有兩種isoforms，分別是分子量97kDa的p97Eps8及68kDa的p68Eps8。在NIH3T3細胞中增加Eps8的表現，能夠促進EGF所誘發的mitogenic signal。先前的研究顯示，在許多人類的腫瘤細胞株中p97Eps8有高度磷酸化的情形，而大量表達p97Eps8於NIH3T3細胞能夠促進細胞EGF receptor所誘導的轉型﹔此外，研究也發現 v-Src transformed cell（IV5）中p97Eps8 和p68Eps8蛋白質表現量增加，並且有高度磷酸化現象。將IV5 細胞中的p97Eps8 knock down，則會抑制細胞增生及動物腫瘤的生長。因此我們推測Eps8蛋白質的表達，在v-Src mediated transformation扮演重要的角色。實驗室先前已經證明v-Src可直接磷酸化p97Eps8於Tyr45和Tyr524，我們進一步想要瞭解這兩個位置的磷酸化是否影響細胞轉型。首先我們將Eps8分別在Tyr45和Tyr524的位置突變為苯丙胺酸（phenylalanine），接著送入wild type -Eps8、Phe45-Eps8和Phe524-Eps8到IV5細胞中，發現在這兩個磷酸化位置的突變會抑制Eps8的酪胺酸磷酸化；此外，將wild type Eps8、Phe45-Eps8和Phe524-Eps8送入 Eps8 knock down 的IV5 細胞，建立穩定表達的細胞株，觀察到Phe45-Eps8和Phe524-Eps8會抑制細胞生長、colony formation及腫瘤生成，並且也會抑制STAT3 Tyr705的磷酸化以及下游Cyclin D1的表達。我們也發現Phe45-Eps8和Phe524-Eps8的細胞株中，IRSp53的蛋白表達減少。由於IRSp53會藉由和Eps8的交互作用活化STAT3以及促進Cyclin D1的表現量，進而影響腫瘤生成。因此Src調控Eps8於Tyr45和Tyr524的磷酸化，可能透過影響IRSp53的蛋白表達，而活化STAT3，造成細胞不正常的分裂。

As an EGF receptor substrate, Eps8 (EGFR pathway substrate no.8) enhances EGF-dependent mitogenesis and tumorigenesis. The presence of tyrosyl phosphorylation of p97Eps8 in several human tumor cell lines suggests its involvement in human cancer formation. As a Src substrate, Eps8 participates in v-Src-mediated cell proliferation and tumorigenesis. To address whether Src-mediated Eps8 phosphorylation is involved in cell transformation, site-directed mutagenesis indicated that Tyr45 and Tyr524 of Eps8 are the major Src phosphorylation residues on Eps8 in IV5 cells. In addition, as compared to wild type-Eps8, ectopically expressed Phe45-Eps8 or Phe524-Eps8 reduced cell proliferation in culture dish, soft agar and tumor growth in mice. Furthermore, the phosphorylation of STAT3 Tyr705 and Cyclin D1 expression were decreased in Phe45-Eps8- or Phe524-Eps8-expressing cells. Notably, we found IRSp53, which affects tumor growth through its interaction with Eps8, is reduced in Phe45-Eps8- and Phe524-Eps8-expressing cells. Thus, Src-mediated Tyr45 and Tyr524 phosphorylation on Eps8 may lead to IRSp53 overexpression and the following STAT3 activation that culminated to abnormal cell proliferation.

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