CAAT區/增强子结合蛋白 beta（C/EBP beta）不同轉譯產物的調控

Abstract

CAAT區/增强子结合蛋白 beta (C/EBP beta) 是一個重要的轉錄因子，而且在許多細胞反應扮演重要的角色。當內質網 (endoplasmic reticulum) 內未折疊或錯折疊蛋白積聚時，就會對內質網產生壓力，也就是所謂的 ER stress 。在 ER stress 的情況下， C/EBP beta 的表現量會增加。 C/EBP beta 有三個轉譯產物，皆從同一條 mRNA 轉譯出來，其中有兩個是轉錄活化因子 (LAP* 和 LAP)，另一個則是轉錄抑制因子 (LIP)。三個轉譯產物之間的相對量對於 C/EBP beta 下游基因的調控很重要。本篇發現了在 thapsigargin 和低濃度的 anisomycin 環境下， C/EBP beta 三個轉譯產物的表現量會改變。而低濃度的 anisomycin 已知亦會產生 ER stress 。我們也發現在 anisomycin 的處理下，用 C/EBP beta 的抗體可偵測到有未知的 C/EBP beta 產物表現出來。除此之外，有一個小的 open reading frame (sORF) 穿插在 C/EBP beta 的 mRNA 會影響 LAP 跟 LIP 的表現。之前研究對於 sORF 如何調控下游 LIP 的轉譯，有兩種說法： 一個模式是核糖體的 leaky scanning ，而另一種是 re-initiation 。在本篇，我們的結果支持 LIP 的轉譯是經由 re-initiation 模式.

C/EBP beta is a transcription factor that belongs to the C/EBP family and plays important roles in many cellular responses. C/EBP beta expression increases during endoplasmic reticulum (ER) stress, which is triggered by the accumulation of unfolded proteins in the ER. Three isoforms, LAP*, LAP and LIP, are translated at alternative initiation codons of an intronless mRNA. LAP* and LAP are transcriptional activators, while LIP is a dominant-negative repressor. The relative expression of three isoforms is critical for C/EBP beta-mediated gene transcription. This report showed that the expression of C/EBP beta isoforms changed in response to thapsigargin and low dose anisomycin, which can trigger ER stress. We also found novel protein bands triggered by anisomycin treatment. An out-of-frame AUG codon of small open reading frame (sORF) in mRNA has been reported that regulated LAP and LIP expression. In this report, our data supported the model that translation of LIP is mediated by re-initiation model, in which the 40S ribosomal subunit remains attached to the mRNA after termination of the uORF, and re-initiates scanning.