GATA-Dependent Expression of the Interleukin-1 Receptor-Related T1 Gene in Mast Cells

The murine delayed-early serum-responsive gene T1 encodes glycoproteins of the interleukin-1 receptor family. Transcriptional initiation in fibroblasts is regulated by c-Fos and gives rise to a rare 5-kb mRNA and an abundant 2.7-kb mRNA. These transcripts are translated into a receptor-like membrane-anchored protein and a secreted protein consisting only of the ectodomain. In mast cells, T1 gene transcription is initiated 10.5 kb further upstream than in fibroblasts and gives rise predominantly to the 5-kb transcript under normal growth conditions. Here we demonstrate that calcium ionophore stimulation of mast cells resulted in an upregulation of T1 gene expression and a switch from the long to the short T1 transcript. This was paralleled by the disappearance of the receptor-type T1 protein on the mast cell surface and the secretion of large amounts of the truncated T1 protein. c-Fos and a T1 enhancer, which have previously been identified to be essential for T1 expression in fibroblasts, were not required for calcium ionophore-mediated T1 gene upregulation. Overexpression of the transcription factor GATA-1 in mast cells caused elevated T1 synthesis. Three GATA elements were identified in the minimal GATA-responsive mast cell promoter. Mutational analysis revealed that all three GATA elements are involved in T1 gene expression. Point mutations within the middle GATA element eliminated promoter activity completely, while mutations of the distal and proximal GATA binding sites reduced promoter strength by factors of 2 and 5, respectively. Exogenous expression of GATA-1 was not sufficient to activate the mast cell-specific promoter in NIH 3T3 fibroblasts.

小鼠延迟早期血清应答基因T1编码属于白细胞介素-1受体家族（interleukin-1 receptor family）的糖蛋白。成纤维细胞中的转录起始受c-Fos调控，可产生丰度极低的5kb mRNA与丰度较高的2.7kb mRNA。这两种转录本分别翻译为类受体膜锚定蛋白与仅含胞外域的分泌型蛋白。在肥大细胞中，T1基因的转录起始位点较成纤维细胞上游10.5kb，正常生长条件下主要产生5kb转录本。本研究证实，对肥大细胞施加钙离子载体（calcium ionophore）刺激可上调T1基因的表达，并使转录本从长型T1转录本转换为短型T1转录本。该变化伴随肥大细胞表面受体型T1蛋白的消失，以及大量截短型T1蛋白的分泌。此前已证实，成纤维细胞中T1基因的表达依赖c-Fos与T1增强子，但钙离子载体介导的T1基因上调并不需要这两者。在肥大细胞中过表达转录因子GATA-1可提升T1的合成水平。在肥大细胞GATA应答型最小启动子中，共鉴定出3个GATA元件。突变分析结果显示，这3个GATA元件均参与T1基因的表达调控。中间GATA元件内的点突变可完全消除启动子活性，而远端与近端GATA结合位点的突变则分别使启动子活性强度下降至原来的1/2与1/5。在NIH 3T3成纤维细胞中外源表达GATA-1，不足以激活肥大细胞特异性启动子。