Protein Tyrosine Phosphatase CD148-Mediated Inhibition of T-Cell Receptor Signal Transduction Is Associated with Reduced LAT and Phospholipase Cγ1 Phosphorylation

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cγ1 (PLCγ1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCγ1 and LAT.

本研究探讨了受体样蛋白酪氨酸磷酸酶CD148（receptor-like protein tyrosine phosphatase CD148）在T细胞活化中的作用。在Jurkat T细胞系中过表达CD148，可在T细胞受体（T-cell receptor, TCR）刺激后抑制活化T细胞核因子的活化，但无法通过异源表达的G蛋白偶联受体——人毒蕈碱受体亚型1——的刺激产生该抑制效应。本研究采用四环素诱导表达系统，证实当CD148的表达水平接近活化原代T细胞中的内源水平时，其可抑制TCR介导的Ras通路与钙信号通路的活化。上述抑制效应依赖于CD148的磷酸酶活性。对TCR诱导的蛋白质酪氨酸磷酸化的分析显示，绝大多数磷酸化蛋白的水平不受CD148表达的影响。但在TCR刺激后，表达CD148的细胞中磷脂酶Cγ1（phospholipase Cγ1, PLCγ1）与LAT呈现显著的低磷酸化状态，而Slp-76与Itk的磷酸化水平仅出现轻度下降。基于上述结果，本研究提出CD148可通过干扰PLCγ1与LAT的磷酸化及功能，负向调控TCR信号通路。