ABSTRACT
TRX is also released from various types of mammalian cells despite the absence of a typical secretory signal sequence5,6 and has been shown to have several cytokine-and chemokine-like activities.4,7 However, the mechanism of TRX release is still unknown. To better understand the mechanism, we generated the Jurkat stable transfectant cells expressing wild-type TRX and a mutant TRX (C32S/C35S), which has two cysteines displaced by serine in the active site of TRX.8 We tested whether the redox-active site of TRX is involved in the release of TRX in response to oxidative stress, e.g., by exposure to H2O2 (Figure 1.3A). The wild-type TRX was rapidly released from the Jurkat transfectant expressing wild-type TRX but not from the mutant TRX C32S/C35S. The result suggested that the intact sequence of a redox-active site (-CGPC-) in the TRX protein is indispensable for its active release from T lymphocytes by oxidative stress. We also found that this TRX release is regulated by extracellular TRX (Figure 1.3B). TRX release by exposure to H2O2 was inhibited in a dose-dependent manner by the presence of rTRX-WT. The result suggested the presence of target molecules
on the plasma membrane, which sense the concentration of TRX in extracellular fraction. Next, we studied the physiological functions of extracellular TRX. The pretreatment with rTRX suppressed H2O2-induced apoptosis. As shown in Figure 1.3C, extracellular rTRX attenuated ROS production and ROS-mediated apoptosis via caspase-3 activation. Based on these results, we speculated that the release of TRX from T lymphocytes is regulated by a negative feedback loop that senses the concentration of TRX in and/or outside the cells for the purpose of maintaining the physiological condition of the cells.
