Fujita Wakako, Kuroiwa Yusuke
Laboratory of Pharmacotherapeutics, Faculty of Pharmacy, Juntendo University, Chiba 279-0013, Japan.
Department of Medical Pharmacology, Nagasaki University Graduate School of Biomedical Sciences, Nagasaki 852-8523, Japan.
Int J Mol Sci. 2024 Dec 21;25(24):13676. doi: 10.3390/ijms252413676.
The receptor transporter protein 4 (RTP4) is a receptor chaperone protein that targets class A G-protein coupled receptor (GPCR)s. Recently, it has been found to play a role in peripheral inflammatory regulation, as one of the interferon-stimulated genes (ISGs). However, the detailed role of RTP4 in response to inflammatory stress in the central nervous system has not yet been fully understood. While we have previously examined the role of RTP4 in the brain, particularly in neuronal cells, this study focuses on its role in microglial cells, immunoreactive cells in the brain that are involved in inflammation. For this, we examined the changes in the RTP4 levels in the microglial cells after exposure to inflammatory stress. We found that lipopolysaccharide (LPS) treatment (0.1~1 µg/mL, 24 h) significantly upregulated the mRNA levels in the microglial cell line, SIM-A9. Furthermore, the mRNA levels and extracellular levels of IFN-β were also increased by LPS treatment. This upregulation was reversed by treatment with neutralizing antibodies targeting either the interferon receptor (IFNR) or toll-like receptor 4 (TLR4), and with a TLR4 selective inhibitor, or a Janus kinase (JAK) inhibitor. On the other hand, the mitogen-activated protein kinase kinase (MEK) inhibitor, U0126, significantly enhanced the increase in RTP4 mRNA following LPS treatment, whereas the PKC inhibitor, calphostin C, had no effect. These findings suggest that in microglial cells, LPS-induced inflammatory stress activates TLR4, leading to the production of type I IFN, the activation of IFN receptor and JAK, and finally, the induction of gene expression. Based on these results, we speculate that RTP4 functions as an inflammation-responsive molecule in the brain. However, further research is needed to fully understand its role.
受体转运蛋白4（RTP4）是一种靶向A类G蛋白偶联受体（GPCR）的受体伴侣蛋白。最近，它被发现作为干扰素刺激基因（ISG）之一，在外周炎症调节中发挥作用。然而，RTP4在中枢神经系统对炎症应激反应中的具体作用尚未完全明确。虽然我们之前研究了RTP4在大脑中的作用，特别是在神经元细胞中的作用，但本研究聚焦于其在小胶质细胞中的作用，小胶质细胞是大脑中参与炎症的免疫反应细胞。为此，我们检测了小胶质细胞在暴露于炎症应激后RTP4水平的变化。我们发现脂多糖（LPS）处理（0.1~1μg/mL，24小时）显著上调了小胶质细胞系SIM-A9中的mRNA水平。此外，LPS处理还增加了IFN-β的mRNA水平和细胞外水平。用靶向干扰素受体（IFNR）或Toll样受体4（TLR4）的中和抗体、TLR4选择性抑制剂或Janus激酶（JAK）抑制剂处理可逆转这种上调。另一方面，丝裂原活化蛋白激酶激酶（MEK）抑制剂U0126显著增强了LPS处理后RTP4 mRNA的增加，而蛋白激酶C（PKC）抑制剂calphostin C则没有作用。这些发现表明，在小胶质细胞中，LPS诱导的炎症应激激活TLR4，导致I型干扰素的产生、干扰素受体和JAK的激活，最终诱导基因表达。基于这些结果，我们推测RTP4在大脑中作为一种炎症反应分子发挥作用。然而，需要进一步研究以全面了解其作用。
