Department of Molecular Immunology and Inflammation, Research Institute, National Center for Global Health and Medicine, 1-21-1 Toyama, Shinjuku-ku, Tokyo, Japan.
Department of Physiology, Faculty of Medicine, Toho University, 5-21-16 Omorinishi, Ota-ku, Tokyo, Japan.
Int Immunol. 2021 Jun 18;33(7):399-406. doi: 10.1093/intimm/dxab006.
Solute carrier family 15 member 4 (SLC15A4) is an endolysosome-resident amino acid transporter that regulates innate immune responses, and is genetically associated with inflammatory diseases such as systemic lupus erythematosus (SLE) and colitis. SLC15A4-deficient mice showed the amelioration of symptoms of these model diseases, and thus SLC15A4 is a promising therapeutic target of SLE and colitis. For developing a SLC15A4-based therapeutic strategy, understanding human SLC15A4's properties is essential. Here, we characterized human SLC15A4 and demonstrated that human SLC15A4 possessed pH- and temperature-dependent activity for the transportation of dipeptides or tripeptides. Human SLC15A4 localized in LAMP1+ compartments and constitutively associated with Raptor and LAMTORs. We also investigated SLC15A4's role in inflammatory responses using the human plasmacytoid dendritic cell line, CAL-1. Knock down (KD) of the SLC15A4 gene in CAL-1 (SLC15A4-KD CAL-1) impaired Toll-like receptor (TLR) 7/8 or TLR9-triggered type I interferon (IFN-I) production and mTORC1 activity, indicating that human SLC15A4 is critical for TLR7/8/9-mediated inflammatory signaling. We also examined SLC15A4's role in the autophagy response since SLC15A4 loss caused the decrease of mTORC1 activity, which greatly influences autophagy. We found that SLC15A4 was not required for autophagy induction, but was critical for autophagy sustainability. Notably, SLC15A4-KD CAL-1 severely decreased mitochondrial membrane potential in starvation conditions. Our findings revealed that SLC15A4 plays a key role in mitochondrial integrity in human cells, which might benefit immune cells in fulfilling their functions in an inflammatory milieu.
溶质载体家族 15 成员 4（SLC15A4）是一种内溶酶体驻留的氨基酸转运体，可调节先天免疫反应，并且与炎症性疾病（如系统性红斑狼疮（SLE）和结肠炎）具有遗传相关性。SLC15A4 缺陷型小鼠表现出这些模型疾病症状的改善，因此 SLC15A4 是 SLE 和结肠炎的有前途的治疗靶标。为了开发基于 SLC15A4 的治疗策略，了解人类 SLC15A4 的特性至关重要。在这里，我们对人类 SLC15A4 进行了表征，并证明人类 SLC15A4 具有 pH 和温度依赖性的二肽或三肽运输活性。人类 SLC15A4 定位于 LAMP1+隔室中，并与 Raptor 和 LAMTORs 组成性相关。我们还使用人浆细胞样树突状细胞系 CAL-1 研究了 SLC15A4 在炎症反应中的作用。CAL-1 中的 SLC15A4 基因敲低（KD）（SLC15A4-KD CAL-1）损害 Toll 样受体（TLR）7/8 或 TLR9 触发的 I 型干扰素（IFN-I）产生和 mTORC1 活性，表明人类 SLC15A4 对 TLR7/8/9 介导的炎症信号至关重要。我们还研究了 SLC15A4 在自噬反应中的作用，因为 SLC15A4 的缺失导致 mTORC1 活性的降低，这极大地影响自噬。我们发现 SLC15A4 对于自噬诱导不是必需的，但对于自噬的可持续性至关重要。值得注意的是，SLC15A4-KD CAL-1 在饥饿条件下严重降低线粒体膜电位。我们的研究结果表明，SLC15A4 在人类细胞中线粒体完整性中起关键作用，这可能有益于免疫细胞在炎症环境中发挥其功能。
