Intestinal Amino Acid Availability via PEPT‑1 Affects TORC1/2 Signaling and the Unfolded Protein Response

The intestinal peptide transporter PEPT-1 plays an important role in development, growth, reproduction, and stress tolerance in Caenorhabditis elegans, as revealed by the severe phenotype of the pept-1-deficient strain. The reduced number of offspring and increased stress resistance were shown to result from changes in the insulin/IGF-signaling cascade. To further elucidate the regulatory network behind the phenotypic alterations in PEPT1-deficient animals, a quantitative proteome analysis combined with transcriptome profiling was applied. Various target genes of XBP-1, the major mediator of the unfolded protein response, were found to be downregulated at the mRNA and protein levels, accompanied by a reduction of spliced xbp-1 mRNA. Proteome analysis also revealed a markedly reduced content of numerous ribosomal proteins. This was associated with a reduction in the protein synthesis rate in pept-1 C. elegans, a process that is strictly regulated by the TOR (target of rapamycine) complex, the cellular sensor for free amino acids. These data argue for a central role of PEPT-1 in cellular amino acid homeostasis. In PEPT-1 deficiency, amino acid levels dropped systematically, leading to alterations in protein synthesis and in the IRE-1/XBP-1 pathway.肠道肽转运蛋白PEPT-1（PEPT-1）在秀丽隐杆线虫（Caenorhabditis elegans）的发育、生长、繁殖与应激耐受过程中发挥关键作用，这一结论可由pept-1基因缺陷菌株的严重表型得以佐证。研究证实，子代数量减少与应激抗性增强均源于胰岛素/IGF信号级联反应的改变。为进一步阐明PEPT-1缺陷型动物表型改变背后的调控网络，本研究采用了定量蛋白质组学分析结合转录组谱分析的研究策略。未折叠蛋白反应（unfolded protein response）的核心介导因子XBP-1的诸多靶基因在mRNA与蛋白质水平均呈现下调表达，同时伴随剪接型xbp-1 mRNA的含量降低。蛋白质组学分析还显示，大量核糖体蛋白的含量显著降低。这一现象与pept-1缺陷型秀丽隐杆线虫的蛋白质合成速率下降存在关联，而该过程严格受TOR（target of rapamycine）复合体调控——该复合体是细胞内游离氨基酸的感受器。上述数据表明，PEPT-1在细胞氨基酸稳态中占据核心地位。在PEPT-1缺失状态下，氨基酸水平呈系统性下降，最终引发蛋白质合成与IRE-1/XBP-1通路的改变。