BACKGROUND: The endoplasmic reticulum (ER) plays a central role in protein homeostasis by facilitating the folding, modification, and quality control of secretory and membrane proteins. Disruption of ER function results in protein misfolding and ER stress, which activate the unfolded protein response (UPR). While the three canonical UPR branches, inositol-requiring enzyme 1 (IRE1), protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6), have been extensively studied, the mechanisms that coordinate their activities and ultimately dictate survival or death remain poorly understood. Transmembrane P24 trafficking protein 9 (TMED9), a cargo receptor that cycles between the ER and Golgi, has been implicated in protein quality control under pathological conditions, but its physiological role in ER proteostasis and UPR signaling is unclear. METHODS: The ER stress response was studied in cellular human models including normal epithelial cells and patient-derived pediatric glioma cultures. To define the regulatory mechanisms dictating TMED9 expression, quantitative Reverse Transcription polymerase chain reaction (qRT-PCR), luciferase reporter assay, and western blotting were employed. To elucidate TMED9 function, loss-of-function approaches, including clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9-mediated knockout and small interfering RNA knockdown were used in combination with RNA-seq and live imaging. Protein stability was tested by pulse-chase experiments, ubiquitination, and degradation analyses. To study the implications of TMED9 activation, we screened curated gene expression datasets from the European Molecular Biology Laboratory- European Bioinformatics Institute (EMBL-EBI) Expression Atlas and employed live-cell imaging-based assays and functional assays (cell viability, apoptosis, migration, and self-renewal). RESULTS: Our study uncovers a physiological role for TMED9 in ER proteostasis and UPR signaling. We show that, under ER stress, TMED9 expression is transcriptionally induced by the IRE1-spliced X-box binding protein 1 (XBP1s) pathway via a conserved unfolded protein response element (UPRE)-like element in its promoter. Removal of TMED9 selectively impairs ATF6 activation without altering IRE1 or PERK signaling, resulting in increased sensitivity to ER stress-induced apoptosis. Mechanistically, we identify TMED9 as a stress-induced stabilizer of ATF6 that prevents its ubiquitin-dependent proteasomal degradation. Functionally, TMED9 regulation is exploited by tumor cells, which sustain IRE1-XBP1s activity to upregulate TMED9, thereby enhancing survival under ER stress conditions. CONCLUSIONS: Collectively, our findings establish TMED9 as a critical regulator of ER stress adaptation. TMED9 emerges as a molecular mediator that links IRE1-dependent transcriptional response to ATF6 stabilization, ultimately supporting increased secretory demand under stress conditions and in cancer development.
Lenchisky et al. (Thu,) studied this question.