Orlistat Suppresses Small Intestinal Neuroendocrine Tumor Growth Through ATF3-Associated Stress and Ferroptosis-Related Responses—In Vitro and Cell Line Xenograft Evidence

Background/Objectives: The incidence of small intestinal neuroendocrine tumors (SI-NETs) has increased in recent years, while effective therapeutic options remain limited. This study aimed to investigate the antitumor effects of orlistat on SI-NETs and elucidate the underlying mechanisms. Methods: The effects of orlistat were evaluated using STC-1 cells, patient-derived primary SI-NET cells, and xenograft models. Functional assays, RNA sequencing, lipidomic profiling, ATAC sequencing, and molecular analyses were performed to assess tumor-related phenotypes and investigate the underlying mechanisms. Results: Orlistat inhibited cell proliferation, migration, and invasion, induced G1-phase arrest and apoptosis, and suppressed tumor growth in vivo. Mechanistically, orlistat triggered endoplasmic reticulum stress, inhibited PI3K/AKT/mTOR signaling, and upregulated activating transcription factor 3 (ATF3). Lipidomic profiling revealed alterations in lipid metabolism. Increased reactive oxygen species and lipid peroxidation, together with changes in ferroptosis-related proteins, indicated the involvement of ferroptosis-associated pathways. ATF3 overexpression reproduced the antitumor and ferroptosis-associated effects of orlistat. Integrated ATAC-seq and RNA-seq analyses identified CHAC1 as a potential downstream effector of ATF3. Conversely, ATF3 knockdown reduced CHAC1 expression and attenuated the effects of orlistat both in vitro and in vivo. Conclusions: Orlistat suppresses SI-NET progression and is associated with changes in endoplasmic reticulum stress, ATF3 expression, and ferroptosis-related responses, with CHAC1 identified as a potential downstream effector of ATF3. These findings support further investigation of orlistat as a potential therapeutic strategy for SI-NETs.