Abstract PO4-08-01: Methylsterol monooxygenase 1 (MSMO1) modulates endoplasmic reticulum stress through cholesterol metabolic reprogramming to alter chemotherapy sensitivity in breast cancer
Authors: Ren Hengyu , Wangxu Liren , Chi Yayun , Wu Jiong 
Published: May 2024

Abstract
Background
Endoplasmic reticulum stress (ER stress) is a process in which cells activate unfolded protein response and other signaling pathways in response to misfolded and unfolded protein aggregation and calcium ion balance disturbance in the endoplasmic reticulum cavity. Depending on the intensity of endoplasmic reticulum stress, it can not only induce chaperone expression to play a protective effect, but also induce apoptosis independently. Metabolic reprogramming is one of the characteristics of tumor, which refers to the metabolic changes that cells make in response to various stimuli. Here, we demonstrate that methylsterol monooxygenase 1 (MSMO1) regulates cellular cholesterol metabolism and influences breast cancer susceptibility to chemotherapy by regulating the relative intracellular T-MAS content.
Methods
RNA-seq was performed on core needle biopsy samples from 26 patients receiving neoadjuvant chemotherapy for breast cancer to screen for genes that may influence neoadjuvant chemotherapy sensitivity. Cell proliferation, CCK8 assay and apoptosis assay were performed for MSMO1 phenotype study. Transmission electron microscopy (TEM) was conducted to visualize the structure of endoplasmic reticulum. Western Blot was used to detect the activation level of the unfolded protein response pathway. The relative content of sterols in MSMO1 knockdown cells was detected by targeted lipid mass spectrometry. AO/PI staining of breast cancer organoids was performed to verify the effect of MSMO1 metabolic substrate T-MAS on chemotherapy drug sensitivity.
Results
RNA-seq of core needle biopsy samples from breast cancer patients receiving neoadjuvant chemotherapy suggested that MSMO1 was highly expressed in non-pCR patients, and clinical survival data suggested that MSMO1 was associated with poor prognosis in patients with all types of breast cancer, suggesting that MSMO1 might be related to chemotherapy sensitivity of breast cancer. Phenotypic experiments revealed that MSMO1 regulates chemotherapy sensitivity of breast cancer both in vivo and in vitro. Ectopic overexpression of MSMO1 promoted cancer cell resistance to chemotherapy. To uncover the underlying mechanisms, RNA-seq of MSMO1 knock-down cells were carried out and identified that MSMO1 was associated with ER stress. TEM and western blot assay indicated that downregulation of MSMO1 expression leads to endoplasmic reticulum swelling and activation of the unfolded protein response. Considering that MSMO1 is a cholesterol metabolizing enzyme, targeted lipids mass spectrometry was performed to detect the changes in the contents of various intermediate metabolites in the cholesterol pathway, which indicated that the relative content of T-MAS, a metabolic substrate of MSMO1, was significantly up-regulated in MSMO1 knockdown cells. Then, in order to verify whether this metabolite can independently regulate ER stress, breast cancer cells were given T-MAS externally, and TEM and Western Blot analysis indicated that T-MAS could induce ER stress and activate the unfolded protein response independently. Apoptosis assay and AO/PI staining of breast cancer organoids revealed that T-MAS can improve the sensitivity of breast cancer to chemotherapy drugs.
Conclusion
In summary, these results shed light on the role of MSMO1 in cholesterol metabolism reprogramming, which adjust the endoplasmic reticulum stress state of cells by changing the relative content of intracellular T-MAS, and thereby influencing the sensitivity of breast cancer chemotherapy. As an intermediate metabolite of cholesterol metabolism, T-MAS has potential clinical application value of chemotherapy sensitization.
Citation Format: Hengyu Ren, Liren Wangxu, Yayun Chi, Jiong Wu. Methylsterol monooxygenase 1 (MSMO1) modulates endoplasmic reticulum stress through cholesterol metabolic reprogramming to alter chemotherapy sensitivity in breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-08-01.