Carboxylesterase 2 induces mitochondrial dysfunction via disrupting lipid homeostasis in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is characterized by high recurrence and metastasis and places a heavy burden on societies worldwide. Cancer cells thrive in a changing microenvironment by reprogramming lipidomic metabolic processes to provide nutrients and energy, activate oncogenic signaling pathways, and manage redox homeostasis to avoid lipotoxicity. The mechanism by which OSCC cells maintain lipid homeostasis during malignant progression is unclear.

The altered expression of fatty acid (FA) metabolism genes in OSCC, compared with that in normal tissues, and in OSCC patients with or without recurrence or metastasis were determined using public data from the TCGA and GEO databases. Immunohistochemistry was performed to examine the carboxylesterase 2 (CES2) protein level in our own cohort. CCK-8 and Transwell assays and an in vivo xenograft model were used to evaluate the biological functions of CES2. Mass spectrometry and RNA sequencing were performed to determine the lipidome and transcriptome alterations induced by CES2. Mitochondrial mass, mtDNA content, mitochondrial membrane potential, ROS levels, and oxygen consumption and apoptosis rates were evaluated to determine the effects of CES2 on mitochondrial function in OSCC.

CES2 was downregulated in OSCC patients, especially those with recurrence or metastasis. CES2^high OSCC patients showed better overall survival than CES2^low OSCC patients. Restoring CES2 expression reduced OSCC cell viability and suppressed their migration and invasion in vitro, and it inhibited OSCC tumor growth in vivo. CES2 reprogrammed lipid metabolism in OSCC cells by hydrolyzing neutral lipid diacylglycerols (DGs) to release free fatty acids and reduce the membrane structure lipid phospholipids (PLs) synthesis. Free FAs were converted to acyl-carnitines (CARs) and transferred to mitochondria for oxidation, which induced reactive oxygen species (ROS) accumulation, mitochondrial damage, and apoptosis activation. Furthermore, the reduction in signaling lipids, e.g., DGs, PLs and substrates, suppressed PI3K/AKT/MYC signaling pathways. Restoring MYC rescued the diminished cell viability, suppressed migratory and invasive abilities, damaged mitochondria and reduced apoptosis rate induced by CES2.

We demonstrated that CES2 downregulation plays an important role in OSCC by maintaining lipid homeostasis and reducing lipotoxicity during tumor progression and may provide a potential therapeutic target for OSCC.