Esposito, Marianna (2025) Revealing a novel FAM134B-calcium axis: redefining the link between ER-phagy and colorectal cancer. [Tesi di dottorato]

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Abstract

The endoplasmic reticulum (ER) is a multifunctional organelle essential for protein folding, lipid synthesis, and calcium (Ca²⁺) storage. Its structural and functional integrity relies on selective quality-control mechanisms, among which ER-phagy plays a central role. The ER-phagy receptor FAM134B is crucial for ER membrane remodeling and turnover. Initially identified in colorectal cancer (CRC) under the name JK-1, its specific contribution to CRC pathophysiology has remained elusive. Here, we explored the molecular and functional role of FAM134B in a panel of CRC cellular models: CCD 841 CoN (non-tumoral), Caco-2, SW480, SW48, and HCT116. Proteomic analyses revealed a consistent and specific interaction between FAM134B and the Ca²⁺ pump ATP2A2/SERCA2, pointing to a mechanistic connection between ER-phagy and Ca²⁺ homeostasis. To investigate this relationship, we generated FAM134B knockout cell lines. Loss of FAM134B caused an enlarged ER morphology, heightened sensitivity to SERCA inhibition, and enhanced Ca²⁺ release from the ER upon thapsigargin (TG) treatment. These disturbances led to a mitochondrial Ca²⁺ overload, increased ER–mitochondria contact sites, mitochondrial fragmentation, and impaired respiratory capacity. Functionally, FAM134B deficiency triggered a robust activation of the intrinsic apoptotic pathway. Remarkably, re-expression of FAM134B in knockout cells restored Ca²⁺ dynamics, mitochondria respiration and reduced apoptotic vulnerability. Altogether, these findings unveil a previously unrecognized role of FAM134B as a key coordinator of Ca²⁺ signaling at the ER-mitochondria interface, essential to maintain organelle integrity and cell survival under stress conditions.

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