Noncanonical function of Claudin-1 mitigates lethal endoplasmic reticulum stress to drive prostate cancer lineage plasticity and chemoresistance

Neuroendocrine prostate cancer (NEPC) is a lethal subtype of castration-resistant prostate cancer (CRPC) driven by therapy-induced lineage plasticity. While endoplasmic reticulum (ER) homeostasis is critical for cell fate determination and therapeutic response, the molecular mechanisms linking ER stress regulation to lineage plasticity and chemoresistance remain poorly defined. Claudin-1 (CLDN1), a canonical tight junction protein, has been implicated in cancer progression, but its noncanonical functions in intracellular signaling and organelle homeostasis remain largely unexplored. We integrated multi-cohort transcriptome analysis with in-house RNA sequencing to identify key regulators of NEPC and docetaxel resistance. Mechanistic studies were performed using co-immunoprecipitation coupled with mass spectrometry, site-directed mutagenesis, ubiquitination assays, and transmission electron microscopy. Functional validation was conducted in vitro using gain- and loss-of-function approaches and in vivo using subcutaneous xenograft models. We identify CLDN1 as a key regulator of both neuroendocrine differentiation (NED) and docetaxel resistance in prostate cancer. Mechanistically, CLDN1 promotes the recruitment of the deubiquitinase USP7 to the ER membrane, where it mediates site-specific deubiquitination of the ER chaperone calnexin (CANX) at conserved lysine residues K118, K170, and K199. This post-translational modification stabilizes CANX protein, enhances ER protein folding capacity, and mitigates lethal ER stress induced by chemotherapy. Pharmacological activation of ER stress with tunicamycin effectively reverses CLDN1-induced NED and chemoresistance in vivo. Clinical tissue analysis further confirms a significant positive correlation between CLDN1 expression and neuroendocrine markers in human prostate cancer specimens. Our study reveals a previously unrecognized noncanonical function of CLDN1 that strengthens the interaction between USP7 and CANX to regulate ER homeostasis. The CLDN1-USP7-CANX axis represents a novel molecular link between tight junction proteins, organelle homeostasis, and tumor lineage plasticity, providing both fundamental insights into cancer cell reprogramming and a promising therapeutic target for treatment-resistant prostate cancer.