Beyond chaperoning: HSPA8 modulates alternative splicing of PPP2R2D to drive apoptotic evasion and macrophage-associated immune microenvironment remodeling in triple-negative breast cancer.

INTRODUCTION

The molecular chaperone HSPA8 is classically recognized for its role in protein homeostasis, and while its upregulation correlates with poor prognosis in triple-negative breast cancer (TNBC), whether HSPA8 exerts non-canonical functions beyond chaperoning remains unknown.

METHODS

Through integrative transcriptomic profiling, RNA immunoprecipitation (RIP), isoform-specific rescue experiments, and in vivo xenograft models, we investigated the functional and mechanistic role of HSPA8 in TNBC pathogenesis.

RESULTS

We unveil a paradigm-shifting function for HSPA8 as a modulator of alternative splicing of the PP2A regulatory subunit gene PPP2R2D, promoting exon 2 retention to favor the full-length PPP2R2D-2 isoform over the truncated PPP2R2D-1. These two isoforms exhibit functionally distinct roles: only PPP2R2D-2 possesses pro-survival activity, whereas PPP2R2D-1 lacks this function. Specifically, PPP2R2D-2 rescues HSPA8 depletion-induced apoptosis by modulating the mitochondrial pathway (BAX/BCL-2/cleaved Caspase-3) and restores in vivo tumor growth. Strikingly, this splicing axis also correlates with an immunosuppressive tumor microenvironment, characterized by altered M1/M2 macrophage marker abundance.

DISCUSSION: These findings uncover a dual mechanism that simultaneously drives intrinsic survival signaling and is associated with alterations in the tumor immune microenvironment. They transcend the classical chaperone paradigm by establishing HSPA8 as a non-canonical splicing modulator, and the HSPA8-PPP2R2D-2 axis emerges as a critical vulnerability in TNBC, offering a novel therapeutic angle that concurrently targets tumor cell survival and microenvironmental immunosuppression.