DNA damage-based drugs are widely used in cancer therapy, yet resistance remains a major challenge. In this study, we uncovered a non-DNA repair mechanism contributing to resistance in pancreatic ductal adenocarcinoma (PDAC). We show that in gemcitabine-resistant PDAC cells, CIP2A undergoes ubiquitin-mediated degradation, resulting in enhanced PP2A phosphatase activity. This leads to the dephosphorylation of ATR at Ser428 in the cytoplasm, promoting the formation of the prolyl cis-isomeric form of ATR at its Ser428-Pro429 motif. The resulting cis-ATR functions as a mitochondria-targeted antiapoptotic protein (mtATR). Surprisingly, resistant PDAC cells paradoxically accumulated both mtATR and proapoptotic tBid at the mitochondria, forming a stable mtATR-tBid complex that induces a state of apoptotic dormancy. Disrupting this complex, either with the PP2A inhibitor LB-100 or a cytoplasmic ATR-specific antibody, reactivates the pre-accumulated tBid and restores apoptosis in resistant PDAC cells. In an orthotopic PDAC mouse model, LB-100 alone significantly inhibit gemcitabine-resistant tumor growth by disrupting the mtATR-tBid complex. These findings reveal a previously unrecognized mechanism of resistance to DNA damage-based therapies and identify a novel action mechanism of LB-100, characterized by the CIP2A degradation-mediated PP2A-mtATR-tBid axis. By targeting mtATR-tBid-mediated apoptotic dormancy, this strategy offers a promising approach to restore apoptotic sensitivity in drug-resistant cancers.
