Abstract

HERPUD1 is a protein of the endoplasmic reticulum (ER) that is sensitive to the unfolded protein response (UPR) induced during ER stress and has been linked to ER stress tolerance in cancer cells. Many tumors, including triple-negative breast cancer (TNBC), which lacks an effective treatment, display UPR activity as a malignancy trait. However, whether HERPUD1 provides an ER-dependent mechanistic link for tumorigenic agents and/or potential therapeutic targets remains unknown. To address these possibilities, we first analyzed HERPUD1 expression in breast cancer (BC) biopsies via immunohistochemistry and immunofluorescence, revealing significantly higher levels in BC, including luminal A and TNBC, compared to non-malignant tissue. In TNBC, in addition to epithelial cells, HERPUD1 associated with inflammatory infiltrates, highlighting its potential role in tumor progression. Palmitic acid (PA), a dietary saturated fatty acid, is an obesity-associated tumor risk factor that induces ER stress and activates UPR. Interestingly, MDA-MB-231 cells, but not other BC cell lines, specifically upregulate HERPUD1 together with XBP1s and ATF4, key UPR factors, in response to PA, whereas TG treatment elevated HERPUD1 across all tested cell lines. HERPUD1 silencing reduced TNBC cell proliferation, migration, and invasion while enhancing doxorubicin (DOX) cytotoxicity, in both 2D and 3D cell culture models. HERPUD1 ablation also elevated UPR activation under TG. In contrast, PA-induced stress led to reduced UPR activation and lower IL-6 and IL-8 levels in the absence of HERPUD1 expression. We identified CK2 as a kinase that regulates HERPUD1 stability via Ser-59 phosphorylation. Strikingly, inhibition of CK2 with CX-4945 not only reduced HERPUD1 levels but also increased the sensitivity of BC cells to DOX. HERPUD1-S59D phosphomimetic mutants showed opposite effects. Our findings establish HERPUD1 as a key mediator of PA-driven aggressiveness, dependent on the lipid-handling capacity of TNBC cells and reveals a mechanistic to lipid stress and tumor progression. © The Author(s) 2025.