Resistance to HER2 tyrosine-kinase inhibitor Lapatinib (Lap) is one of the leading causes of cancer treatment failure in HER2+ breast cancer (BC), associated with an aggressive tumor phenotype. Cryptotanshinone (Cry) is a natural terpene molecule that could function as a chemosensitizer by disturbing estrogen receptor (ERα) signaling and inhibiting the protein translation factor-4A, eIF4A. Therefore, we evaluated Cry dual regulation on eIF4A and ERα. This study aimed to elucidate the underlying mechanisms of Lap chemoresistance and the impact of Cry on them. We generated two Lap-resistant BT474 cell HER2+ variants named BT474^LapRV1 and BT474^LapRV2 with high chemoresistance levels, with 7- and 11-fold increases in EC₅₀, respectively, compared to BT474 parental cells. We found a PDCD4-p70S6Kβ axis association with Lap chemoresistance. However, a concomitant down-regulation of the RAF-MEK-ERK cell survival pathway and NF-κB was found in the chemoresistant cell variants; this phenomenon was exacerbated by joint treatment of Cry and Lap under a Lap plasmatic reported concentration. Optimized calcium management was identified as a compensatory mechanism contributing to chemoresistance, as determined by the higher expression of calcium pumps PMCA1/4 and SERCA2. Contrary to expectations, a combination of Lap and Cry did not affect the chemoresistance despite the ERα down-regulation; Cry-eIF4A binding possibly dampens this condition. Results indicated the pro-survival eIF4A/STAT/Bcl-xl pathway and that the down-regulation of the MAPK-NF-κB might function as an adaptive mechanism; this response may be compensated by calcium homeostasis in chemoresistance, highlighting new adaptations in HER2+ cells that lead to chemoresistance.

Breast cancer (BC) remains one of the leading causes of cancer-related death in women worldwide. HER2-positive BC accounts for approximately 15–20% of all cases and is associated with aggressive disease and poor prognosis. Although Lapatinib, a dual tyrosine kinase inhibitor (TKI), is a cornerstone in HER2+ BC therapy, the emergence of chemoresistance severely limits its long-term efficacy, a phenomenon that dyslipidemias may influence. We characterized a Lapatinib-resistant cell model (BT474LapRV2) to explore the impact of low-density lipoproteins (LDL) on chemoresistance mechanisms in HER2+ BC. LDL uptake was evaluated by flow cytometry and confocal microscopy. Functional assays, qPCR, Western blotting, and molecular docking were employed to assess changes in proliferation, apoptosis, and the regulation of drug metabolism and efflux pathways. LDL treatment promoted cell proliferation and resistance to apoptosis in both parental (BT474Par) and resistant BT474LapRV2 cells. This effect was associated with differential modulation of cholesterol-regulating proteins (LDLR, SREBP2), drug-metabolizing enzymes (CYP3A4, SULT1A1), and drug efflux transporters (ABCB1, ABCC1). Notably, LDL selectively upregulated SULT1A1 in resistant cells, suggesting a metabolic shift toward sulfation-mediated inactivation of Lapatinib. Molecular docking confirmed strong binding affinity of Lapatinib and its metabolites to SULT1A1, ABCC1, and ABCB1, supporting their role in drug detoxification and exportation. In parallel, LDL induced reactivation of HER2 expression in resistant cells, enhancing survival signaling. Data demonstrated that LDL facilitates Lapatinib resistance in HER2+ BC by remodeling drug and lipid metabolic pathways, notably through SULT1A1 overexpression and HER2 reactivation. These findings highlight LDL as a metabolic driver of chemoresistance and support the therapeutic potential of targeting lipid metabolism and phase II drug enzymes in resistant HER2+ BC.