3D Breast Cancer Spheroids Reveal Architecture-Dependent HER2 Expression and Signaling

Background: Three-dimensional (3D) culture systems offer a physiologically relevant alternative to monolayers for studying tumor organization, signaling, and drug response. HER2-positive breast cancers (BCa) account for 15–30% of BCa cases and benefit from HER2-targeted therapies, yet predictive in vitro models remain limited. Objective: To generate and compare 3D spheroids from two HER2+ BCa cell lines, SKBR3 and BT474, and investigate how 3D architecture influences HER2 distribution, intracellular signaling, and cellular organization. Methods: Spheroids were reproducibly generated from SKBR3 and BT474 cells and analyzed after 4 days of culture. Cell viability was evaluated using live/dead staining, HER2 distribution was assessed by confocal microscopy and quantified on cryosections, and protein expression/phosphorylation was measured by Western blotting. Epithelial and EMT markers were visualized by immunofluorescence, and ultrastructural features were examined by transmission electron microscopy (TEM). Results: Both cell lines formed viable spheroids with distinct architectures: SKBR3 spheroids were loose and heterogeneous, whereas BT474 spheroids were compact and highly spherical. Confocal and cryosection imaging showed consistent membrane HER2 localization with a progressive signal decrease toward the core of the spheroids, more pronounced in BT474. Western blotting revealed divergent HER2 expression and AKT phosphorylation: SKBR3 spheroids displayed increased HER2 but reduced pAKT, while BT474 spheroids showed reduced HER2 and pAKT levels. EpCAM and E-cadherin staining revealed cell line–specific epithelial organization, and TEM demonstrated differences in intercellular spacing and mitochondrial morphology reflecting spheroid compactness. Conclusion: 3D architecture profoundly influences HER2 distribution, signaling, and structural organization in HER2+ BCa spheroids. This model provides a robust platform for investigating architecture-dependent molecular processes, with potential applications in drug response, receptor trafficking, and targeted therapy evaluation.