Tuneable hydrogel stiffness in a 3D in vitro model induces epithelial to mesenchymal transition in MCF7 but not MDA-MB-231 breast cancer cells

The study of in vitro models of breast cancer is crucial for understanding and treating the malignancy in patients, with 3D in vitro models providing researchers with more biomimetic systems to overcome limitations of current to 2D cultures and in vivo animal models. Ex vivo patient tissues have shown that malignant breast tissues are stiffer than healthy or benign tissues, and that the stiffness corresponds with increasing tumour grade. Stiffening of the breast tumour environment alters tumour cell phenotype and facilitates tumour progression, invasion and metastasis. Better understanding of the relationship between extracellular matrix stiffness and breast cancer cell phenotype, and how that is important in the initiation of metastasis, should lead to designing 3D models that mimic the breast tumour microenvironment at different stages of breast cancer progression. This study investigated phenotypic response of two breast cancer cell lines that are representative of clinical breast cancer subtypes (MCF7, Luminal A; MDA-MB-231, Triple Negative Breast Cancer) in gelatin-methacryloyl (GelMA) hydrogels of varying stiffness. A visible light photoinitiation system was adopted to provide a tuneable photocrosslinking platform to systematically control hydrogel stiffness and tumour microenvironment. This allowed rapid fabrication of biocompatible hydrogels supporting high cell viability over long-term culture. The impact of a clinically relevant range of microenvironmental stiffness on breast cancer cell behaviour and phenotype was examined over a 21-day culture period using GelMA hydrogels. Results showed that MCF7 cells cultured for 21 days in high stiffness hydrogels (10 wt%; 28 kPa) responded by downregulating the epithelial marker E-cadherin and upregulating mesenchymal markers N-cadherin and Vimentin, whereas MDA-MB-231 cells showed no changes in EMT-markers when cultured in hydrogels of corresponding stiffness (10 wt%; 33 kPa). Culturing both cell lines in soft hydrogels (5 wt%; 11 kPa) maintained their phenotype over 21 days, highlighting the importance of controlling hydrogel mechanical properties when studying breast cancer cell phenotype.