Abstract 1800 HER2-Positive Breast Cancer Organoids Exhibit Distinct Reactions to Activin-A and HER2 Targeted Cancer Treatments

HER2-positive breast cancer (BC) is a highly heterogeneous disease where patients have a higher risk of recurrence and metastasis, and it is associated with high mortality rates. Within the tumor microenvironment (TM), tumor cells influence normal cells to alter their behavior, aiding tumor progression. Mesenchymal stem cells and cancer-associated fibroblasts are known to increase cytosolic secretion of Activin-A, a ligand from the transforming growth factor-beta (TGFβ) superfamily. Previous studies in our laboratory have demonstrated that populations of BC cells exhibit altered expression levels of Vimentin (VIM) and GRP78, in response to exogenous Activin-A. Therefore, is feasible to investigate the role of Activin-A in enriching aggressive HER2-positive BC populations towards a therapy/stress-resistant and metastatic state. The aim of our study was to develop organoid models derived from HER2-positive BC cell lines, BT-474 and UACC-812. The organoids were cultured in Cultrex hydrogel to resemble the TM and were treated at different stages of growth with Activin-A in combination with Traztuzumab or Lapatinib to assess whether Activin-A altered the organoids response to both drugs. Images of the organoids were obtained for 10 days and the area an eccentricity were measured for both cell models. Our results showed that BT-474 organoids displayed a higher population percentage of low area and high eccentricity organoids when treated with Activin-A and Lapatinib, while no significant changes were observed for UACC-812 organoids. Most dual treatments did not alter the dynamics of most organoid populations, but they did change the expression of VIM and GRP78. Immunoblotting analyses showed that BT-474 organoids treated at early stages of growth with Activin-A exhibited increased expression levels of GRP78. While for UACC-812, GRP78 expression increased at late stages of growth after treatment with Activin-A. Additionally, endogenous levels of Activin-A were measured through an ELISA assay for both organoid models demonstrating an increase in expression of Activin-A for BT-474 and no significant differences for UACC-812. These findings suggest that HER2-positive BC organoids populations display variable responses to Traztuzumab and Lapatinib in combination with Activin-A . While Activin-A treatment altered molecular markers important for tumor progression suggesting possible differences at the molecular level.