Abstract B38: Altered p53 status confers molecular subtype-dependent distinct interactions with fibroblasts in breast cancer

Different molecular subtypes of breast cancer have distinct interactions with stromal and immune cells in the cancer microenvironment. However, the role of specific somatic mutations in mediating stromal interactions is not well understood. Conversely, the role of stromal interactions in modulating the biological effects of tumor genetic events is understudied. This project hypothesized that (a) p53 mutations may modulate stromal-epithelial interactions, and (b) stromal-epithelial interactions may potentiate the effects of p53 loss. The p53 gene is mutated in 50% of breast cancers, with 84% of basal-like breast cancers harboring p53 mutations, compared to just 12-32% of luminal breast cancers. The influence of p53 loss on stromal-epithelial interactions across different molecular subtypes may have important implications for tumor progression. To study how gene expression of fibroblast-epithelial cocultures differ in luminal vs. basal-like breast cancers as a function of p53 status, we employed RNAi-mediated p53 knockdown (KD) in a panel of four epithelial cell lines (two basal-like, two luminal). Cocultures were carried out with p53-wildtype or KD cell lines maintained in direct physical contact with fibroblasts for 48 hours before RNA isolation. Results of gene expression analysis show that p53 knockdown significantly altered cell-cell interactions in a subtype-specific manner. Comparing cocultures of p53-wildtype to cocultures of p53 KD basal-like cells, we identified 1600 genes whose expression was significantly altered. Among luminal breast cancer cocultures, the impact of p53 loss on gene expression was far more limited, with no genes identified at the same level of significance. Among basal-likes, p53 loss-associated gene expression occurred in immune response pathways, including IL-8 signaling, IL-15 signaling LXR/RXR activation, TREM1 signaling, B cell receptor signaling. In addition, Wnt/β-catenin Signaling was altered in coculture due to p53 loss. Other recent studies have indicated that p53 may modulate Wnt signaling and EMT via a microRNA-dependent mechanism, and these findings suggest that the p53-Wnt axis is also modulated by stromal-epithelial interactions. These observations strongly suggest that the interactions of epithelial cells with fibroblasts are significantly affected by p53 status. Given the high frequency of basal-like tumors with p53 mutations, it is important to understand how stromal-epithelial interactions potentiate the effec