Development of a fully "humanized" xenograft model of breast cancer
Until now there has been a distinct lack of a truly representative breast cancer model. The development of a complex heterologous multi-compartment xenograft model incorporating the relevant stromal elements will provide a realistic alternative to currently available chimeric xenograft models. The recent ability to immortalize primary human mammary endothelial cells and fibroblasts by the insertion of the hTERT and a temperature sensitive mutant variant of SV40 LT has made this possible. We have commenced the development of an organotypic, 3-compartment xenograft model of human breast cancer. The immortalized mammary stromal cells provide a viable and much needed tumour microenvironment of human origin for tumour proliferation. Results have confirmed the crucial importance of stromal cell support as well as tumour-stromal interactions in tumourigenesis. MCF-7 cells when xenografted in numbers insufficient to produce tumours alone, consistently produced tumours when combined with stromal cells. Also, heterologous xenografts produced not only faster growing but larger tumours than tumour cell lines alone. Immunohistochemical analysis using human cell specific markers demonstrated that initial tumour growth was supported by the stromal cells before the recruitment of host vasculature. The xenografts were composed predominantly of proliferating tumour cells, host ECM and vasculature. Surprisingly the LT antigen expressing immortalized stromal cells did not survive, proliferate or form organized structures in this environment for any reasonable time period. To conclude, these findings parallel observations made in 3-D organotypic cultures which indicates that progression of the epithelial cell tumour is not cell autonomous. Rather, that tumourigenesis is promoted, and probably sometimes induced, by the anomalies in the surrounding stroma and microenvironment. With further refinement and adjustments this model can be utilized as a credible pre-clinical model for the development and testing of new therapeutic strategies, such as those that target breast cancer stroma itself. This is very timely as stroma is now emerging as the dominant factor in modulating epithelial morphogenesis and mitogenesis.