The microenvironment influences the progression of an epithelial malignancy. To examine the effect of fibroblasts on epithelial cells by direct cell-cell contact in vitro, a coculture system was designed to assess cell death and proliferation of two cell populations when grown together. We used a green fluorescent dye to stain fibroblasts and distinguish them from unstained epithelial cells by a flow cytometer. We show that tumor cell death is 5-fold less when cocultured with normal human prostatic fibroblasts than when cultured alone. In contrast, proliferation of tumor cells was similar when cocultured with normal human prostatic fibroblasts or when grown alone. The reduction in tumor cell death during coculture appears to play a significant role in promoting tumor formation. Combination of prostatic fibroblasts with LNCaP xenografts formed large tumors at a high frequency with a low apoptotic index in vivo, whereas, LNCaP xenografts alone formed small infrequent tumors with a high apoptotic index. Therefore, prostatic fibroblasts promote tumor formation by retarding the apoptotic pathways in tumor cells.

The present study demonstrates that fibroblasts associated with carcinomas stimulate tumor progression of initiated nontumorigenic epithelial cells both in an in vivo tissue recombination system and in an in vitro coculture system. Human prostatic carcinoma-associated fibroblasts grown with initiated human prostatic epithelial cells dramatically stimulated growth and altered histology of the epithelial population. This effect was not detected when normal prostatic fibroblasts were grown with the initiated epithelial cells under the same experimental conditions. In contrast, carcinoma-associated fibroblasts did not affect growth of normal human prostatic epithelial cells under identical conditions. From these data, we conclude that in this human prostate cancer model, carcinoma-associated fibroblasts stimulate progression of tumorigenesis. Thus, carcinoma-associated fibroblasts can direct tumor progression of an initiated prostate epithelial cell.