Figure 1. Tumors from the MPA-induced breast cancer model used in this study.

Hormone-dependent mammary carcinomas induced by MPA are maintained by syngeneic passages in MPA-treated or untreated BALB/c mice. Occasionally hormone independent (HI) variants arise. These HI variants show different PR isoform ratios. C4-2-HI and 59-HI express higher levels of PRB than PRA and are resistant to MFP treatment (Western blots in the right panel). C4-HI and 59-2-HI show higher levels of PRA than PRB (Western blots in the left panel) and are MFP responsive tumors. These four tumors were selected for this study. The model is reviewed in Lanari et al., [32].