Targeting progesterone receptors in breast cancer.

Hormone receptors represent the earliest biomarkers used in breast cancer not only as prognosis markers but, in addition, to decide treatment. However, mostly estrogen receptors have been used as therapeutic targets. There is compelling evidence indicating that progesterone receptors (PRs) play a hierarchical role in breast cancer growth and that they might be potentially used to improve the success of endocrine treatments. The two PR isoforms, PR-A and PR-B, play differential roles in regulating gene expression. Tumors overexpressing one or other PR isoform may respond different to endocrine treatment. In this chapter, we highlight the evidence regarding progestins as promoters or inhibitors of cell proliferation in order to understand the dual role of PR in regulating tumor growth, underscoring thus the need of biomarkers to identify which patients may benefit with an antiprogestin/progestin treatment.