The meaning of cell sensitivity in presence of progesterone: Progesterone receptor is sensitive to progesterone, this sensitivity is due to the estrogen producing the progestorone receptors (it needs progesterone to bring out). But the supplement of progesterone inhibit the proliferation of estrogen receptors, and reducing their content.
Reference:
1. Antiestrogen action of progesterone in breast tissue, Mauvais-Jarvis P, Kuttenn F, Gompel A. Department of Reproductive Endocrinology, Necker Faculty of Medicine, Paris, France.