A paracrine role for the epithelial progesterone receptor in mammary gland development

Abstract

Recently generated progesterone receptor (PR)-negative (PR-/-) mice provide an excellent model for dissecting the role of progesterone in the development of the mammary gland during puberty and pregnancy. However, the full extent of the mammary gland defect in these mice caused by the absence of the PR cannot be assessed, because PR-/- mice do not exhibit estrous cycles and fail to become pregnant. To circumvent this difficulty, we have transplanted PR-/- breasts into wild-type mice, and we have demonstrated that the development of the mammary gland in the absence of the PR is arrested at the stage of the simple ductal system found in the young virgin mouse. Mammary transplants lacking the PR in the stromal compartment give rise to normal alveolar growth, whereas transplants containing PR-/- epithelium conserve the abnormal phenotype. Chimeric epithelia in which PR-/- cells are in close vicinity to PR wild-type cells go through complete alveolar development to which the PR-/- cells contribute. Together, these results indicate that progesterone acts by a paracrine mechanism on a subset of mammary epithelial cells to allow for alveolar growth and that expression of the PR is not required in all the cells of the mammary epithelium in order for alveolar development to proceed normally.

Figure 1

Whole breast transplantation. Whole-mount preparations of the PR^−/− (Left) and PR^+/+ (Center) whole breast implant and endogenous mammary gland (Right) derived from RAG1^−/− recipient mouse after parturition.

Figure 2

Transplantation of engrafted fat pads. Whole-mount preparations of transplanted reconstituted breasts. Fat pads from PR^−/− or PR^+/+ mice were engrafted with ROSA26 (β-galactosidase⁺) PR^+/+ primary mammary epithelial cells and transplanted onto the abdominal muscle wall of PR^+/+.RAG1^−/− recipients. the reconstituted mammary glands were removed from the recipients after parturition and stained with X-Gal before whole-mounting.

Figure 3

Transplantation of epithelium. Whole-mount preparations of mammary glands from PR^+/+.RAG1^−/− recipients. (Left) Preparation derived from a recipient after parturition. (Right) Preparation derived from a virgin mouse. (Top) Transplanted PR^−/− epithelium. (Middle) Transplanted PR^+/+ epithelium. (Bottom) Endogenous mammary gland.

Figure 4

Rescue of the PR^−/− phenotype in PR^−/− and PR^+/+ chimeric epithelia. (Left) Whole-mount preparation of cleared PR^+/+.RAG1^−/− fat pad implanted with a mixture of PR^−/− (red) epithelium and ROSA26.PR^+/+ epithelium (blue) in a 1:1 ratio. The engrafted mammary gland was removed after the recipient had given birth, subjected to X-Gal staining, and whole-mounted.(Bar in Upper corresponds to 2 mm; bar in Lower, to 200 μm). (Center) Whole-mount preparation of cleared PR^+/+.RAG1^−/− fat pad injected with a mixture of PR^−/−.ROSA26 (blue) epithelium and PR^+/+ epithelium (red) injected in a 1:10 ratio, treated as for Left. (Bar corresponds to 200 μm.) (Right) Adjacent histological sections of an area with PR^−/−.ROSA26 alveolar structures. (Upper) Expression of β-casein in wt and PR^−/−.ROSA26 alveoli. (Lower) Control without primary antibody. Arrow indicates PR^−/−.ROSA26 alveolus expressing β-casein. (Bar corresponds to 50 μm.)