Progesterone, via its nuclear receptor, is mandatory not only for the induction and specification of mammary gland ductal side-branching and lobuloalveologenesis but also for carcinogen-induced mammary tumorigenesis. Notwithstanding these recent advances, a more comprehensive molecular explanation of progesterone-induced mammary morphogenesis is contingent upon the identification and characterization of mammary molecular targets that are responsive to the progesterone signal. Toward this goal, we report that calcitonin, a 32 amino acid peptide hormone involved in calcium homeostasis, is exclusively expressed in, and secreted from, luminal epithelial cells within the mammary gland of the pregnant mouse, and, importantly, its expression is progesterone-dependent. Conversely, the calcitonin receptor is present during all stages of post-natal mammary development examined, is localized to the myoepithelial cell lineage, and is not regulated by progesterone. Because calcitonin induction spatiotemporally correlates with increases in progesterone-induced mammary gland proliferation and structural remodeling, we posit that calcitonin - through its receptor - may be involved in one or both of these progesterone-dependent processes.

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