Parathyroid hormone-related protein specifies the mammary mesenchyme and regulates embryonic mammary development

Abstract

Parathyroid Hormone related Protein (PTHrP) is a critical regulator of mammary gland morphogenesis in the mouse embryo. Loss of PTHrP, or its receptor, PTHR1, results in arrested mammary buds at day 15 of embryonic development (E15). In contrast, overexpression of PTHrP converts the ventral epidermis into hairless nipple skin. PTHrP signaling appears to be critical for mammary mesenchyme specification, which in turn maintains mammary epithelial identity, directs bud outgrowth, disrupts the male mammary rudiment and specifies the formation of the nipple. In the embryonic mammary bud, PTHrP exerts its effects on morphogenesis, in part, through epithelial-stromal crosstalk mediated by Wnt and BMP signaling. Recently, PTHLH has been identified as a strong candidate for a novel breast cancer susceptibility locus, although PTHrP's role in breast cancer has not been clearly defined. The effects of PTHrP on the growth of the embryonic mammary rudiment and its invasion into the dermis may, in turn, have connections to the role of PTHrP in breast cancer.

Figure 1. Expression of PTHrP-lacZ in the mammary placodes

X-gal staining is seen in the mammary placode (A) and in cells of the mammary line (A, arrow). At E12.5, staining is observed in all five placodes (B) and along a “tail” of cells extending outwards from the placodes (arrowheads). Staining is also observed in the cells between the mammary placodes (B, arrows). FL= forelimb bud, HL= hindlimb bud.

Figure 2. Schematic representation of the consequences of PTHrP/PTHR1 deficiency

Mammary epithelium (pink, ME) is specified in WT, PTHrP^−/− and PTHR1^−/− embryos. Loss of PTHrP and PTHR1 results in reduced mammary mesenchyme (blue, MM) that does not support outgrowth of the mammary rudiment (MR) or formation of the nipple. Instead, the mammary buds revert to an epidermal fate (reverting bud, RB).

Figure 3. Downstream signaling engaged by PTHrP during embryonic mammary development

PTHrP is secreted from the mammary epithelium (pink) and acts on the mammary mesenchyme (blue) to activate signaling via β-catenin. This pathway directs sexual dimorphism, in part via regulation of AR expression, and possibly regulates the response to BMP signaling. PTHrP signaling, via PTHR1, also upregulates BMPR1a and sensitizes mammary mesenchymal cells to BMP4, possibly upregulating BMP signaling in the mammary mesenchyme. This pathway is required for bud outgrowth, via MMP2 and for inhibiting hair folliculogenesis in the nipple, via Msx2. PTHrP signaling also maintains mammary epithelial identity via signaling in the mammary mesenchyme, although the exact mechanism remains unknown.