Estrogen modulates mesenchyme-epidermis interactions in the adult nipple.

Maintenance of specialized epidermis requires signals from the underlying mesenchyme; however, the specific pathways involved remain to be identified. By recombining cells from the ventral skin of the K14-PTHrP transgenic mice [which overexpress parathyroid hormone-related protein (PTHrP) in their developing epidermis and mammary glands] with those from wild type, we show that transgenic stroma is sufficient to reprogram wild-type keratinocytes into nipple-like epidermis. To identify candidate nipple-specific signaling factors, we compared gene expression signatures of sorted Pdgfrα-positive ventral K14-PTHrP and wild-type fibroblasts, identifying differentially expressed transcripts that are involved in WNT, HGF, TGFß, IGF, BMP, FGF and estrogen signaling. Considering that some of the growth factor pathways are targets for estrogen regulation, we examined the upstream role of this hormone in maintaining the nipple. Ablation of estrogen signaling through ovariectomy produced nipples with abnormally thin epidermis, and we identified TGFß as a negatively regulated target of estrogen signaling. Estrogen treatment represses Tgfß1 at the transcript and protein levels in K14-PTHrP fibroblasts in vitro, while ovariectomy increases Tgfb1 levels in K14-PTHrP ventral skin. Moreover, ectopic delivery of Tgfß1 protein into nipple connective tissue reduced epidermal proliferation. Taken together, these results show that specialized nipple epidermis is maintained by estrogen-induced repression of TGFß signaling in the local fibroblasts.

Expansion of specialized epidermis induced by hormonal state and mechanical strain.

In mammals, some sites of specialized skin such as the palms, soles, and lips grow proportionally with the animal. However, other types of specialized skin such as the nipple and anal/genital region are dramatically altered with changes of reproductive status. The specific cell types that mediate the growth of these sites have not been identified. In the mouse, we observed a dramatic expansion of the specialized epidermis of the nipple, coupled to changes in connective tissue and hair shaft density, which we designate as areola formation. During this process thymidine analog uptake was elevated in the epidermis and hair follicles. Although there were no changes in connective tissue cell proliferation, we did observe an altered expression of extracellular matrix genes. In addition, the fibroblasts of the virgin nipple areola and region showed increased transcript and protein levels for estrogen, progesterone, relaxin, and oxytocin relative to those of ventral skin. To determine the role of pregnancy, lactation hormonal milieu, and localized mechanical strain on areola formation, we created models that separated these stimuli and evaluated changes in gross structure, proliferation and protein expression. While modest increases of epidermal proliferation and remodeling of connective tissue occurred as a result of individual stimuli, areola formation required exposure to pregnancy hormones, as well as mechanical strain.

Parathyroid hormone hormone-related protein and the PTH receptor regulate angiogenesis of the skin.

In developing organs, parathyroid hormone (PTH)/parathyroid hormone-related protein (PTHrP) receptor (PPR) signaling inhibits proliferation and differentiation of mesenchyme-derived cell types resulting in control of morphogenic events. Previous studies using PPR agonists and antagonists as well as transgenic overexpression of the PPR ligand PTHrP have suggested that this ligand receptor combination might regulate the anagen to catagen transition of the hair cycle. To further understand the precise role of PTHrP and the PPR in the hair cycle, we have evaluated hair growth in the traditional K14-PTHrP (KrP) and an inducible bitransgenic PTHrP mice. High levels of PTHrP trangene expression limited to the adult hair cycle resulted in the production of shorter hair shafts. Morphometric analysis indicated that reduced proliferation in the matrix preceded the appearance of thinner hair follicles and shafts during late anagen. CD31 staining revealed that the late anagen hair follicles of the KrP mice were surrounded by reduced numbers of smaller diameter capillaries as compared to controls. Moreover, the fetal skins of the PTHrP and PPR knockouts (KOs) had reciprocal increases in the length, diameter, and density of capillaries. Finally, crossing the KrP transgene onto a thrombospondin-1 KO background reversed the vascular changes as well as the delayed catagen exhibited by these mice. Taken together, these findings suggest that PTHrP's influence on the hair cycle is mediated in part by its effects on angiogenesis.