Parathyroid hormone-related peptide medulates signal pathways in skin and hair follicle cells

Abstract: Parathyroid hormone‐related protein (PTHrP) is secreted by skin epithelial cells and is thought to play an important role in the development and function of the hair follicle. It was hypothesized that PTHrP binds to receptors in dermal papilla cells and modulates intracellular signaling systems in these cells. We tested the effects of PTHrP on protein synthesis, protein kinase A (PKA) and protein kinase C (PKC) activities as well as tyrosine phosphorylation in rat vibrissa dermal papilla and capsular fibroblast cells. Cells were cultured in the presence or absence of the N‐terminal peptide PTHrP1‐34 for 48 h and detergent extracts prepared. Proteins were separated by electrophoresis. Phosphotyrosine and the PTH/PTHrP receptor immunoreactivity was identified by Western blot analysis. PKC and PKA activities in the cells were measured using colorimetric enzyme assays. Extracts of both dermal papilla cells and capsular fibroblasts displayed immunoreactivity to the PTH/PTHrP receptor. Electrophoresis showed that PTHrP treatment reduced the density of a 50‐kDa protein in dermal papilla cells but not in capsular fibroblasts. Media conditioned by the cells showed similar changes, indicating that the PTHrP‐modulated 50‐kDa protein was secreted. Furthermore, 2‐D gel electrophoresis indicated that the protein had a number of phosphorylation sites. Western analysis with antiphosphotyrosine antibodies confirmed a significant decrease in the intensity of a phosphorylated 50‐kDa protein in papilla cells and papilla cell‐conditioned medium. PKC and PKA activities of papilla cells were unaffected by PTHrP. However, activities of PKC were increased and PKA reduced in capsular fibroblasts following peptide treatment. These cell‐specific effects showed that endogenous PTHrP may activate different intracellular pathways in mesenchymal cells of skin and elicit changes in levels of locally secreted proteins that specifically modulate normal follicular function.