The neurosensory tachykinins substance P and neurokinin A directly induce keratinocyte nerve growth factor

Nerve growth factor is an essential neurotrophic factor required for the growth and maintenance of cutaneous sensory nerves. In the skin, keratinocytes are a significant source of nerve growth factor; however, the regulation of cutaneous nerve growth factor production still remains to be fully understood. In this study we tested the hypothesis that neuropeptides released by cutaneous sensory nerves have the capacity to modulate directly the expression of keratinocyte nerve growth factor, which would have important implications for the maintenance and repair of nerves in the skin. In order to address this question experimentally we examined the effect of the neuropeptides, substance P and neurokinin A, on nerve growth factor expression in human keratinocytes and the murine keratinocyte PAM 212 cell line by quantitative reverse transcriptase-polymerase chain reaction, enzyme-linked immunosorbent assay, and the PC-12 nerve growth factor bioassay. The results of these studies indicated that substance P and neurokinin A can directly induce nerve growth factor mRNA expression and the secretion of bioactive nerve growth factor protein in both human and murine keratinocytes. The specificity of these responses was demonstrated using neuropeptide receptor antagonists and nerve growth factor blocking antibodies. Additional studies also demonstrated a significant in vivo upregulation of keratinocyte nerve growth factor expression in murine epidermis after the topical application of the neuropeptide releasing agent capsaicin. This is the first report demonstrating the induction of cutaneous nerve growth factor by sensory nerve-derived neuropeptides such as substance P and neurokinin A. This direct effect of the neurosensory system on keratinocyte nerve growth factor production may have important consequences for the maintenance and regeneration of cutaneous nerves in normal skin and during inflammation and wound healing.