Calcium-Inducible MAPK/AP-1 Signaling Drives Semaphorin 3A Expression in Normal Human Epidermal Keratinocytes

Epidermal keratinocytes are primarily involved in the expression of semaphorin (Sema) 3A, which is involved in the regulation of cutaneous innervation. However, the mechanisms underlying the intracellular signaling of Sema3A expression in keratinocytes remain unknown. We herein investigated the signaling mechanisms for the induction of Sema3A expression in normal human epidermal keratinocytes (NHEKs). Sema3A expression is transiently increased in calcium-stimulated NHEKs, whereas it is markedly decreased in terminally differentiated NHEKs. Sema3A mRNA is mainly localized in the stratum basale and stratum suprabasale of the epidermis. We cloned the 5'-flanking region of the Sema3A gene and identified a critical region for Sema3A promoter activity within -134 base pairs of the start codon. We found transcription factor binding sites, including that for activator protein (AP)-1, in this region. Sema3A expression was increased by the co-overexpression of JunB and Fra-2 in the presence of 0.1 or 1.4 mM calcium. The calcium-mediated transient upregulation of Sema3A expression was significantly suppressed by mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) 1/2 or AP-1 inhibitors. These results demonstrate that the calcium-mediated transient upregulation of Sema3A in NHEKs is involved in the MEK/ERK and AP-1 signaling axis. Therefore, Sema3A mRNA may be expressed in the lower epidermis under controlled conditions by calcium via the MAPK-AP-1 axis.