Biological effects of SLURP-1 on human keratinocytes.

A novel paradigm of keratinocyte (KC) regulation via nicotinic acetylcholine receptors (nAChR) has been discovered in studies of SLURP (secreted mammalian Ly-6/urokinase-type plasminogen activator receptor-related protein)-1 in Mal de Meleda. We cloned human SLURP-1 and produced recombinant protein and the monoclonal antibody 336H12-1A3 that visualized native SLURP-1. SLURP-1 ligated the conventional ligand-binding site of KC nAChR, showing a higher affinity to the [(3)H]nicotine-, compared with the [(3)H]epibatidine-sensitive nAChR. SLURP-1 significantly (p<0.05) increased the activities of caspases 3 and 8, and the number of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling-positive cells. The pro-apoptotic activity of SLURP-1 exceeded that of tumor necrosis factor-alpha, suggesting the involvement of separate pathways. In a series of real-time PCR and in-cell western experiments, SLURP-1 significantly (p<0.05) upregulated expression of transglutaminase type I cytokeratin 10, p21, and caspase-3. In the presence of the agonist carbachol, the effects of SLURP-1 on gene expression were augmented, which is in keeping with the notion that SLURP-1 acts as an allosteric agonist at the KC nAChR. Thus, the changes in the cell state induced by SLURP-1 could result from nAChR-mediated effects on the KC gene expression. These results suggest that the biological role of SLURP-1 in the epidermis is to provide fine tuning of the physiologic regulation of KC functions through the cholinergic pathways.