PPARγ mediates innate immunity by regulating the 1α,25-dihydroxyvitamin D3 induced hBD-3 and cathelicidin in human keratinocytes

Background: Production of antimicrobial peptides (AMPS) is the primary mechanism by which skin innate immunity protects against infection. Hormonally active vitamin D3 (1 alpha,25-dihydroxyvitamin D3; 1,25D(3)) is a vital regulator of skin innate immunity, and has been shown to increase the expression and function of AMPs. Objective: PPAR gamma is a ligand-activated nuclear receptor and plays a role in keratinocyte differentiation and cutaneous homeostasis. In this study, we investigate whether 1,25D(3)-activated PPAR gamma signaling regulates AMP expression in keratinocytes. Methods: Subconfluent keratinocytes were treated with 1,25D(3) for the indicated times. The mRNA and protein levels of AMPs were detected by RT-PCR and Western blot, and the DNA binding activation of PPAR gamma, VDRE and AP-1 was investigated by EMSA. To examine the role of PPAR gamma, the recombinant adenovirus carrying a dominant-negative form of PPAR gamma (dn-PPAR gamma) was constructed and transfected into keratinocytes. Results: We show here that 1,25D(3) significantly enhances hBD-3 and cathelicidin expression in keratinocytes. Expression of dn-PPAR gamma did not affect binding to the vitamin D-responsive element (VDRE), which is crucial for cathelicidin induction by VD3; however, it did decrease 1,25D(3) induction of both hBD-3 and cathelicidin. Inhibition of the p38, ERK, and JNK signaling pathways blocked hBD-3 expression, whereas only p38 inhibition suppressed cathelicidin induction. dn-PPAR gamma had no effect on ERK and JNK activity, but inhibited p38 phosphorylation and suppressed 1,25D(3)-induced AP-1 activation via effects on Fra1 and c-Fos proteins. Conclusions: In conclusion, PPAR gamma regulates the 1,25D(3)-induced hBD-3 and cathelicidin expression in keratinocytes through the regulation of AP-1 and p38 activity. (C) 2010 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.