Act1 mediates the effect of IL-17A on cathelicidin expression in keratinocytes. Act1 is a recently identified adaptor protein involved in IL-17RA signaling. To investigate the role of Act1 in IL-17A increased cathelicidin NHEK were transfected with siRNA to decrease Act1 expression before stimulation with IL-17A (10 ng/ml) and 1,25D3 (10−8 M). Silencing of Act1 was confirmed by quantitative real-time PCR. A, The siRNA suppression of Act1 blocked enhancement of cathelicidin by IL-17A in the presence of 1,25D3. B, 1,25D3-induced cathelicidin was not affected. C, NHEK were transfected with siRNA oligonucleotides for Act1 and stimulated with 1,25D3 (10−8 M), IL-17A (10 ng/ml), or the combination. Again, silencing of Act1 blocked induction of HBD2 (top) and IL-6 (bottom). Data are mean ± SD of a single experiment performed in triplicate and are representative of three independent experiments. **, p <0.01, determined by Student’s t test. D, The corresponding cathelicidin peptide hCAP18 expression levels are displayed as observed by Western blot. Similar to cathelicidin mRNA expression, silencing of Act1 blocked IL-17A-enhanced cathelicidin peptide.