The APRIL-induced expression of interleukin-8 (IL-8) requires the activity of mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB). (a) THP-1 cells were pre-treated with 10 μm of U0126 (U), SB203580 (SB), Jun N-terminal kinase (JNK) inhibitor (JI), or negative control of JNK inhibitor (J–) for 30 min and the cells were stimulated with anti-APRIL monoclonal antibody (mAb; 1 μg/ml). Culture supernatants were collected after 24 hr to measure IL-8 concentrations using enzyme-linked immunosorbent assay. C, no treatment control; L, 1 μg/ml lipopolysaccharide as a positive control; D, 0·1% dimethyl sulphoxide as a vehicle control. D, THP-1 cells were pre-treated for 30 min with 20 and 40 μm TPCK, 5 and 10 mm ethyl pyruvate (EP), or 0·5 and 1 mm of sulfasalzine (SSZ) and stimulated with anti-APRIL mAb (1 μg/ml). Culture supernatants were collected after 24 hr to measure IL-8 concentrations. Results are representative for three independent experiments.