The root of Stellera chamaejasme L. is a traditional Chinese herb termed Rui Xiang Lang Du and has been used to treat solid tumors, tuberculosis and psoriasis. Exactly how S. chamaejasme L. regulates cellular responses remains unclear. We examined four biflavonoids isolated from S. chamaejasme L., including isochamaejasmin, two of its stereo-isomers and a methyl derivative, in functional assays originally designed to screen ligands for the G protein-coupled formyl peptide receptor-like 1 (FPRL1). Isochamaejasmin was found to induce the expression of a nuclear factor (NF)-kappaB-directed reporter gene in transfected HeLa cells with an EC50 of 3.23 microM, independently of FPRL1. The isochamaejasmin-stimulated NF-kappaB reporter activity was accompanied by nuclear translocation of NF-kappaB proteins and was blocked by a dominant-negative construct of IkappaBalpha. Isochamaejasmin also induced time-dependent phosphorylation of the mitogen-activated protein kinases extracellular signal-regulated kinase 1/2 and p38, and a novel protein kinase C (PKCdelta). Likewise, inhibition of these kinases with the respective pharmacological inhibitors significantly reduced the isochamaejasmin-stimulated NF-kappaB activation. It is noteworthy that the two stereoisomers and the methyl derivative did not induce detectable activation of NF-kappaB and were more cytotoxic than isochamaejasmin, which could partially rescue cycloheximide-induced apoptosis. Inhibition of NF-kappaB activation reversed the anti-apoptotic effect of isochamaejasmin. These results provide the first evidence for a potential mechanism of action by S. chamaejasme L., and indicate that structurally similar compounds derived from S. chamaejasme L. may have different pharmacological properties.