Background and purpose: Extracts of Plumbago zeylanica containing suberosin exhibit anti-inflammatory activity. We purified suberosin from such extracts and studied its effects on a set of key regulatory events in the proliferation of human peripheral blood mononuclear cells (PBMC) stimulated by phytohemagglutinin (PHA). Experimental approach: Proliferation of PBMC in culture was measured by uptake of 3 H-thymidine; production of cytokines and cyclins by Western blotting and RT-PCR. Transcription factors NF-AT and NF-kB were assayed by immunocytochemistry and EMSA. Key results: Suberosin suppressed PHA-induced PBMC proliferation and arrested cell cycle progression from the G 1 transition to the S phase. Suberosin suppressed, in activated PBMC, transcripts of interleukin-2 (IL-2), interferon-g (IFN-g), and cyclins D3, E, A, and B. DNA binding activity and nuclear translocation of NF-AT and NF-kB induced by PHA were blocked by suberosin. Suberosin decreased the rise in intracellular Ca 2 þ concentration ([Ca 2 þ ] i ) in PBMC stimulated with PHA. Suberosin did not affect phosphorylation of p38 and JNK but did reduce activation of ERK in PHA-treated PBMC. Pharmacological inhibitors of NF-kB, NF-AT, and ERK decreased expression of mRNA for the cyclins, IL-2, and IFN-g and cell proliferation in PBMC activated by PHA. Conclusions and Implications: The inhibitory effects of suberosin on PHA-induced PBMC proliferation, were mediated, at least in part, through reduction of [Ca 2 þ ] i , ERK, NF-AT, and NF-kB activation, and early gene expression in PBMC including cyclins and cytokines, and arrest of cell cycle progression in the cells. Our observations provide an explanation for the antiinflammatory activity of P. zeylanica.

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