Flavopiridol Inhibits NF-κB Activation Induced by Various Carcinogens and Inflammatory Agents through Inhibition of IκBα Kinase and p65 Phosphorylation

Abstract

Flavopiridol, a synthetic flavone closely related to a compound originally isolated from the stem bark of the native Indian plant Dysoxylum binectariferum, has been found to inhibit cyclin-dependent kinases, induce apoptosis, suppress inflammation, and modulate the immune response. Because several genes in which expression is altered by flavopiridol are regulated by NF-kappaB, we propose that this flavone must affect the activation of NF-kappaB. For this report, we investigated the effect of flavopiridol on NF-kappaB activation by various carcinogens and inflammatory agents. Flavopiridol suppressed tumor necrosis factor (TNF)-activation of NF-kappaB in a dose- and time-dependent manner in several cell types, with optimum inhibition occurring upon treatment of cells with 100 nm flavopiridol for 6 h. This effect was mediated through inhibition of IkappaBalpha kinase, phosphorylation, ubiquitination, and degradation of IkappaBalpha (an inhibitor of NF-kappaB), and suppression of phosphorylation, acylation, and nuclear translocation of the p65 subunit of NF-kappaB. Besides TNF, flavopiridol also suppressed NF-kappaB activated by a carcinogen (cigarette smoke condensate), tumor promoters (phorbol myristate acetate and okadaic acid), and an inflammatory agent (H2O2). TNF-induced NF-kappaB-dependent reporter gene transcription was also suppressed by this flavone. NF-kappaB reporter activity induced by TNF receptor 1, TNF receptor-associated death domain, TNF receptor-associated factor-2, NF-kappaB-inducing kinase, and IkappaBalpha kinase, were all blocked by flavopiridol but not that activated by p65. Furthermore, flavopiridol suppressed TNF-induced activation of Akt. Flavopiridol also inhibited the expression of the TNF-induced NF-kappaB-regulated gene products cyclin D1, cyclooxygenase-2, and matrix metalloproteinase-9. Overall, our results indicated that flavopiridol inhibits activation of NF-kappaB and NF-kappaB-regulated gene expression, which may explain the ability of flavopiridol to suppress inflammation, modulate the immune response, and regulate cell growth.