5-Fluorouracil suppression of NF-kappa B is mediated by the inhibition of I kappa B kinase activity in human salivary gland cancer cells

We have recently shown that 5-Fluorouracil (5-FU) suppresses the transcript ion factor NF-kappaB in human salivary gland cancer cells (cl-1) by mediati ng upregulation of I kappaB-alpha expression. However, the precise mechanis m involved in this action has not yet been elucidated. I kappaB kinases (IK K-alpha and IKK-beta) are the key components of the IKK complex that mediat es activation of NF-kappaB in response to external stimuli such as cytokine s. In addition, NF-kappaB-inducing kinase (NIK) and mitogen-activated prote in kinase kinase kinase 1 (MEKK-1), both of which are the upstream kinases for the IKKs, interact with and activate the IKKs. Thus, we investigated th e molecular mechanisms involved in the suppression of NF-kappaB by 5-FU. Al though 5-FU did not affect the expression levels of IKKs, NIK, or MEKK-1, I RK activity in cl-1 cells was suppressed at both 6 h and 12 h after treatme nt with 2 mug/ml 5-FU. Moreover, when cells were treated with various conce ntrations of 5-FU for 12 h, the concentration of 2 mug/ml efficiently inhib ited the IKK activity as compared to 1, 5, or 10 mug/ml. The expression of Fas-associated death domain-like interleukin 1-converting enzyme-inhibitory protein (FLIP), which acts as an inhibitor of an initiator caspase (caspas e-8), was down-regulated by 5-FU treatment in cl-1 cells. Apoptosis, as evi denced by cleavage of poly(ADP-ribose) polymerase through the action of an executioner caspase (caspase-3), was also clearly observed. Thus, these res ults suggest that 5-FU induction of apoptosis in cl-1 cells may be mediated by suppression of NF-kappaB via inhibition of IKK activity. (C) 2001 Acade mic Press.