Human mantle cell lymphoma (MCL), an aggressive B cell non-Hodgkin's lymphoma, is characterized by the overexpression of cyclin D1 which plays an essential role in the survival and proliferation of MCL. Because of MCL's resistance to current chemotherapy, novel approaches are needed. Since MCL cells are known to overexpress NF-kappaB regulated gene products (including cyclin D1), we used curcumin, a pharmacologically safe agent, to target NF-kappaB in a variety of MCL cell lines. All four MCL cell lines examined had overexpression of cyclin D1, constitutive active NF-kappaB and IkappaB kinase and phosphorylated forms of IkappaBalpha and p65. This correlated with expression of TNF, IkappaBalpha, Bcl-2, Bcl-xl, COX-2 and IL-6, all regulated by NF-kappaB. On treatment of cells with curcumin, however, downregulated constitutive active NF-kappaB and inhibited the consitutively active IkappaBalpha kinase (IKK), and phosphorylation of IkappaBalpha and p65. Curcumin also inhibited constitutive activation of Akt, needed for IKK activation. Consequently, the expression of all NF-kappaB-regulated gene products, were downregulated by the polyphenol leading to the suppression of proliferation, cell cycle arrest at the G1/S phase of the cell cycle and induction of apoptosis as indicated by caspase activation, PARP cleavage, and annexin V staining. That NF-kappaB activation is directly linked to the proliferation of cells, is also indicated by the observation that peptide derived from the IKK/NEMO-binding domain and p65 suppressed the constitutive active NF-kappaB complex and inhibited the proliferation of MCL cells. Constitutive NF-kappaB activation was found to be due to TNF, as anti-TNF antibodies inhibited both NF-kappaB activation and proliferation of cells. Overall, our results indicate that curcumin inhibits the constitutive NF-kappaB and IKK leading to suppression of expression of NF-kappaB-regulated gene products that results in the suppression of proliferation, cell cycle arrest, and induction of apoptosis in MCL.