Small molecule inhibitors of IκB kinase are selectively toxic for subgroups of diffuse large B-cell lymphoma defined by gene expression profiling

Constitutive activation of the NF-κB pathway is required for survival of the activated B cell-like (ABC) subgroup of diffuse large B-cell lymphoma (DLBCL). Here we show that a small molecule IκB kinase (IKK) inhibitor, PS-1145, and related compounds are toxic for ABC DLBCL cell lines but not for cell lines derived from the other prevalent form of DLBCL, germinal center 1 cell-like DLBCL. Treatment of ABC lines with these inhibitors rapidly induced a series of gene expression changes that were attributable to cessation of constitutive IKK activity, similar to changes induced by acute expression of genetic inhibitors of NF-κB, confirming the effectiveness and specificity of this compound. Before cell death, inhibition of IKK also induced features of apoptosis and an arrest in the G₁ phase of the cell cycle. To test further the specificity of this toxicity, an inducible form of NF-κB was created by fusing the p65 NF-κB subunit with the ligand-binding domain of the estrogen receptor (p65-ERD). In the presence of tamoxifen, p65-ERB reversed the toxicity of IKK inhibition and restored expression of many NF-κB target genes. Another subgroup of DLBCL, primary mediastinal B-cell lymphoma (PMBL), also expresses NF-κB target genes, and treatment of a PMBL cell line with an IKK inhibitor was toxic and induced gene expression changes of a distinct group of NF-κB target genes. These studies validate the NF-κB pathway as a promising therapeutic target in ABC DLBCL, PMBL, and other lymphomas that depend on the activity of NF-κB for survival and proliferation.