The NF-kB pathway has become a particular focus of the Staudt laboratory due to its recurrent involvement in various lymphoid cancers. The laboratory demonstrated that multiple myeloma has frequent engagement of the NF-kB pathway due to diverse genetic abnormalities in regulators of the pathway, including amplification or translocation of NIK, deletion or somatic mutation of TRAF3, deletion of the locus encoding c-IAP1 and c-IAP2 deletion, deletion of the CYLD, and overexpression of CD40, NFkB1. The laboratory demonstrated that NIK overexpression and TRAF3 inactivation were responsible for constitutive activation of the classical NF-kB pathway in the multiple myeloma. Inhibition of IkappaB kinase beta, the critical kinase in the classical NF-kB pathway, was lethal to many myeloma cell lines. The laboratory developed a gene expression signature of NF-kB pathway activation in multiple myeloma and showed that the majority of primary myeloma cases have NF-kB activation in the malignant cells and proposed that this pathway is a promising new target for therapy of myeloma. To identify further therapeutic targets in multiple myeloma, the laboratory conducted an RNA interference-based genetic screen for genes required for the proliferation and survival of myeloma cells. This loss-of-function screen utilized a library of retroviral vectors expressing small hairpin RNAs (shRNAs), which mediate RNA interference. shRNAs targeting IRF4 were toxic for multiple myeloma cell lines but not to lymphoma cell lines. IRF4 is a lymphoid-restricted transcription factor that is required for B cell activation and for differentiation of mature B cells into plasma cells. IRF4 inactivation was toxic to 10 different myeloma cell lines representing most of the recurrent genetic subtypes of myeloma. Notably, IRF4 is not genetically abnormal in most myeloma cases. Therefore, the dependence of myeloma cells on IRF4 is an prime example of non-oncogene addiction, a phenomenon wherein cancer cells become dependent upon normal cellular proteins for their survival. To understand the molecular basis for this non-oncogene addiction, the Staudt laboratory combined gene expression profiling and genome-wide chromatin immunoprecipitation to determine the target genes activated by IRF4. Among the 35 genes that were directly activated by IRF4 were genes that encode regulators of the cell cycle, metabolism and energy, general transcription, cell death, and plasma cell function. Some of these targets are highly expressed in normal activated B cells while other are instead expressed highly in normal plasma cells. This indicates that myelomas are addicted to an aberrant genetic network regulated by IRF4. Of special interest was the proto-oncogene MYC, which is frequently overexpresed in multiple myeloma due to chromosomal translocation or amplification. The Staudt laboratory demonstrated that IRF4 directly transactivates MYC and MYC in turn transactives IRF4, thereby forming a positive autoregulatory loop. Consistent with this concept, myelomas have higher expression of both MYC and IRF4 than normal plasma cells. IRF4 emerges from these experiments as an attractive new therapeutic target with potential in all forms of multiple myeloma, regardless of underlying genetic abnormality. Since IRF4 deficient mice have discrete defects in B cell activation and plasma cell generation, therapeutic targeting of IRF4 would be predicted to have defined and manageable on-target side effects. Modulating aberrant transcription of oncogenes is a relatively unexplored opportunity in cancer therapeutics. In 10% of multiple myelomas, the initiating oncogenic event is translocation of MAF, a transcriptional activator of key target genes such as cyclinD2. Our prior work showed that MAF is upregulated in an additional 30% of MM cases, albeit by an unknown mechanism. We recently discovered a common mechanism inducing MAF transcription in both instances. The second mode of MAF transcription occurred in myelomas with MMSET translocation, and these cases overexpressed MAF target genes. MMSET knockdown decreased MAF transcription and cell viability. A small molecule screen found an inhibitor of MEK, which activates ERK-MAP kinases, reduced MAF mRNA in cells representing MMSET or MAF subgroups. ERK activates transcription of FOS, one subunit of the AP-1 transcription factor. By chromatin immunoprecipitation, FOS bound the MAF promoter, and MEK inhibition decreased this interaction. MEK inhibition selectively induced apoptosis in MAF-expressing myelomas, and FOS inactivation was similarly toxic. Re-expression of MAF rescued cells from death induced by MMSET depletion, MEK inhibition, or FOS inactivation. The data presented herein demonstrate that the MEK-ERK pathway regulates MAF transcription, providing molecular rationale for clinical evaluation of MEK inhibitors in MAF-expressing myeloma. This clinical trial has been initiated in collaboration with Drs. Ola Landgren and Tina Annunizata of the CCR. Recent RNA interference screens in multiple myeloma revealed caspase-10 as an essential gene that blocks an autophagic form of cell death in multiple myeloma. In other settings caspase-10 is a fully active protease that initiates apoptosis. In multiple myeloma, caspase-10 forms a complex with c-FlipL, creating a partially active protease. The reason multiple myeloma cells have this unusual caspase-10 isoform is that the genes encoding caspase-10 and c-FlipL are both direct targets of IRF4. When caspase-10 is inhibited genetically or pharmacologically, autophagy is induced, followed by a non-apoptotic form of cell death. The mechanism of caspase-10 action in multiple myeloma relates to its ability to cleave and inactivate BCLAF1. BCLAF1 was originally identified as a BCL2-binding protein. We discovered that it potently induces autophagic cell death when ectopically expressed in cells. It does so by blocking the ability of BCL2 and related family members from sequestering beclin, a key inducer of autophagy. Thus, by inactivating BCLAF1, caspase-10 promotes the binding of beclin to BCL2, thereby blocking autophagy. These studies suggest that caspase-10 inhibitors might be developed for the therapy of multiple myeloma.
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