Global gene expression response to BET inhibition in two cancer cell lines

The MYC transcription factor is a master regulator of diverse cancer pathways and somatic cell reprogramming. MYC is a compelling therapeutic target that exhibits cancer-specific cellular effects. Pharmacologic inhibition of MYC function has proven challenging due to its numerous modes of forced expression and the difficulty of disrupting protein-DNA interactions. Here we demonstrate the rapid and potent abrogation of MYC gene transcription by representative small molecule bromodomain inhibitors of the BET family of chromatin adaptors. This transcriptional suppression of MYC was observed in the context of the natural, chromosomally translocated, and amplified gene locus. Inhibition of BET bromodomain-promoter interactions and subsequent reduction of MYC transcript and protein levels resulted in G1 arrest and extensive apoptosis in a variety of leukemia and lymphoma cell lines. Exogenous expression of MYC from an artificial promoter that is resistant to BET regulation significantly protected cells from growth suppression by BET inhibitors and revealed that MYC exerts a direct and tight control of key pro-growth and anti-apoptotic target genes. Transcriptional profiling of two cells after 4 and 8 hours of treatment with BET inhibitor shows that both MYC and its targets are strongly down-regulated. We thus demonstrate that pharmacologic inhibition of MYC is achievable through targeting BET bromodomains, and suggest that such inhibitors may have broad clinical applicability given the widespread pathogenetic role of MYC in cancer. LP-1, a multiple myeloma cell line, and Raji, a Burkitt lymphoma cell line, were treated with BET inhibitor and an inactive enantiomer for 4 or 8 hours. Two biological replicates of each sample were profiled on exon arrays.