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The bromodomain and extra terminal domain (BET) family members, including BRD2, BRD3, and BRD4, act as epigenetic readers to regulate gene expression. Indoleamine 2,3-dioxygenase 1 (IDO1) is an enzyme that participates in tumor immune escape primarily by catalyzing tryptophan to  l -kynurenine. Here, we report that  IDO1  is a new target gene of the BET family. RNA profiling showed that compound 9, a new BET inhibitor, reduced  IDO1  mRNA up to seven times in Ty-82 cells. IDO1 differentially expressed in tumor cells and its expression could be induced with interferon gamma (IFN-γ). BET inhibitors (ABBV-075, JQ1, and OTX015) inhibited both constitutive and IFN-γ-inducible expression of  IDO1 . Similarly, reduction of BRD2, BRD3, or BRD4 decreased  IDO1  expression. All these BET family members bound to the  IDO1  promoter via the acetylated histone H3. JQ1 led to their release and reduced enrichment of RNA polymerase II (Pol II) on the promoter. IFN-γ increased the binding of BRD2, BRD3, BRD4, and Pol II on the  IDO1  promoter by increasing the acetylation of histone H3, which could be prevented by JQ1 partially or even completely. Furthermore, both JQ1 and OTX015 decreased the production of  l -kynurenine. The combination of BET inhibitors with the IDO1 inhibitor further reduced  l -kynurenine, though only marginally. Importantly, the BET inhibitor ABBV-075 significantly inhibited the growth of human Ty-82 xenografts in nude mice and reduced both protein and mRNA levels of  IDO1  in the xenografts. This finding lays a basis for the potential combination of BET inhibitors and IDO1 inhibitors for the treatment of IDO1-expressing cancers.

Inhibition of the BET family reduces its new target gene IDO1 expression and the production of l-kynurenine