Ferritinophagy is required for the induction of ferroptosis by the bromodomain protein BRD4 inhibitor (+)-JQ1 in cancer cells

(+)-JQ1 is an inhibitor of the tumor-driver bromodomain protein BRD4 and produces satisfactory effects because it efficiently increases apoptosis. Ferroptosis is an oxidative cell death program differing from apoptosis. Ferroptosis is characterized by high levels of iron and reactive oxygen species and has been confirmed to suppress tumor growth. In this study, BRD4 expression in cancer and its influence on the prognosis of cancer patients were analyzed using data from public databases. In addition, the effect of the BRD4 inhibitor (+)-JQ1 on ferroptosis was investigated via a series of in vitro assays. A nude mouse model was used to evaluate the function of (+)-JQ1 in ferroptosis in vivo. The potential mechanisms by which (+)-JQ1 regulates ferroptosis were explored. The results showed that BRD4 expression levels were higher in cancer tissues than in normal tissues and were related to poor prognosis in cancer patients. Furthermore, ferroptosis was induced under (+)-JQ1 treatment and BRD4 knockdown, indicating that (+)-JQ1 induces ferroptosis via BRD4 inhibition. Moreover, the anticancer effect of (+)-JQ1 was enhanced by ferroptosis inducers. Further studies confirmed that (+)-JQ1 induced ferroptosis via ferritinophagy, which featured autophagy enhancement by (+)-JQ1 and increased iron levels. Subsequently, the reactive oxygen species levels were increased by iron via the Fenton reaction, leading to ferroptosis. In addition, expression of the ferroptosis-associated genes GPX4, SLC7A11, and SLC3A2 was downregulated under (+)-JQ1 treatment and BRD4 knockdown, indicating that (+)-JQ1 may regulate ferroptosis by controlling the expression of ferroptosis-associated genes regulated by BRD4. Finally, (+)-JQ1 regulated ferritinophagy and the expression of ferroptosis-associated genes via epigenetic inhibition of BRD4 by suppressing the expression of the histone methyltransferase G9a or enhancing the expression of the histone deacetylase SIRT1. In summary, the BRD4 inhibitor (+)-JQ1 induces ferroptosis via ferritinophagy or the regulation of ferroptosis-associated genes through epigenetic repression of BRD4.