Bromodomain and extraterminal domain (BET) protein inhibitors are emerging as promising anti-cancer therapies. The gene encoding the E3 ubiquitin ligase substrate-binding adaptor speckle-type POZ protein (SPOP) is most frequently mutated in prostate cancer. Here we demonstrate that wild-type SPOP binds to and induces ubiquitination and proteasomal degradation of BET proteins (BRD2, BRD3 and BRD4) by recognizing a common degron motif. In contrast, prostate cancer-associated SPOP mutants impair binding and proteasomal degradation of BET proteins, thus inducing their accumulation in prostate cancer cells and patient specimens. Transcriptome and BRD4 cistrome analyses reveal that SPOP mutation enhances BRD4-dependent expression of GTPase RAC1 and cholesterol biosynthesis genes and AKT-mTORC1 activation. SPOP mutant expression confers BET inhibitor resistance and this effect can be overcome by AKT inhibitors. Thus, SPOP mutations promote AKT-mTORC1 activation and intrinsic BET inhibitor resistance by stabilizing BET proteins, suggesting that SPOP mutation can be an effective biomarker to guide BET inhibitor-oriented therapy of prostate cancer.

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Nuclear FOXO proteins act as tumor suppressors by transcriptionally activating genes involved in apoptosis and cell cycle arrest, and these anticancer functions are inhibited by AKT-induced phosphorylation and cytoplasmic sequestration of FOXOs. We found that, after AKT-mediated phosphorylation at serine 319, FOXO1 binds to IQGAP1, a hub for activation of the MAPK pathway, and impedes IQGAP1-dependent phosphorylation of ERK1/2 (pERK1/2). Conversely, decreased FOXO1 expression increases pERK1/2 in cancer cell lines and correlates with increased pERK1/2 levels in patient specimens and disease progression. Treatment of cancer cells with PI3K inhibitors or taxane causes FOXO1 localization in the nucleus, increased expression of pERK1/2, and drug resistance. These effects are reversed by administering a small FOXO1-derived phospho-mimicking peptide inhibitor and in mice. Our results show a tumor suppressor role of AKT-phosphorylated FOXO1 in the cytoplasm and suggest that this function of FOXO1 can be harnessed to overcome chemoresistance in cancer.