Intrinsic BET inhibitor resistance in SPOP-mutated prostate cancer is mediated by BET protein stabilization and AKT–mTORC1 activation

Abstract

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.

Targeting Bromodomain and Extra-Terminal (BET) Family Proteins in Castration-Resistant Prostate Cancer (CRPC)

“…This is likely to reflect the demonstration that patients with lower BRD4 expression are more likely to present with local disease (lymph node negative, no distant disease) and receive radical treatment; an observation that warrants further investigation. SPOP mutant cancers have been identified as a subgroup of prostate cancer with increased BRD4 expression (31,32). Although we were unable to confirm these findings, this is likely explained by the small numbers (4 cases) of SPOP mutant cases and different patient population.…”

“…Nuclear BRD4 expression (continuous variable; per 100 HS) in CRPC biopsies was not significantly associated with OS (HR 0.63, 95% CI 0.32-1.24; p=0.18). Although SPOP mutant PC have been associated with increased BRD4 protein expression, we found no clear association between SPOP mutations (4 of 39 cases with SPOP status) and BRD4 protein expression (Supplementary Figure S3) (31,32). Taken together, these data suggest that higher nuclear BRD4 expression at diagnosis, but not CRPC, is associated with poorer patient outcome.…”

“…BRD4 is a critical coregulator of AR (29,30). Furthermore, increased BRD4 protein stability in substrate-binding adaptor speckle-type POZ protein (SPOP) mutant prostate cancers is associated with AR signaling (31,32). BET inhibitors (BETi) and proteolysis targeting chimera (PROTAC) induced BET protein degraders have anti-tumor activity in CRPC models (29,30,(33)(34)(35)(36), attenuating AR signaling and potently reducing C-MYC expression (29,30,33,34,36).…”

AZD5153 Inhibits Prostate Cancer Cell Growth in Vitro and in Vivo

“…Cancer-associated SPOP mutations are observed in the meprin and TRAF (Tumor necrosis factor receptor-associated factor) homology (MATH) domain, inhibiting substrate binding [40]. Recent studies have demonstrated that BET proteins, including BRD2, BRD3 and BRD4, are the targets of SPOP [18][19][20]. Wild-type SPOP binds to the a degron motif in the BET proteins, causing their ubiquitination and proteasomal degradation [18][19][20].…”

“…A very recent study by Zhang et al, has implied that AKT activation might be important for the resistance of BRD4 inhibitors [18]. First, two known AKT inhibitors, AKTi-1/2 [24] and MK-2206 [37], were utilized.…”