Androgen Receptor Is the Key Transcriptional Mediator of the Tumor Suppressor SPOP in Prostate Cancer

Abstract

Somatic missense mutations in the substrate-binding pocket of the E3 ubiquitin ligase adaptor SPOP are present in up to 15% of human prostate adenocarcinomas (PC), but are rare in other malignancies suggesting a prostate-specific mechanism of action. SPOP promotes ubiquitination and degradation of several protein substrates, including the androgen receptor (AR) coactivator factor SRC-3. However, the relative contributions that SPOP substrates may contribute to the pathophysiology of SPOP-mutant (mt) PC is unknown. Using an unbiased bioinformatics approach, we determined that the gene expression profile of PC cells engineered to express mt-SPOP overlaps greatly with the gene signature of both SRC-3 and AR transcriptional output, with a stronger effect on AR than SRC-3. This finding suggests that in addition to its SRC-3-mediated effects, SPOP also exerts SRC-3-independent effects that are AR mediated. Indeed, we found that wild-type (wt) but not PC-associated mutants of SPOP promoted AR ubiquitination and degradation, acting directly through a SPOP-binding motif in the hinge region of AR. In support of these results, tumor xenografts composed of PC cells expressing mt-SPOP expressed higher AR protein levels and grew faster than tumors composed of PC cells expressing wt-SPOP. Further, genetic ablation of SPOP was sufficient to increase AR protein levels in mouse prostate. Examination of public human PC datasets confirmed a strong link between transcriptomic profiles of mt-SPOP and AR. Overall, our studies highlight the AR axis as the key transcriptional output of SPOP in PC, and they provide an explanation for the prostate-specific tumor suppressor role of wt-SPOP.

Androgen Signaling in Prostate Cancer

“…An example of this relationship is underscored by speckle-type POZ protein (SPOP) missense mutations in prostate cancer (Berger et al 2011;Barbieri et al 2012;Grasso et al 2012). SPOP, which is an E3 ubiquitin ligase normally involved in the degradation and turnover of AR as well as SRC3, may incur mutations that lead to increased AR protein levels and liberation of AR-mediated gene transcription Geng et al 2014). Interestingly, ARVs that lack the hinge region required for interaction with SPOP are resistant to degradation .…”

Degrons in cancer

“…Recognition of the ERG degron is facilitated by a shallow groove on the MATH domain where the ERG degron motif interacts through an extended conformation. Mutations in this evolutionarily conserved and structurally important region lead to the loss of E3 ligase function toward SPOP-dependent targets (194,195). As with translocations that eliminate the function of the ERG degron, mutations in SPOP that compromise substrate binding result in increased ERG in prostate cancer cells.…”

Higher‐order oligomerization promotes localization of SPOP to liquid nuclear speckles

“…Truncated SPOP constructs encoding the BTB and BACK domains self‐associate into higher‐order oligomers (Errington et al , 2012) that possess increased ubiquitination efficiency, supporting the functional importance of oligomerization (Zhuang et al , 2009; Errington et al , 2012). Mutations within the MATH domain perturb interactions with substrates (Geng et al , 2013, 2014; Theurillat et al , 2014; Zeng et al , 2014; Zhang et al , 2015). Mutations within both self‐association domains are also found in cancers [Appendix Fig S1 and www.cbioportal.org (Cerami et al , 2012; Gao et al , 2013)], further supporting a functional role for higher‐order oligomerization, but their pathological mechanism is unclear.…”