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2017 ; 36
(33
): 4767-4777
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SPOP regulates prostate epithelial cell proliferation and promotes ubiquitination
and turnover of c-MYC oncoprotein
#MMPMID28414305
Geng C
; Kaochar S
; Li M
; Rajapakshe K
; Fiskus W
; Dong J
; Foley C
; Dong B
; Zhang L
; Kwon OJ
; Shah SS
; Bolaki M
; Xin L
; Ittmann M
; O'Malley BW
; Coarfa C
; Mitsiades N
Oncogene
2017[Aug]; 36
(33
): 4767-4777
PMID28414305
show ga
The E3 ubiquitin ligase adaptor speckle-type POZ protein (SPOP) is frequently
dysregulated in prostate adenocarcinoma (PC), via either somatic mutations or
mRNA downregulation, suggesting an important tumour suppressor function. To
examine its physiologic role in the prostate epithelium in vivo, we generated
mice with prostate-specific biallelic ablation of Spop. These mice exhibited
increased prostate mass, prostate epithelial cell proliferation, and expression
of c-MYC protein compared to littermate controls, and eventually developed
prostatic intraepithelial neoplasia (PIN). We found that SPOP(WT) can physically
interact with c-MYC protein and, upon exogenous expression in vitro, can promote
c-MYC ubiquitination and degradation. This effect was attenuated in PC cells by
introducing PC-associated SPOP mutants or upon knockdown of SPOP via
short-hairpin-RNA, suggesting that SPOP inactivation directly increases c-MYC
protein levels. Gene Set Enrichment Analysis revealed enrichment of Myc-induced
genes in transcriptomic signatures associated with SPOP(MT). Likewise, we
observed strong inverse correlation between c-MYC activity and SPOP mRNA levels
in two independent PC patient cohorts. The core SPOP(MT);MYC(High) transcriptomic
response, defined by the overlap between the SPOP(MT) and c-MYC transcriptomic
programmes, was also associated with inferior clinical outcome in human PCs.
Finally, the organoid-forming capacity of Spop-null murine prostate cells was
more sensitive to c-MYC inhibition than that of Spop-WT cells, suggesting that
c-MYC upregulation functionally contributes to the proliferative phenotype of
Spop knock-out prostates. Taken together, our data highlight SPOP as an important
regulator of luminal epithelial cell proliferation and c-MYC expression in
prostate physiology, identify c-MYC as a novel bona fide SPOP substrate, and help
explain the frequent inactivation of SPOP in human PC. We propose
SPOP(MT)-induced stabilization of c-MYC protein as a novel mechanism that can
increase total c-MYC levels in PC cells, in addition to amplification of c-MYC
locus.