PTEN Linear Ubiquitination Regulates Prostate Cancer Progression

As a new type ubiquitination,linear ubiquitination is generated by peptide-bond formation between the N-terminal methionine of one ubiquitin and the C-terminal glycine of another ubiquitin.These polyubiquitin chains are catalyzed by the linear ubiquitin chain assembly complex(LUBAC).LUBAC,which is composed of HOIP(HOIL1-interacting protein;also known as RNF31),HOIL-1L(haem-oxidized IRP2 ubiquitin ligase 1L;also known as RBCK1)and Sharpin(SHANK-associated RH domain-interacting protein;also known as SIPL1),is the sole E3 ubiquitin ligase known to specifically generate linear ubiquitin chains.Linear ubiquitin chains have been established as important players of apoptosis,immune response and inflammatory signaling.Moreover,evidences show that linear ubiquitination is also involved in tumorigenesis,but the precise molecular mechanism remains largely unknown.PTEN is a well-known tumor suppressor.It is subject to multiple protein post-translational modifications(PTMs)including phosphorylation,ubiquitination,SUMOylation,acetylation and so on.These PTMs interfere with PTEN stability,subcellular localization or enzymatic activity to regulate the PI3K/AKT signaling pathway and tumorigenesis.To date,PTEN linear ubiquitination has not yet been reported and its specific function is unknown.In this paper,we identified linear ubiquitination of PTEN for the first time and researched its role in prostate cancer progression.(1)LUBAC-?,which is composed of HOIP and Sharpin,directly interacted with PTEN and promoted PTEN linear ubiquitination.Moreover,OTULIN(OTU deubiquitinase with linear linkage specificity)was identified as a deubiquitinase to remove linear polyubiquitin chains of PTEN.(2)Mutations in the PTEN gene are frequent in prostate cancer.The cancer-associated mutants,PTEN-R173 H,PTEN-Y68 H,and PTEN-Y27 N,exhibited remarkably high linear ubiquitination,which suggested that PTEN linear ubiquitination might be involved in the regulation of prostate cancer progression.(3)The analysis of clinical database concluded that both the m RNA and protein levels of HOIP were high in prostate cancer.Function experiments revealed that HOIP activated AKT to promote cell migration,colony-formation and xenograft tumor growth.HOIP E3 enzymatic activity was required for above process,indicating that linear ubiquitin chains catalyzed by HOIP played a key role in cancer progression.(4)Furthermore,when PTEN was knocked out,HOIP had no effect on AKT activation and its ability in promoting tumorigenesis was significantly impaired,which showed that HOIP promoted the tumor progression in a PTEN-dependent manner.These results also indicated that PTEN,as a substrate protein of HOIP,was conjugated with linear polyubiquitin chains to inhibite its ability in regulating PI3 K / AKT pathway.(5)Some clinical PTEN mutants with high linear ubiquitination impaired its ability in inhibiting AKT phosphorylation and then promoted prostate cancer progression,which suggested that loss of function in PTEN mutations had close correlation with its high linear ubiquitination.These results might provide a guide for the diagnosis and treatment of clinical prostate cancer.