Transcriptional regulation and ubiquitination-dependent regulation of HnRNPK oncogenic function in prostate...

Heterogeneous nuclear ribonucleoprotein K (HnRNPK) is a nucleic acid-binding protein that regulates diverse biological events. Pathologically, HnRNPK proteins are frequently overexpressed and clinically correlated with poor prognosis in various types of human cancers and are therefore pursued as attractive therapeutic targets for select patients. However, both the transcriptional regulation and degradation of HnRNPK in prostate cancer remain poorly understood.

qRT-PCR was used to detect the expression of HnRNPK mRNA and miRNA; Immunoblots and immunohistochemical assays were used to determine the levels of HnRNPK and other proteins. Flow cytometry was used to investigate cell cycle stage. MTS and clonogenic assays were used to investigate cell proliferation. Immunoprecipitation was used to analyse the interaction between SPOP and HnRNPK. A prostate carcinoma xenograft mouse model was used to detect the in vivo effects of HnRNPK and miRNA.

In the present study, we noted that HnRNPK emerged as an important player in the carcinogenesis process of prostate cancer. miR-206 and miR-613 suppressed HnRNPK expression by targeting its 3'-UTR in PrCa cell lines in which HnRNPK is overexpressed. To explore the potential biological function, proliferation and colony formation of PrCa cells in vitro and tumor growth in vivo were also dramatically suppressed upon reintroduction of miR-206/miR-613. We have further provided evidence that Cullin 3 SPOP is a novel upstream E3 ubiquitin ligase complex that governs HnRNPK protein stability and oncogenic functions by promoting the degradation of HnRNPK in polyubiquitination-dependent proteolysis in the prostate cancer setting. Moreover, prostate cancer-associated SPOP mutants fail to interact with and promote the destruction of HnRNPK proteins.

Our findings reveal new posttranscriptional and posttranslational modification mechanisms of HnRNPK regulation via miR-206/miR-613 and SPOP, respectively. More importantly, given the critical oncogenic role of HnRNPK and the high frequency of SPOP mutations in prostate cancer, our results provide a molecular rationale for the clinical investigation of novel strategies to combat prostate cancer based on SPOP genetic status.