Poly r(C) Binding Protein 1 Regulates Posttranscriptional Expression of the Ubiquitin Ligase TRIM56 in Ovarian Cancer.

Our earlier work has shown that the E3 ligase TRIM56 messenger RNA (mRNA) level and vimentin protein expression followed an inverse correlation in ovarian carcinoma patients; however, the regulatory mechanisms underlying TRIM56 expression is unclear. Steady state expression of TRIM56 mRNA expression in the normal ovarian cell line Moody and ovarian cancer cell lines SKOV-3, A2780, and Caov-3 were not significantly different; however, TRIM56 protein expression was significantly lower in the ovarian cancer cell lines compared to the Moody cell line. Polysome profiling showed significant increase in translation of TRIM56 messenger RNA in the Moody cells compared to the SKOV-3 cells. We performed RNA-affinity pulldown using biotinylated TRIM56 5 'and 3'-UTR and postnuclear extracts from Moody and SKOV-3 cells. Whereas no notable difference was observed in affinity pull-down obtained with the 5'-UTR, there was obvious difference in protein binding patterns with the 3'-UTR. Mass spectrometry was used to determine the most differentially binding protein as poly r (c) binding protein 1 (PCBP1). PCBP1 expression and binding to the 3'-UTR was both higher in SKOV-3 cells compared to the Moody cells. Silencing of TRIM56 in Moody cells cause an increase in in vitro migration and invasion, and a similar effect was mimicked by overexpression of PCBP1. Conversely, silencing of PCBP1 or overexpression of TRIM56 in SKOV-3 cells significantly decreased in vitro migration and invasion. In xenograft assays, SKOV-3 cells stably overexpressing shRNA targeting PCBP1 decreased metastasis, whereas shRNA-targeting TRIM56 potentiated detection of metastatic lesions, compared to the parental SKOV-3 cells themselves. Taken together our results reveal a yet undefined posttranscriptional regulatory mechanism underlying low expression of TRIM56 in ovarian cancer.