Abstract 4061: TRIM-NHL proteins: New potential targets for cancer therapy

Abstract

Abstract Exploring the interactions between DNA viral and cellular proteins has revealed critical tumor target such as p53, E2F, and PI-3 kinase. Using a genetic approach, we have discovered that an adenoviral protein, E4-ORF3, dominantly suppresses p53 activated transcription (but not p53 induction) in response to oncogenic/genotoxic stresses. E4-ORF3 does not inactivate p53 directly, but binds and mislocalizes ‘Tripartite motif (TRIM)’ proteins, such as PML and Tif1a which are also targeted by chromosomal translocations in human tumors. Using an unbiased proteomics approach, we have identified TRIM32 as a novel cellular protein that binds to p53 and that is also a target of E4-ORF3. Using immunofluorescence, we show that p53 and TRIM32 colocalize in a punctuate cytoplasmic pattern. We hypothesized that TRIM32 may act as a novel ubiquitin ligase for p53. Consistent with this hypothesis, treatment with the proteasome inhibitor MG132 stabilizes and increases TRIM32-p53 protein interaction complexes in pulldowns of cellular lysates. Our preliminary data suggest that TRIM32 binds to p53 and targets it for ubiquitination, independently of Mdm2. siRNA-mediated knock-down of TRIM32 results in the stabilization of p53, which increases the levels of p53 and results in p53 transcriptional activation of downstream effectors. Taken together, our data suggest TRIM32 is a novel E3-ubiquitin ligase for p53, which may play an important role in regulating p53 tumor suppressor functions. Given the effects of TRIM32 in regulating p53 and also as a target of a DNA tumor virus protein, E4-ORF3, we hypothesized that TRIM32 may also be targeted in tumorigenesis. TRIM32 together with TRIM2 and TRIM3 belong to the TRIM-NHL subgroup. Despite their well conserved modular structure, no common biological role has yet been discovered for TRIM proteins. TRIM2, TRIM3 and TRIM32 are mainly expressed in the brain and encode structural homologues of Drosophila brain tumor (brat) implicated in progenitor cell proliferation control and cancer stem cell suppression. Interestingly, 25% of human glioblastoma (GBM) patients exhibit loss of heterozygosity for TRIM3, suggesting it is a critical tumor suppressor. TRIM32 is known to be required and sufficient for suppressing proliferation and inducing neuronal differentiation in mouse neural progenitors. Based on these data we compared the expression of TRIM2, TRIM3 and TRIM32 in normal brain tissue versus tumor brain tissues -human and murine- and found that their expression was dramatically downregulated in tumor versus normal samples, suggesting that their loss of expression is required for gliomagenesis. Preliminary results suggest that restoring the expression of these TRIM-NHL proteins can delay tumor formation in a GBM xenograft mouse model. Together these results shed new light on the functions of TRIM32 and TRIM-NHL proteins in general as potentially novel therapeutic targets for the treatment of brain cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4061. doi:10.1158/1538-7445.AM2011-4061