The Putative PAX8/PPAR  Fusion Oncoprotein Exhibits Partial Tumor Suppressor Activity through Up-Regulation of Micro-RNA-122 and Dominant-Negative PPAR  Activity

Abstract

IntroductionFollicular thyroid carcinoma (FTC) accounts for up to 20% of all thyroid cancers but is responsible for up to 40% of the deaths associated with this disease.1 In contrast to the wealth of mechanistic information available regarding the pathogenesis of papillary thyroid carcinoma (PTC), the triggering mechanisms of FTC development have remained obscure. PAX8/PPARγ rearrangements, which express the PAX8/PPARγ fusion protein (PPFP), are found in about 36% of FTC.2 Because such rearrangements are associated with oncogenesis in other systems, 3 PPFP has been implicated in the pathogenesis of FTC. Furthermore, our group and others have confirmed that PPFP acts as an oncogene in vitro, with increased rates of cell growth and attachmentindependent growth and decreased rates of apoptosis of several cell lines in vitro. 4,5 However, PPFP fails to transform normal thyrocytes in a transgenic animal, 6 suggesting that other oncogenic events are required to elicit its oncogenic actions. Consequently, the precise role of PPFP in the pathogenesis of FTC remains uncertain.We have demonstrated that the presence of PPFP in human tumors is associated with reduced CD-31 staining, suggesting decreased neovascularization.2 This finding is congruent with our earlier study of a cohort of 54 FTC patients, in which the 31 tumors that expressed PPFP exhibited favorable prognostic indicators, including better tumor Submitted 27- Dec-2010; revised 22-Feb-2011; accepted 27-Feb-2011 AbstractIn vitro studies have demonstrated that the PAX8/PPARγ fusion protein (PPFP), which occurs frequently in follicular thyroid carcinomas (FTC), exhibits oncogenic activity. However, paradoxically, a meta-analysis of extant tumor outcome studies indicates that 68% of FTC-expressing PPFP are minimally invasive compared to only 32% of those lacking PPFP (χ 2 = 6.86, P = 0.008), suggesting that PPFP favorably impacts FTC outcomes. In studies designed to distinguish benign thyroid neoplasms from thyroid carcinomas, the previously identified tumor suppressor miR-122, a major liver micro-RNA (miR) that is decreased in hepatocellular carcinoma, was increased 8.9-fold (P < 0.05) in all FTC versus normal, 9.2-fold in FTC versus FA (P < 0.05), and 16.8-fold (P < 0.001) in FTC + PPFP versus FTC -PPFP. Constitutive expression of PPFP in the FTC-derived cell line WRO (WRO-PPFP) caused a 5-fold increase of miR-122 expression (P < 0.05) and a striking 5.1-fold reduction (P < 0.0001) in tumor progression compared to WRO-vector cells in a mouse xenograft model. Constitutive expression of either miR-122 or a dominant-negative PPARγ mutant in WRO cells was less effective than PPFP at inhibiting xenograft tumor progression (1.8-fold [P < 0.001] and 1.7-fold [P < 0.03], respectively). PPFP-induced up-regulation of miR-122 expression was independent of its known dominant-negative PPARγ activity. Up-regulation of miR-122 negatively regulates ADAM-17, a known downstream target, in thyroid cells, suggesting an antiangiogenic mechanism in thyroid carcinoma. This latter ...

miRNAs with the Potential to Distinguish Follicular Thyroid Carcinomas from Benign Follicular Thyroid Tumors: Results of a Meta-analysis

“…One miRNA ( miR-886-5p ) was removed from the group of 8 confi rmed miRNAs: miR-886-5p , miR-146a-5p , miR-200b-3p , miR-138-5p , miR-187-3p , miR-224-5p , miR-574-3p , and miR-222-3p since it represents a small polymerase III transcript contained in the vault particles of eukaryotic cells [ 23 ] . Despite the partial cross confi rmation according to the fold change criteria the statistical signifi cance of the markers is very low: 3 signifi cant miRNAs in [ 13 ] , 4 in [ 14 ] , and 6 in [ 15 ] out of 27 in total. Furthermore, only miR-885-5p showed a p-value lower than 0.05 in 2 independent data sets [ 13 , 14 ] .…”

“…Furthermore, only miR-885-5p showed a p-value lower than 0.05 in 2 independent data sets [ 13 , 14 ] . The FDRs for the 27 studied miRNAs were 0.77 ± 0.10 [ 13 ] , 0.77 ± 0.16 [ 14 ] , and 0.66 ± 0.25 [ 15 ] in the respective data sets when given in the mean ± standard deviation form.…”

“…All 3 available data sets were generated from fresh-frozen tissue samples. The Reddi et al [ 14 ] data set was obtained from the authors of the publication after a data share request. Sample numbers are given in • ▶ …”

“…For this analysis the intensity data of the surrounding tissue in the second channel were not considered. All 3 data sets hybridized to diff erent chips -Exiqon (Rossing et al [ 13 ] ), Illumina (Reddi et al [ 14 ] ), and IPMC custom microarrays (Lassalle et al [ 15 ] ) were normalized with quintile normalization in the R/Bioconductor programming environment [ 20 ] by the preprocess Core library. Furthermore, log2 transformation was applied to diminish the eff ect of skewed microarray intensity distributions.…”

MECHANISMS IN ENDOCRINOLOGY: MicroRNA in diagnostics and therapy of thyroid cancer

“…This upregulation was associated with the presence of the PAX8/PPARg fusion protein, which, paradoxically, led to a minimally invasive behavior of tumor. Functional studies revealed that overexpression of miR-122 in a mouse xenograft model resulted in a significant reduction of tumor progression (78).…”