Role of PATZ1 as a potential tumor suppressor gene in thyroid cancer

Abstract

The POZ/BTB and AT-hook-containing zinc finger protein 1 (PATZ1), is a transcriptional regulatory factor that belongs to the POZ and Zinc Finger (POK) family of transcriptional factor involved in chromatin remodeling, protein-protein interaction, transcriptional regulation and targeting of proteins to distinct nuclear substructures. PATZ1 has been shown to regulate expression of different genes and its transcriptional activity, as for other POK proteins, depends on the POZ domain, which mediates oligomers formation. Therefore, it has been suggested that it may not be a typical transactivator but an architectural transcription factor, thus functioning either as activator or repressor depending on the presence of proteins able to interact with it, which could be different depending on the cellular context. The physiological role of PATZ1 is still not completely known, but it has been demonstrated that PATZ1 is important in testis development, in spermatogenesis and it plays a key role in the development of the central nervous system (CNS), and of the cardiac out-flow tract (OFT). Several studies suggest a role for PATZ1 in cancer but its cancer-related function is still debated between being a tumor suppressor or an oncogene. To better understand the role of PATZ1 in human cancer we focused on thyroid tumors, which represent a useful model to study the multi-step carcinogenesis. In this study quantitative real-time PCR (qRT-PCR) analysis showed that PATZ1 is down-regulated in human thyroid tumors with respect to normal thyroid gland, with the expression levels being inversely correlated with the degree of de-differentiation and malignancy. Immunohistochemical analysis confirmed this result showing the reduction of PATZ1 nuclear expression and its cellular delocalization from the nucleus to the cytoplasm proceeding from the normal to malignant human thyroid tissues. Subsequently, we conducted functional studies on two cell lines derived from human papillary thyroid carcinomas (TPC-1 and BC-PAP) and one cell line derived from a human anaplastic thyroid tumor (FRO), in which PATZ1 is strongly down-regulated, in comparison with the same cells in which the presence of PATZ1 has been restored by stable transfection, identifying noticeable differences associated with the neoplastic phenotype. Indeed, the restoration of PATZ1 expression impairs the proliferation capacity of BC-PAP and FRO cells and induces apoptosis in FRO cells. Moreover, PATZ1 expression inhibits cell migration and invasion in all the three cells lines analyzed and suppress the tumorigenicity of FRO cells both in vitro and in vivo. Furthermore, as PATZ is essentially a transcriptional regulator, we looked for the genes specifically regulated by it, which could explain its role in counteracting thyroid cell transformation. For this purpose we focused our attention on BAX, PUMA and PERP genes, that are all targets of p53 and have a role in cellular migration and/or apoptosis induction. We found that the expression of all three of them was up-regulated in thyroid cell lines re-expressing, PATZ1 compared to the parental control cells. Moreover, ChIP and EMSA assay demonstrated that PATZ1 is able to bind their promoters. Taken together these findings support the hypothesis that PATZ1 may act as a tumor-suppressor in thyroid cancer, likely inhibiting cellular migration and/or enhancing apoptosis through the up-regulation of BAX, PUMA and PERP expression. These studies open a road to investigate the diagnostic value and the prognostic impact of PATZ1 in thyroid neoplasia, as well as the efficacy of a thyroid cancer therapy based on the re-expression of the PATZ1 gene.