Floriana Forzati

HMGA proteins up-regulate CCNB2 gene in mouse and human pituitary adenomas

The high mobility group As (HMGAs) belong to a family of nonhistone nuclear proteins that orchestrate the assembly of nucleoprotein complexes. Through a complex network of protein-DNA and protein-protein interaction, they play important roles in gene transcription, recombination, and chromatin structure. This protein family is involved, through different mechanisms, in both benign and malignant neoplasias. We have recently reported that transgenic mice carrying the Hmga1 or Hmga2 genes under transcriptional control of the cytomegalovirus promoter develop pituitary adenomas secreting prolactin and growth hormone. We have shown that the mechanism of the HMGA2-induced pituitary adenoma is based on the increased E2F1 activity. The expression profile of mouse normal pituitary glands and adenomas induced in HMGA transgenic mice revealed an increased expression of the ccnb2 gene, coding for the cyclin B2 protein, in the neoplastic tissues compared with the normal pituitary gland. Here, we show, by electrophoretic mobility shift assay and chromatin immunoprecipitation, a direct binding of HMGA proteins to the promoter of ccnb2 gene, whereas luciferase assays showed that HMGAs are able to up-regulate ccnb2 promoter activity. Finally, we report an increased CCNB2 expression in human pituitary adenomas of different histotypes that is directly correlated with HMGA1 and HMGA2 expression. Because cyclin B2 is involved in the regulation of the cell cycle, these results taken together indicate that HMGA-induced cyclin B2 overexpression gives an important contribution to experimental and human pituitary tumorigenesis.

Loss of the CBX7 Gene Expression Correlates with a Highly Malignant Phenotype in Thyroid Cancer

Using gene expression profiling, we found that the CBX7 gene was drastically down-regulated in six thyroid carcinoma cell lines versus control cells. The aims of this study were to determine whether CBX7 is related to the thyroid cancer phenotype and to try to identify new tools for the diagnosis and prognosis of thyroid cancer. We thus evaluated CBX7 expression in various snap-frozen and paraffin-embedded thyroid carcinoma tissues of different degrees of malignancy by quantitative reverse transcription-PCR and immunohistochemistry, respectively. CBX7 expression progressively decreased with malignancy grade and neoplasia stage. Indeed, it decreased in an increasing percentage of cases going from benign adenomas to papillary (PTC), follicular, and anaplastic (ATC) thyroid carcinomas. This finding coincides with results obtained in rat and mouse models of thyroid carcinogenesis. CBX7 loss of heterozygosity occurred in 36.8% of PTC and in 68.7% of ATC. Restoration of CBX7 expression in thyroid cancer cells reduced growth rate, with a retention in the G(1) phase of the cell cycle, suggesting that CBX7 can contribute to the proliferation of the transformed thyroid cells. In conclusion, loss of CBX7 expression correlates with a highly malignant phenotype in thyroid cancer patients.

CBX7 is a tumor suppressor in mice and humans

The CBX7 gene encodes a polycomb group protein that is known to be downregulated in many types of human cancers, although the role of this protein in carcinogenesis remains unclear. To shed light on this issue, we generated mice null for Cbx7. Mouse embryonic fibroblasts derived from these mice had a higher growth rate and reduced susceptibility to senescence compared with their WT counterparts. This was associated with upregulated expression of multiple cell cycle components, including cyclin E, which is known to play a key role in lung carcinogenesis in humans. Adult Cbx7-KO mice developed liver and lung adenomas and carcinomas. In in vivo and in vitro experiments, we demonstrated that CBX7 bound to the CCNE1 promoter in a complex that included HDAC2 and negatively regulated CCNE1 expression. Finally, we found that the lack of CBX7 protein expression in human lung carcinomas correlated with CCNE1 overexpression. These data suggest that CBX7 is a tumor suppressor and that its loss plays a key role in the pathogenesis of cancer.

Identification of a New Pathway for Tumor Progression MicroRNA-181b Up-Regulation and CBX7 Down-Regulation by HMGA1 Protein

High mobility group A (HMGA) overexpression plays a critical role in neoplastic transformation. To investigate whether HMGA acts by regulating the expression of microRNAs, we analyzed the microRNA expression profile of human breast adenocarcinoma cells (MCF7) transfected with the HMGA1 gene, which results in a highly malignant phenotype. Among the microRNAs induced by HMGA1, we focused on miR-181b, which was overexpressed in several malignant neoplasias including breast carcinomas. We show that miR-181b regulates CBX7 protein levels, which are down-regulated in cancer, and promotes cell cycle progression. We also demonstrate that CBX7, being negatively regulated by HMGA, is able to negatively regulate miR-181b expression. Finally, there was a direct correlation between HMGA1 and miR-181b expression and an inverse correlation between HMGA1 and CBX7 expression in human breast carcinomas. These data indicate the presence of a novel pathway involving HMGA1, miR-181b, and CBX7, which leads to breast cancer progression.

Regulation of microRNA expression by HMGA1 proteins

The High Mobility Group proteins HMGA1 are nuclear architectural factors that play a critical role in a wide range of biological processes. Since recent studies have identified the microRNAs (miRNAs) as important regulators of gene expression, modulating critical cellular functions such as proliferation, apoptosis and differentiation, the aim of our work was to identify the miRNAs that are physiologically regulated by HMGA1 proteins. To this purpose, we have analysed the miRNA expression profile of mouse embryonic fibroblasts (MEFs) carrying two, one or no Hmga1 functional alleles using a microarray (miRNA microarray). By this approach, we found a miRNA expression profile that differentiates Hmga1-null MEFs from the wild-type ones. In particular, a significant decrease in miR-196a-2, miR-101b, miR-331 and miR-29a was detected in homozygous Hmga1-knockout MEFs in comparison with wild-type cells. Consistently, these miRNAs are downregulated in most of the analysed tissues of Hmga1-null mice in comparison with the wild-type mice. ChIP assay shows that HMGA1 is able to bind regions upstream of these miRNAs. Moreover, we identified the HMGA2 gene product as a putative target of miR-196a-2, suggesting that HMGA1 proteins are able to downregulate the expression of the other member of the HMGA family through the regulation of the miR-196a-2 expression. Finally, ATXN1 and STC1 gene products have been identified as targets of miR-101b. Therefore, it is reasonable to hypothesize that HMGA1 proteins are involved in several functions by regulating miRNA expression.

Hmga1/Hmga2 double knock-out mice display a “superpygmy” phenotype

ABSTRACT The HMGA1 and HMGA2 genes code for proteins belonging to the High Mobility Group A family. Several genes are negatively or positively regulated by both these proteins, but a number of genes are specifically regulated by only one of them. Indeed, knock-out of the Hmga1 and Hmga2 genes leads to different phenotypes: cardiac hypertrophy and type 2 diabetes in the former case, and a large reduction in body size and amount of fat tissue in the latter case. Therefore, to better elucidate the functions of the Hmga genes, we crossed Hmga1-null mice with mice null for Hmga2. The Hmga1−/−/Hmga2−/− mice showed reduced vitality and a very small size (75% smaller than the wild-type mice); they were even smaller than pygmy Hmga2-null mice. The drastic reduction in E2F1 activity, and consequently in the expression of the E2F-dependent genes involved in cell cycle regulation, likely accounts for some phenotypic features of the Hmga1−/−/Hmga2−/− mice.

HMGA1-pseudogene overexpression contributes to cancer progression

Two pseudogenes for HMGA1, whose overexpression has a critical role in cancer progression, have been identified. They act as decoy for miRNAs that are able to target the HMGA1 gene then enhancing cell proliferation and migration. Moreover, these pseudogenes contain sequences that are potential target sites for cancer-related miRNAs. Interestingly, HMGA1 pseudogenes are highly expressed in human anaplastic thyroid carcinomas, that is one of the most aggressive tumor in mankind, but almost undetectable in well differentiated thyroid carcinomas.

Tumor suppressor activity of CBX7 in lung carcinogenesis.

The generation of knockout mice for the Cbx7 gene validates the tumor suppressor role of CBX7, whose expression is lost in several human malignancies. Indeed, these mice developed liver and lung adenomas and carcinomas. Cyclin E overexpression due to the lack of Cbx7 negative regulation of its expression likely accounts for the phenotype of the Cbx7-KO mice. A similar mechanism is likely involved in human lung carcinogenesis, since cyclin E upregulation associated with the loss of CBX7 expression has been observed in most of the human lung carcinomas analyzed.

The Mia/Cd-rap gene expression is downregulated by the high-mobility group A proteins in mouse pituitary adenomas

The high-mobility group A (HMGA) family of proteins orchestrates the assembly of nucleoprotein structures playing important roles in gene transcription, recombination, and chromatin structure through a complex network of protein-DNA and protein-protein interactions. Recently, we have generated transgenic mice carrying wild type or truncated HMGA2 genes under the transcriptional control of the cytomegalovirus promoter. These mice developed pituitary adenomas secreting prolactin and GH mainly due to an increased E2F1 activity, directly consequent to the HMGA2 overexpression. To identify other genes involved in the process of pituitary tumorigenesis induced by the HMGA2 gene, in this study we have analyzed the gene expression profile of three HMGA2-pituitary adenomas in comparison with a pool of ten normal pituitary glands from control mice, using the Affymetrix MG MU11K oligonucleotide array representing approximately 13,000 unique genes. We have identified 82 transcripts that increased and 72 transcripts that decreased at least four-fold in all the mice pituitary adenomas analyzed compared with normal pituitary glands. Among these genes, we focused our attention on the Mia/Cd-rap gene, whose expression was essentially suppressed in all of the pituitary adenomas tested by the microarray. We demonstrated that the HMGA proteins directly bind to the promoter of the Mia/Cd-rap gene and are able to downregulate its expression. In order to understand a possible role of Mia/Cd-rap in pituitary cell growth, we performed a colony assay in GH3 and GH4 cells. Interestingly, Mia/Cd-rap expression inhibits their proliferation, suggesting a potential tumor suppressor role of Mia/Cd-rap in pituitary cells.

CBX7 Modulates the Expression of Genes Critical for Cancer Progression

Background We have previously shown that the expression of CBX7 is drastically decreased in several human carcinomas and that its expression progressively decreases with the appearance of a highly malignant phenotype. The aim of our study has been to investigate the mechanism by which the loss of CBX7 expression may contribute to the emergence of a more malignant phenotype. Methods We analyzed the gene expression profile of a thyroid carcinoma cell line after the restoration of CBX7 and, then, analyzed the transcriptional regulation of identified genes. Finally, we evaluated the expression of CBX7 and regulated genes in a panel of thyroid and lung carcinomas. Results We found that CBX7 negatively or positively regulates the expression of several genes (such as SPP1, SPINK1, STEAP1, and FOS, FOSB, EGR1, respectively) associated to cancer progression, by interacting with their promoter regions and modulating their transcriptional activity. Quantitative RT-PCR analyses in human thyroid and lung carcinoma tissues revealed a negative correlation between CBX7 and its down-regulated genes, while a positive correlation was observed with up-regulated genes. Conclusion In conclusion, the loss of CBX7 expression might play a critical role in advanced stages of carcinogenesis by deregulating the expression of specific effector genes.