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2012/2013 HMGA1 is an oncogene encoding for an architectural transcription factor that affects fundamental cell processes, leading to neoplastic transformation. The two main mechanisms by which HMGA1 protein is known to be involved in cancer concern the regulation of gene expression by altering DNA structure and interacting with a conspicuous number of transcription factors. Here we provide evidence of an additional level of gene expression regulation exploited by HMGA1 to exert its oncogenic activity. Starting from protein-protein interaction data showing that HMGA1 interacts with histones, we show that HMGA1 regulates gene expression by affecting the epigenetic status of cancer cells. In particular, it modulates the signalling cascade mediated by the RAS/RAF/MEKK/ERK/RSK2 pathway regulating the levels of histone H3 phosphorylation at Serine 10 and Serine 28. We demonstrate that the down-regulation of these two H3 post-translational modifications by HMGA1 silencing and by inhibitors of the RAS/RAF/MEKK/ERK pathway is linked to cell migration decrease and morphological changes resembling the mesenchymal to epithelial transition. HMGA1 è un oncogene codificante per un fattore trascrizionale architetturale che influenza fondamentali processi cellulari, portando alla trasformazione neoplastica. I due principali meccanismi tramite cui la proteina HMGA1 è nota essere coinvolta nel cancro riguardano la regolazione dell’espressione genica tramite l’alterazione della struttura del DNA e l’interazione con un cospicuo numero di fattori di trascrizione. Qui forniamo la prova di un addizionale livello di regolazione dell’espressione genica sfruttato da HMGA1 per esercitare la sua attività oncogenica. Partendo da dati d’interazione proteina-proteina che mostrano che HMGA1 interagisce con gli istoni, mostriamo che HMGA1 regola l’espressione genica influenzando lo stato epigenetico delle cellule cancerose. In particolare, essa modula la cascata di segnalazione mediata dalla via di RAS/RAF/MEKK/ERK/RSK2 regolando i livelli di fosforilazione dell’istone H3 sulla Serina 10 e sulla Serina 28. Noi dimostriamo che la down-regolazione di queste due modificazioni post-traduzionali di H3 tramite il silenziamento di HMGA1 e l’utilizzo di inibitori della via di RAS/RAF/MEKK/ERK/RSK2 è correlata alla diminuzione della migrazione cellulare e a cambiamenti morfologici che ricordano la transizione mesenchimo-epiteliale. XXV Ciclo 1983

Cancer is a very heterogeneous disease, and biological variability adds a further level of complexity, thus limiting the ability to identify new genes involved in cancer development. Oncogenes whose expression levels control cell aggressiveness are very useful for developing cellular models that permit differential expression screenings in isogenic contexts. HMGA1 protein has this unique property because it is a master regulator in breast cancer cells that control the transition from a nontumorigenic epithelial-like phenotype toward a highly aggressive mesenchymal-like one. The proteins extracted from HMGA1-silenced and control MDA-MB-231 cells were analyzed using label-free shotgun mass spectrometry. The differentially expressed proteins were cross-referenced with DNA microarray data obtained using the same cellular model and the overlapping genes were filtered for factors linked to poor prognosis in breast cancer gene expression meta-data sets, resulting in an HMGA1 protein signature composed of 21 members (HRS, HMGA1 reduced signature). This signature had a prognostic value (overall survival, relapse-free survival, and distant metastasis-free survival) in breast cancer. qRT-PCR, Western blot, and immunohistochemistry analyses validated the link of three members of this signature (KIFC1, LRRC59, and TRIP13) with HMGA1 expression levels both in vitro and in vivo and wound healing assays demonstrated that these three proteins are involved in modulating tumor cell motility. Combining proteomic and genomic data with the aid of bioinformatic tools, our results highlight the potential involvement in neoplastic transformation of a restricted list of factors with an as-yet-unexplored role in cancer. These factors are druggable targets that could be exploited for the development of new, targeted therapeutic approaches in triple-negative breast cancer.

The HMGA1 architectural transcription factor is an oncogene overexpressed in the vast majority of human cancers. HMGA1 is a highly connected node in the nuclear molecular network and the key aspect of HMGA1 involvement in cancer development is that HMGA1 simultaneously confers cells multiple oncogenic hits, ranging from global chromatin structural and gene expression modifications up to the direct functional alterations of key cellular proteins. Interestingly, HMGA1 also modulates DNA damage repair pathways. In this work, we provide evidences linking HMGA1 with Non-Homologous End Joining DNA repair. We show that HMGA1 is in complex with and is a substrate for DNA-PK. HMGA1 enhances Ligase IV activity and it counteracts the repressive histone H1 activity towards DNA ends ligation. Moreover, breast cancer cells overexpressing HMGA1 show a faster recovery upon induction of DNA double-strand breaks, which is associated with a higher survival. These data suggest that resistance to DNA-damaging agents in cancer cells could be partially attributed to HMGA1 overexpression thus highlighting the relevance of considering HMGA1 expression levels in the selection of valuable and effective pharmacological regimens.

High Mobility Group A are non-histone nuclear proteins that regulate chromatin plasticity and accessibility, playing an important role both in physiology and pathology. Their activity is controlled by transcriptional, post-transcriptional and post-translational mechanisms. In this study we provide evidence for a novel modulatory mechanism for HMGA functions. We show that HMGAs are complexed in vivo with the histone chaperone nucleophosmin (NPM1), that this interaction requires the histone-binding domain of NPM1 and that NPM1 modulates both DNA-binding affinity and specificity of HMGAs. By focusing on two human genes whose expression is directly regulated by HMGA1, the Insulin receptor (INSR) and the Insulin-like growth factor-binding protein 1 (IGFBP1) genes, we demonstrated that occupancy of their promoters by HMGA1 was NPM1-dependent, reflecting a mechanism in which the activity of these cis-regulatory elements is directly modulated by NPM1 leading to changes in gene expression. HMGAs need short stretches of AT-rich nucleosome-free regions to bind to DNA. Therefore, many putative HMGA binding sites are present within the genome. Our findings indicate that NPM1, by exerting a chaperoning activity towards HMGAs, may act as a master regulator in the control of DNA occupancy by these proteins and hence in HMGA-mediated gene expression.

The architectural factors HMGA are highly connected hubs in the chromatin network and affect key cellular functions. HMGA have a causal involvement in cancer development; in fact, truncated or chimeric HMGA forms, resulting from chromosomal rearrangements, lack the constitutively phosphorylated acidic C-terminal tail and display increased oncogenic potential, suggesting a functional role for this domain. HMGA belong to the intrinsically disordered protein category, and this prevents the use of classical approaches to obtain structural data. Therefore, we combined limited proteolysis, ion mobility separation-mass spectrometry (IMS-MS), and electrospray ionization-mass spectrometry (ESI-MS) to obtain structural information regarding full length and C-terminal truncated HMGA forms. Limited proteolysis indicates that HMGA acidic tail shields the inner portions of the protein. IMS-MS and ESI-MS show that HMGA proteins can assume a compact form and that the degree of compactness is dependent upon the presence of the acidic tail and its constitutive phosphorylations. Moreover, we demonstrate that C-terminal truncated forms and wild type proteins are post-translationally modified in a different manner. Therefore, we propose that the acidic tail and its phosphorylation could affect HMGA post-translational modification status and likely their activity. Finally, the mass spectrometry-based approach adopted here proves to be a valuable new tool to obtain structural data regarding intrinsically disordered proteins.

Nuclear functions rely on the activity of a plethora of factors which mostly work in highly coordinated molecular networks. The HMGA proteins are chromatin architectural factors which constitute critical hubs in these networks. HMGA are referred to as oncofetal proteins since they are highly expressed and play essential functions both during embryonic development and neoplastic transformation. A particular feature of HMGA is their intrinsically disordered status, which confers on them an unusual plasticity in contacting molecular partners. Indeed these proteins are able to bind to DNA at the level of AT-rich DNA stretches and to interact with several nuclear factors. In the post-genomic era, and with the advent of proteomic tools for the identification of protein-protein interactions, the number of HMGA molecular partners has increased rapidly. This has led to the extension of our knowledge of the functional involvement of HMGA from the transcriptional regulation field to RNA processing, DNA repair, and chromatin remodeling and dynamics. This review focuses mainly on the protein-protein interaction network of HMGA and its functional outcome. HMGA molecular partners have been functionally classified and all the information collected in a freely available database (http://www.bbcm.units.it/ approximately manfiol/INDEX.HTM).