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Plasticity is an essential condition for cancer cells to invade surrounding tissues. The nucleus is the most rigid cellular organelle and it undergoes substantial deformations to get through environmental constrictions. Nuclear stiffness mostly depends on the nuclear lamina and chromatin, which in turn might be affected by nuclear architectural proteins. Among these is the HMGA1 (High Mobility Group A1) protein, a factor that plays a causal role in neoplastic transformation and that is able to disentangle heterochromatic domains by H1 displacement. Here we made use of atomic force microscopy to analyze the stiffness of breast cancer cellular models in which we modulated HMGA1 expression to investigate its role in regulating nuclear plasticity. Since histone H1 is the main modulator of chromatin structure and HMGA1 is a well-established histone H1 competitor, we correlated HMGA1 expression and cellular stiffness with histone H1 expression level, post-translational modifications, and nuclear distribution. Our results showed that HMGA1 expression level correlates with nuclear stiffness, is associated to histone H1 phosphorylation status, and alters both histone H1 chromatin distribution and expression. These data suggest that HMGA1 might promote chromatin relaxation through a histone H1-mediated mechanism strongly impacting on the invasiveness of cancer cells.

As a mediator of insulin-regulated gene expression, the FoxO1 transcription factor represents a master regulator of liver glucose metabolism. We previously reported that the high-mobility group AT-hook 1 (HMGA1) protein, a molecular switch for the insulin receptor gene, functions also as a downstream target of the insulin receptor signaling pathway, representing a critical nuclear mediator of insulin function. Here, we investigated whether a functional relationship existed between FoxO1 and HMGA1, which might help explain insulin-mediated gene transcription in the liver. To this end, as a model study, we investigated the canonical FoxO1-HMGA1-responsive IGFBP1 gene, whose hepatic expression is regulated by insulin. By using a conventional GST-pull down assay combined with co-immunoprecipitation and Fluorescence Resonance Energy Transfer (FRET) analyses, we provide evidence of a physical interaction between FoxO1 and HMGA1. Further investigation with chromatin immunoprecipitation, confocal microscopy, and Fluorescence Recovery After Photobleaching (FRAP) technology indicated a functional significance of this interaction, in both basal and insulin-stimulated states, providing evidence that, by modulating FoxO1 transactivation, HMGA1 is essential for FoxO1-induced IGFBP1 gene expression, and thereby a critical modulator of insulin-mediated FoxO1 regulation in the liver. Collectively, our findings highlight a novel FoxO1/HMGA1-mediated mechanism by which insulin may regulate gene expression and metabolism.

Although the three families of mammalian HMG proteins (HMGA, HMGB and HMGN) participate in many of the same nuclear processes, each family plays its own unique role in modulating chromatin structure and regulating genomic function. This review focuses on the similarities and differences in the mechanisms by which the different HMG families impact chromatin structure and influence cellular phenotype. The biological implications of having three architectural transcription factor families with complementary, but partially overlapping, nuclear functions are discussed., (Copyright 2009 Elsevier B.V. All rights reserved.)

High mobility group (HMG) proteins assume important roles in regulating chromatin dynamics, transcriptional activities of genes and other cellular processes. Post-translational modifications of HMG proteins can alter their interactions with DNA and proteins, and consequently, affect their biological activities. Although the mechanisms through which these modifications are involved in regulating biological processes in different cellular contexts are not fully understood, new insights into these modification "codes" have emerged from the increasing appreciation of the functions of these proteins. In this review, we focus on the chemical modifications of mammalian HMG proteins and highlight their roles in nuclear functions., (Copyright 2009 Elsevier B.V. All rights reserved.)

In plants, the chromatin-associated high mobility group (HMG) proteins occur in two subfamilies termed HMGA and HMGB. The HMGA proteins are characterized by the presence of four AT-hook DNA binding motifs, and the HMGB proteins contain an HMG box DNA binding domain. As architectural factors, the HMG proteins appear to be involved in the regulation of transcription and other DNA-dependent processes. We have examined the subcellular localization of Arabidopsis thaliana HMGA, HMGB1, and HMGB5, revealing that they localize to the cell nucleus. They display a speckled distribution pattern throughout the chromatin of interphase nuclei, whereas none of the proteins associate with condensed mitotic chromosomes. HMGA is targeted to the nucleus by a monopartite nuclear localization signal, while efficient nuclear accumulation of HMGB1/5 requires large portions of the basic N-terminal part of the proteins. The acidic C-terminal domain interferes with nucleolar targeting of HMGB1. Fluorescence recovery after photobleaching experiments revealed that HMGA and HMGB proteins are extremely dynamic in the nucleus, indicating that they bind chromatin only transiently before moving on to the next site, thereby continuously scanning the genome for targets. By contrast, the majority of histone H2B is basically immobile within the nucleus, while linker histone H1.2 is relatively mobile.

Cellular senescence is a stable state of proliferative arrest that provides a barrier to malignant transformation and contributes to the antitumor activity of certain chemotherapies. Senescent cells can accumulate senescence-associated heterochromatic foci (SAHFs), which may provide a chromatin buffer that prevents activation of proliferation-associated genes by mitogenic transcription factors. Surprisingly, we show that the High-Mobility Group A (HMGA) proteins, which can promote tumorigenesis, accumulate on the chromatin of senescent fibroblasts and are essential structural components of SAHFs. HMGA proteins cooperate with the p16(INK4a) tumor suppressor to promote SAHF formation and proliferative arrest and stabilize senescence by contributing to the repression of proliferation-associated genes. These antiproliferative activities are canceled by coexpression of the HDM2 and CDK4 oncogenes, which are often coamplified with HMGA2 in human cancers. Our results identify a component of the senescence machinery that contributes to heterochromatin formation and imply that HMGA proteins also act in tumor suppressor networks.