The High Mobility Group A1 (HMGA1) Chromatin Architectural Factor Modulates Nuclear Stiffness in Breast Cancer Cells.

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.

Activation of human aortic valve interstitial cells by local stiffness involves YAP-dependent transcriptional signaling.

Differentiation of valve interstitial cells (VICs) into pro-calcific cells is one of the central events in calcific aortic valve (AoV) disease (CAVD). While the paracrine pathways and the responsivity of VICs to mechanical compliance of the surrounding environment are well characterized, the molecular programming related to variations in local stiffness, and its link to cytoskeleton dynamics, is less consolidated. By using a simple method to produce 2D poly-acrylamide gels with stiffness controlled with atomic force microscopy (AFM), we manufactured adhesion substrates onto which human VICs from stenotic valves were plated, and subsequently investigated for cytoskeleton dynamics and activation of the mechanosensing-related transcription factor YAP. As a comparison, we employed VICs from patients undergoing valve substitution for valve insufficiency, a non-calcific AoV disease, which does not involve extensive inflammation. While the two VICs types did not differ for basic responses onto substrates with different stiffness values (e.g. adhesion and proliferation), they were subject to a different dynamics of stiffness-dependent YAP nuclear shuttling, revealing for the first time an intracellular force transduction mechanism distinctive for calcific aortic valve disease. In VICs from stenotic valves, YAP nuclear translocation occurred in concert with an increase in cytoskeleton tensioning and loading of the myofibroblast-specific protein αSMA onto the F-actin cytoskeleton. AFM force mapping performed along radial sections of human calcific valve leaflets identified, finally, areas with high and low levels of rigidity within a similar range to those controlling YAP nuclear translocation in vitro. Since VICs juxtaposed to these areas exhibited nuclear localized YAP, we conclude that subtle variations in matrix stiffness are involved in mechanosensing-dependent VICs activation and pathological differentiation in CAVD.

Mechanical cues control mutant p53 stability through a mevalonate-RhoA axis.

Tumour-associated p53 missense mutants act as driver oncogenes affecting cancer progression, metastatic potential and drug resistance (gain-of-function) 1 . Mutant p53 protein stabilization is a prerequisite for gain-of-function manifestation; however, it does not represent an intrinsic property of p53 mutants, but rather requires secondary events 2 . Moreover, mutant p53 protein levels are often heterogeneous even within the same tumour, raising questions on the mechanisms that control local mutant p53 accumulation in some tumour cells but not in their neighbours 2,3 . By investigating the cellular pathways that induce protection of mutant p53 from ubiquitin-mediated proteolysis, we found that HDAC6/Hsp90-dependent mutant p53 accumulation is sustained by RhoA geranylgeranylation downstream of the mevalonate pathway, as well as by RhoA- and actin-dependent transduction of mechanical inputs, such as the stiffness of the extracellular environment. Our results provide evidence for an unpredicted layer of mutant p53 regulation that relies on metabolic and mechanical cues.