Apical cell protrusions cause vertical deformation of the soft cancer nucleus

Cancer cell response to extrinsic and intrinsic mechanical cue: opportunities for tumor apoptosis strategies

Increasing studies have revealed the importance of mechanical cues in tumor progression, invasiveness and drug resistance. During malignant transformation, changes manifest in either the mechanical properties of the tissue or the cellular ability to sense and respond to mechanical signals. The major focus of the review is the subtle correlation between mechanical cues and apoptosis in tumor cells from a mechanobiology perspective. To begin, we focus on the intracellular force, examining the mechanical properties of the cell interior, and outlining the role that the cytoskeleton and intracellular organelle-mediated intracellular forces play in tumor cell apoptosis. This article also elucidates the mechanisms by which extracellular forces guide tumor cell mechanosensing, ultimately triggering the activation of the mechanotransduction pathway and impacting tumor cell apoptosis. Finally, a comprehensive examination of the present status of the design and development of anti-cancer materials targeting mechanotransduction is presented, emphasizing the underlying design principles. Furthermore, the article underscores the need to address several unresolved inquiries to enhance our comprehension of cancer therapeutics that target mechanotransduction.

Recent progress of mechanosensitive mechanism on breast cancer

The mechanical environment is important for tumorigenesis and progression. Tumor cells can sense mechanical signals by mechanosensitive receptors, and these mechanical signals can be converted to biochemical signals to regulate cell behaviors, such as cell differentiation, proliferation, migration, apoptosis, and drug resistance. Here, we summarized the effects of the mechanical microenvironment on breast cancer cell activity, and mechanotransduction mechanism from cellular microenvironment to cell membrane, and finally to the nucleus, and also relative mechanosensitive proteins, ion channels, and signaling pathways were elaborated, therefore the mechanical signal could be transduced to biochemical or molecular signal. Meanwhile, the mechanical models commonly used for biomechanics study in vitro and some quantitative descriptions were listed. It provided an essential theoretical basis for the occurrence and development of mechanosensitive breast cancer, and also some potential drug targets were proposed to treat such disease.

Mechanotherapy in oncology: Targeting nuclear mechanics and mechanotransduction

Mechanotherapy is proposed as a new option for cancer treatment. Increasing evidence suggests that characteristic differences are present in the nuclear mechanics and mechanotransduction of cancer cells compared with those of normal cells. Recent advances in understanding nuclear mechanics and mechanotransduction provide not only further insights into the process of malignant transformation but also useful references for developing new therapeutic approaches. Herein, we present an overview of the alterations of nuclear mechanics and mechanotransduction in cancer cells and highlight their implications in cancer mechanotherapy.

An epigenetic small molecule screen to target abnormal nuclear morphology in human cells

Irregular nuclear shapes are a hallmark of human cancers. Recent studies suggest that alterations to chromatin regulators may cause irregular nuclear morphologies. Here we screened an epigenetic small molecule library consisting of 145 compounds against chromatin regulators, for their ability to revert abnormal nuclear shapes that were induced by gene knockdown in non-cancerous MCF10A human mammary breast epithelial cells. We leveraged a previously validated quantitative Fourier approach to quantify the elliptical Fourier coefficient (EFC ratio) as a measure of nuclear irregularities, which allowed us to perform rigorous statistical analyses of screening data. Top hit compounds fell into three major mode of action categories, targeting three separate epigenetic modulation routes: 1) Histone deacetylase (HDAC) inhibitors; 2) Bromodomain and extra-terminal domain (BET) protein inhibitors; and 3) Methyl-transferase inhibitors. Some of the top hit compounds were also efficacious in reverting nuclear irregularities in MDA-MB-231 triple negative breast cancer cells and in PANC-1 pancreatic cancer cells in a cell type dependent manner. Regularization of nuclear shapes was compound-specific, cell-type specific, and dependent on the specific molecular perturbation that induced nuclear irregularities. Our approach of targeting nuclear abnormalities may be potentially useful in screening new types of cancer therapies targeted toward chromatin structure.

Architectural control of mesenchymal stem cell phenotype through nuclear actin

There is growing appreciation that architectural components of the nucleus regulate gene accessibility by altering chromatin organization. While nuclear membrane connector proteins link the mechanosensitive actin cytoskeleton to the nucleoskeleton, actin’s contribution to the inner architecture of the nucleus remains enigmatic. Control of actin transport into the nucleus, plus the presence of proteins that control actin structure (the actin tool-box) within the nucleus, suggests that nuclear actin may support biomechanical regulation of gene expression. Cellular actin structure is mechanoresponsive: actin cables generated through forces experienced at the plasma membrane transmit force into the nucleus. We posit that dynamic actin remodeling in response to such biomechanical cues provides a novel level of structural control over the epigenetic landscape. We here propose to bring awareness to the fact that mechanical forces can promote actin transfer into the nucleus and control structural arrangements as illustrated in mesenchymal stem cells, thereby modulating lineage commitment.

Viscous shaping of the compliant cell nucleus

The cell nucleus is commonly considered to be a stiff organelle that mechanically resists changes in shape, and this resistance is thought to limit the ability of cells to migrate through pores or spread on surfaces. Generation of stresses on the cell nucleus during migration and nuclear response to these stresses is fundamental to cell migration and mechano-transduction. In this Perspective, we discuss our previous experimental and computational evidence that supports a dynamic model, in which the soft nucleus is irreversibly shaped by viscous stresses generated by the motion of cell boundaries and transmitted through the intervening cytoskeletal network. While the nucleus is commonly modeled as a stiff elastic body, we review how nuclear shape changes on the timescale of migration can be explained by simple geometric constraints of constant nuclear volume and constant surface area of the nuclear lamina. Because the lamina surface area is in excess of that of a sphere of the same volume, these constraints permit dynamic transitions between a wide range of shapes during spreading and migration. The excess surface area allows the nuclear shape changes to mirror those of the cell with little mechanical resistance. Thus, the nucleus can be easily shaped by the moving cell boundaries over a wide range of shape changes and only becomes stiff to more extreme deformations that would require the lamina to stretch or the volume to compress. This model explains how nuclei can easily flatten on surfaces during cell spreading or elongate as cells move through pores until the lamina smooths out and becomes tense.

Nuclear Morphological Abnormalities in Cancer: A Search for Unifying Mechanisms

Irregularities in nuclear shape and/or alterations to nuclear size are a hallmark of malignancy in a broad range of cancer types. Though these abnormalities are commonly used for diagnostic purposes and are often used to assess cancer progression in the clinic, the mechanisms through which they occur are not well understood. Nuclear size alterations in cancer could potentially arise from aneuploidy, changes in osmotic coupling with the cytoplasm, and perturbations to nucleocytoplasmic transport. Nuclear shape changes may occur due to alterations to cell-generated mechanical stresses and/or alterations to nuclear structural components, which balance those stresses, such as the nuclear lamina and chromatin. A better understanding of the mechanisms underlying abnormal nuclear morphology and size may allow the development of new therapeutics to target nuclear aberrations in cancer.

A method for direct imaging of x-z cross-sections of fluorescent samples

The x-z cross-sectional profiles of fluorescent objects can be distorted in confocal microscopy, in large part due to mismatch between the refractive index of the immersion medium of typical high numerical aperture objectives and the refractive index of the medium in which the sample is present. Here, we introduce a method to mount fluorescent samples parallel to the optical axis. This mounting allows direct imaging of what would normally be an x-z cross-section of the object, in the x-y plane of the microscope. With this approach, the x-y cross-sections of fluorescent beads were seen to have significantly lower shape-distortions as compared to x-z cross-sections reconstructed from confocal z-stacks. We further tested the method for imaging of nuclear and cellular heights in cultured cells, and found that they are significantly flatter than previously reported. This approach allows improved imaging of the x-z cross-section of fluorescent samples. LAY DESCRIPTION: Optical distortions are common in confocal microscopy. In particular, the mismatch between the refractive index of the immersion medium of the microscope objective and the refractive index of the sample medium distorts the shapes of fluorescent objects in the x-z plane of the microscope. Here, we introduced a method to eliminate the shape-distortion in the x-z cross-sections. This was achieved by mounting fluorescent samples on vertical glass slides such that the cross-sections orthogonal to the glass surface could be imaged in the x-y plane of the microscope. Our method successfully improved the imaging of nuclear and cellular heights in cultured cells and revealed that the heights were significantly flatter than previously reported with conventional approaches.

High-throughput gene screen reveals modulators of nuclear shape

Irregular nuclear shapes characterized by blebs, lobules, micronuclei, or invaginations are hallmarks of many cancers and human pathologies. Despite the correlation between abnormal nuclear shape and human pathologies, the mechanism by which the cancer nucleus becomes misshapen is not fully understood. Motivated by recent evidence that modifying chromatin condensation can change nuclear morphology, we conducted a high-throughput RNAi screen to identify epigenetic regulators that are required to maintain normal nuclear shape in human breast epithelial MCF-10A cells. We silenced 608 genes in parallel using an epigenetics siRNA library and used an unbiased Fourier analysis approach to quantify nuclear contour irregularity from fluorescent images captured on a high-content microscope. Using this quantitative approach, which we validated with confocal microscopy, we significantly expand the list of epigenetic regulators that impact nuclear morphology.