Mechanobiological cell adaptations to changing microenvironments determine cancer invasiveness: Experimentally validated finite element modeling

Metastases are the leading cause of cancer-associated deaths. A key process in metastasis is cell invasiveness, which is driven and controlled by cancer cell interactions with their microenvironment. We have previously shown that invasive cancer cells forcefully push into and indent physiological stiffness gels to cell-scale depths, where the percentage of indenting cells and their attained depths provide clinically relevant predictions of tumor invasiveness and the potential metastatic risk. The cell-attained, invasive indentation depths are directly affected by gel-microenvironment mechanics, which can concurrently modulate the cells' mechanics and force application capacity, in a complex, coordinated mechanobiological response. As it is impossible to experimentally isolate the different contributions of cell and gel mechanics to cancer cell invasiveness, we perform finite element modeling with literature-based parameters. Under average-scale, cell cytoplasm and nucleus mechanics and cell-applied force levels, increasing gel stiffness 1-50 kPa significantly reduced the attained indentation depth by >200%, while the gel's Poisson ratio reduced depths only by up to 20% and only when the ratio was >0.4; this reveals microenvironment mechanics that can promote invasiveness. Experiments with varying-invasiveness cancer cells exhibited qualitative variations in their responses to gel stiffness increase, for example large/small reduction in indentation depth or increase and then reduction. We quantitatively and qualitatively reproduced the different experimental responses via coordinated changes in cell mechanics and applied force levels. Thus, the different cancer cell capacities to adapt their mechanobiology in response to mechanically changing microenvironments likely determine the varying cancer invasiveness and metastatic risk levels in patients.

Collagens Regulating Adipose Tissue Formation and Functions

The globally increasing prevalence of obesity is associated with the development of metabolic diseases such as type 2 diabetes, dyslipidemia, and fatty liver. Excess adipose tissue (AT) often leads to its malfunction and to a systemic metabolic dysfunction because, in addition to storing lipids, AT is an active endocrine system. Adipocytes are embedded in a unique extracellular matrix (ECM), which provides structural support to the cells as well as participating in the regulation of their functions, such as proliferation and differentiation. Adipocytes have a thin pericellular layer of a specialized ECM, referred to as the basement membrane (BM), which is an important functional unit that lies between cells and tissue stroma. Collagens form a major group of proteins in the ECM, and some of them, especially the BM-associated collagens, support AT functions and participate in the regulation of adipocyte differentiation. In pathological conditions such as obesity, AT often proceeds to fibrosis, characterized by the accumulation of large collagen bundles, which disturbs the natural functions of the AT. In this review, we summarize the current knowledge on the vertebrate collagens that are important for AT development and function and include basic information on some other important ECM components, principally fibronectin, of the AT. We also briefly discuss the function of AT collagens in certain metabolic diseases in which they have been shown to play central roles.

Adipose Tissue Development Relies on Coordinated Extracellular Matrix Remodeling, Angiogenesis, and Adipogenesis

Despite developing prenatally, the adipose tissue is unique in its ability to undergo drastic growth even after reaching its mature size. This development and subsequent maintenance rely on the proper coordination between the vascular niche and the adipose compartment. In this review, the process of adipose tissue development is broken down to explain (1) the ultrastructural matrix remodeling that is undertaken during simultaneous adipogenesis and angiogenesis, (2) the paracrine crosstalk involved during adipose development, (3) the mechanical regulators involved in adipose growth, and (4) the proteolytic and paracrine oversight for matrix remodeling during adipose development. It is crucial to gain a better understanding of the complex relationships that exist between adipose tissue and the vasculature during tissue development to provide insights into the pathological tissue expansion of obesity and to develop improved soft-tissue reconstruction techniques.

Rho/ROCK mechanosensor in adipocyte stiffness and traction force generation

It is increasingly recognized that interaction of adipose cells with extracellular mechanophysical milieus may play a role in regulating adipogenesis and differentiated adipocyte function and such interaction can be mediated by the mechanics of adipose cells. We measured the stiffness and traction force of adipose cells and examined the role of Rho/ROCK, the upstream effector of actin cytoskeletal contractility, in affecting these mechanical properties. Cellular Young's modulus obtained from atomic force microscopy (AFM) was significantly reduced by ROCK inhibitor (Y-27632) but elevated by Rho activator (CN01), for both preadipocytes and differentiated adipocytes. Immunofluorescent imaging suggested this could be attributed to the changes in Rho/ROCK-induced stressed actin filament formation. AFM also confirmed that differentiated adipocytes had higher stiffness than preadipocytes. On the other hand, traction force microscopy (TFM) revealed differentiated adipocytes exerted lower traction forces than preadipocytes. Traction forces of both preadipocytes and adipocytes were decreased by ROCK inhibition, but not significantly altered by Rho activation. Notably, an increasing trend of traction force with respect to cell spreading area was detected, and this trend was substantially amplified by Rho activation. Such traction force-cell area correlation was an order-of-magnitude smaller for differentiated adipocytes relative to preadipocytes, potentially due to disrupted force transmission through cytoskeleton-focal adhesion linkage by lipid droplets. Our work provides new data evidencing the Rho/ROCK control in adipose cell mechanics, laying the groundwork for adipocyte mechanotransduction studies on adipogenesis and adipose tissue remodeling.

Computational modeling reveals a vital role for proximity-driven additive and synergistic cell-cell interactions in increasing cancer invasiveness

Solid-tumor cell invasion typically occurs by collective migration of attached cell-cohorts, yet we show here that indirect cell-interactions through the substrate can also drive invasiveness. We have previously shown that well-spaced, invasive cancer cells push-into and indent gels to depths of 10 µm, while closely adjacent, non-contacting cancer cells may reach up to 18 µm, potentially relying on cell-cell interactions through the gel-substrate. To test that, we developed finite element models of indenting cells, using experimental gel mechanics, cell mechanostructure, and force magnitudes. We show that under 50-350 nN of combined traction and normal forces, a stiff nucleus-region is essential in facilitating 5-10 µm single-cell indentations, while uniformly soft cells attain 1.6-fold smaller indentations. We observe that indentation depths of cells in close proximity (0.5-50 µm distance) increase relative to well-spaced cells, due to additive, continuum mechanics-driven contributions. Specifically, 2-3 cells applying 220 nN normal forces gained up to 3% in depth, which interestingly increased to 7.8% when two cells, 10 µm apart, applied unequal force-magnitudes (i.e., 220 and 350 nN). Such additive, energy-free contributions can reduce cell mechanical energy -output required for invasiveness, yet the experimentally observed 10-18 µm depths likely necessitate synergistic, mechanobiological changes, which may be mechanically triggered. We note that nucleus stiffening or cytoplasm softening by 25-50% increased indentation depths by only 1-7%, while depths increase nearly linearly with force-magnitude even to two-fold levels. Hence, cell-proximity triggered, synergistic and additive cell-interactions through the substrate can drive collective cancer-cell invasiveness, even without direct cell-cell interactions. STATEMENT OF SIGNIFICANCE: Metastatic cancer invasion typically occurs collectively in attached cell-cohorts. We have previously shown increased invasiveness in closely adjacent cancer cells that are able to push-into and indent soft-gels more deeply than single, well-spaced cells. Using finite element models, we reveal mechanisms of cell-proximity driven invasiveness, demonstrating an important role for the stiff nucleus. Cell-proximity can additively induce small increase in indentation depth via continuum mechanics contributions, especially when adjacent cells apply unequal forces, and without requiring increased cell-mechanical-energy-output. Concurrently, proximity-triggered synergistic interactions that produce changes in cell mechanics or capacity for increased force-levels can facilitate deep invasive-indentations. Thus, we reveal concurrent additive and synergistic mechanisms to drive collective cancer-cell invasiveness even without direct cell-cell interactions.

Introduction to force transmission by nonlinear biomaterials

Xiaoming Mao and Yair Shokef introduce the Soft Matter themed collection on force transmission by nonlinear biomaterials.

Mechanical interactions of invasive cancer cells through their substrate evolve from additive to synergistic

Non-contacting, adjacent cancer cells can mechanically interact through their substrate to increase their invasive and migratory capacities that underly metastases-formation. Such mechanical interactions may induce additive or synergistic enhancement of invasiveness, potentially indicating different underlying force-mechanisms. To identify cell-cell-gel interactions, we monitor the time-evolution of three-dimensional traction strains induced by MDA-MB-231 breast cancer cells adhering on physiological-stiffness (1.8 kPa) collagen gels and compare to simulations. Single metastatic cells apply strain energies of 0.2-2 pJ (average 0.51 ± 0.06 pJ) at all observation times (30-174 min) inducing a mechanical volume-of-effect in the collagen gel that is initially (<60 min from seeding) on the cell-volume scale (∼3000 µm³) and on average increases with time from cell seeding. When cells adhere closely adjacent, at short times (<60 min) we distinguish the additive contributions of neighboring cells to the strains, while at longer times strain fields are synergistically amplified and may facilitate increased cooperative/collective cancer-cell-invasiveness. The results of well-spaced and closely adjacent cells at short times match our simulations of additive deformations induced by radially applied strains with experimentally based inverse-distance decay. We thus reveal a time-dependent evolution from additive to synergistic interactions of adjacently adhering cells that may facilitate metastatic invasion.

Mean-field interactions between living cells in linear and nonlinear elastic matrices

Living cells respond to mechanical changes in the matrix surrounding them by applying contractile forces that are in turn transmitted to distant cells. We consider simple effective geometries for the spatial arrangement of cells, we calculate the mechanical work that each cell performs in order to deform the matrix, and study how that energy changes when a contracting cell is surrounded by other cells with similar properties and behavior. Cells regulating the displacements that they generate are attracted to each other in a manner that does not depend on the cell's rigidity. Whereas cells regulating the active stress that they apply repel each other. This repulsion depends on the cell's bulk modulus in spherical geometry, while in cylindrical geometries the interaction depends also on their shear modulus. In nonlinear, strain-stiffening matrices, for displacement regulation, in the presence of other cells, cell contraction is limited due to the divergence of the shear stress. For stress regulation, the interaction energy drops at the nonlinear stiffening stress. Our theoretical work provides insight into matrix-mediated interactions between contractile cells and on the role of their mechanical regulatory behavior.

Modeling force application configurations and morphologies required for cancer cell invasion

We show that cell-applied, normal mechanical stresses are required for cells to penetrate into soft substrates, matching experimental observations in invasive cancer cells, while in-plane traction forces alone reproduce observations in non-cancer/noninvasive cells. Mechanobiological interactions of cells with their microenvironment drive migration and cancer invasion. We have previously shown that invasive cancer cells forcefully and rapidly push into impenetrable, physiological stiffness gels and indent them to cell-scale depths (up to 10 μm); normal, noninvasive cells indent at most to 0.7 μm. Significantly indenting cells signpost increased cancer invasiveness and higher metastatic risk in vitro and in vivo, as verified experimentally in different cancer types, yet the underlying cell-applied, force magnitudes and configurations required to produce the cell-scale gel indentations have yet to be evaluated. Hence, we have developed finite element models of forces applied onto soft, impenetrable gels using experimental cell/gel morphologies, gel mechanics, and force magnitudes. We show that in-plane traction forces can only induce small-scale indentations in soft gels (< 0.7 μm), matching experiments with various single, normal cells. Addition of a normal force (on the scale of experimental traction forces) produced cell-scale indentations that matched observations in invasive cancer cells. We note that normal stresses (force and area) determine the indentation depth, while contact area size and morphology have a minor effect, explaining the origin of experimentally observed cell morphologies. We have thus revealed controlling features facilitating invasive indentations by single cancer cells, which will allow application of our model to complex problems, such as multicellular systems.

Effect of 3D printed polycaprolactone scaffold with a bionic structure on the early stage of fat grafting

Mature adipocytes are sensitive to stress and hypoxia, which are the two major obstacles in large-volume fat grafting. Bionic scaffolds are considered beneficial for fat grafting; however, their mechanism is still unclear. In this study, polycaprolactone scaffolds were fabricated by a 3D-printing technique and compounded with liposuction fat. They were implanted subcutaneously into nude mice. At different times, gross and histological observations were performed to evaluate the retention rates and histological morphologies. Adipocyte viability, apoptosis, and vascularization were analyzed by special immunostaining. Quantitative polymerase chain reaction was used to detect the variations in hypoxia and inflammation. The results showed that the volume and weight retentions in the scaffold group were higher than those in the fat group with the former exhibiting fewer vacuoles and less fibrosis. In immunostaining, elevated CD31+ capillaries, more perilipin+ adipocytes, and fewer TUNEL+ apoptotic cells were observed in the scaffold group by week 4. The lower expression of HIF-1α indicated the alleviation of hypoxia. In conclusion, the scaffold provided mechanical support to resist skin tension, thereby decreasing the interstitial pressure, and improving substance exchange and vascular ingrowth. In this regard, the scaffold attenuated hypoxia and promoted vascularization, making it a feasible method to increase long-term retention in fat grafting using scaffolds with suitable degradation rates and additional vascular maturation stimulation.

Finite element analysis reveals an important role for cell morphology in response to mechanical compression

Mechanical loading naturally controls cell phenotype, development, motility and various other biological functions; however, prolonged or substantial loading can cause cell damage and eventual death. Loading-induced mechanobiological and mechanostructural responses of different cell types affect their morphology and the internal architecture and the mechanics of the cellular components. Using single, mesenchymal stem cells, we have developed a cell-specific three-dimensional finite-element model; cell models were developed from phase-contrast microscopy images. This allowed us to evaluate the mechanostructural response of the naturally occurring variety of cell morphologies to increase sustained compressive loading. We focus on the morphology of the cytoplasm and the nucleus, as the main mechanically responsive elements, and evaluate formation of tensional strains and area changes in cells undergoing increasing uniaxial compressions. Here, we study mesenchymal stem cells as a model, due to their important role in tissue engineering and regenerative medicine; the method and findings are, however, applicable to any cell type. We observe variability in the cell responses to compression, which correlate directly with the morphology of the cells. Specifically, in cells with or without elongated protrusions (i.e., lamellipodia) tensional strains were, respectively, distributed mostly in the thin extensions or concentrated around the stiff nucleus. Thus, through cell-specific computational modeling of mechanical loading we have identified an underlying cause for stiffening (by actin recruitment) along the length of lamellipodia as well as a role for cell morphology in inducing cell-to-cell variability in mechanostructural response to loading.

Adipocytes Migration is Altered Through Differentiation

Adipogenesis is a developmental process in which an elongated preadipocyte differentiates to a round adipocyte along with the accumulation of lipid droplets. In the present study, we focus on the study of cell motility at the single-cell level, toward expanding our knowledge regarding the cytoskeleton alteration during differentiation; since-cell motility is mediated by cytoskeletal components. We used the holographic-microscopy live imaging technique to evaluate, for the first time in the literature, differences between the motility of nondifferentiated preadipocytes and differentiated mature adipocytes in living cell cultures over time. We revealed that mean motility speed of preadipocytes was significantly higher (fourfold) than that of adipocytes, and that the movement of preadipocytes is less consistent and more extensive. Furthermore, we found that preadipocytes tend to migrate to farther distances, while mature adipocytes remain relatively close to their original location. The results presented here are in agreement with the fact that the cytoskeleton of adipocytes is altered during differentiation and similarly, points to the fact that the cell-sensing mechanisms are changing during differentiation. Our research paves the way to gain better insights of the differentiation process and its implications on larger scale systems in the context of obesity.

ECM in Differentiation: A Review of Matrix Structure, Composition and Mechanical Properties

Stem cell regenerative potential owing to the capacity to self-renew as well as differentiate into other cell types is a promising avenue in regenerative medicine. Stem cell niche not only provides physical scaffolding but also possess instructional capacity as it provides a milieu of biophysical and biochemical cues. Extracellular matrix (ECM) has been identified as a major dictator of stem cell lineage, thus understanding the structure of in vivo ECM pertaining to specific tissue differentiation will aid in devising in vitro strategies to improve the differentiation efficiency. In this review, we summarize details about the native architecture, composition and mechanical properties of in vivo ECM of the early embryonic stages and the later adult stages. Native ECM from adult tissues categorized on their origin from respective germ layers are discussed while engineering techniques employed to facilitate differentiation of stem cells into particular lineages are noted. Overall, we emphasize that in vitro strategies need to integrate tissue specific ECM biophysical cues for developing accurate artificial environments for optimizing stem cell differentiation.

Sodium pyruvate pre-treatment prevents cell death due to localised, damaging mechanical strains in the context of pressure ulcers

We demonstrate sodium pyruvate (NaPy) pre-treatment as a successful approach for pressure ulcer (PU) prevention by averting their aetiological origin-cell-level damage and death by large, sustained mechanical loads. We evaluated the NaPy pre-treatment effect on permeability changes in the cell's plasma membrane (PM) following application of in vitro damaging-level strains. Fibroblasts or myoblasts, respectively, models for superficial or deep-tissue damage were grown in 0 or 1 mM NaPy, emulating typical physiological or cell culture conditions. Cells were pre-treated for 4 hours with 0 to 5 mM NaPy prior to 3-hour sustained, damaging-level loads (12% strain). PM permeability was quantified by the cell uptake of small (4 kDa), fluorescent dextran compared with unstrained control using fluorescence-activated cell sorting (FACS). Pre-treatment with 1 mM, and especially 5 mM, NaPy significantly reduces damage to PM integrity. Long-term NaPy pre-exposure can improve protective treatment, affecting fibroblasts and myoblasts differently. Pre-treating with NaPy, a natural cell metabolite, allows cells under damaging-level mechanical loads to maintain their PM integrity, that is, to avoid loss of homeostasis and inevitable, eventual cell death, by preventing initial, microscale stages of PU formation. This pre-treatment may be applied prior to planned periods of immobility, for example, planned surgery or transport, to prolong safe time in a position by preventing initial cell damage that can cascade and lead to PU formation.

Elastic interactions between anisotropically contracting circular cells

We study interactions between biological cells that apply anisotropic active mechanical forces on an elastic substrate. We model the cells as thin disks that along their perimeters apply radial, but angle-dependent forces on the substrate. We obtain analytical expressions for the elastic energy stored in the substrate as a function of the distance between the cells, the Fourier modes of applied forces, and their phase angles. We show how the relative phases of the forces applied by the cells can switch the interaction between attractive and repulsive, and relate our results to those for linear force dipoles. For long enough distances, the interaction energy decays in magnitude as a power law of the cell-cell distance with an integer exponent that generally increases with the Fourier modes of the applied forces.

Maturation of Adipocytes is Suppressed by Fluid Shear Stress

Preadipocytes are mechano-responsive cells and their differentiation to adipocytes may be regulated by various types of physical stimulation. Understanding the mechanism of differentiation, which increases the number of adipocytes and lipid accumulation is important in the study of obesity-related diseases. In this study, we investigated the effects of physical stimulation at different stages of adipogenic differentiation using physiological levels of fluid shear stress. Preadipocytes were treated with dexamethasone, 3-isobutyl-1-methylxanthine and insulin for 3 days (induction period) and incubated for additional 6 days for maturation. Fluid shear stress of 1 Pa at 1 Hz was applied for 1 h at different stages of differentiation. Fluid shear stress applied at the maturation period significantly reduced the expressions of C/enhancer binding protein (EBP)α and peroxisome proliferator-activated receptor (PPAR)γ2 leading to reduced lipid accumulation. Fluid shear stress applied at the early or late stages of the induction period only decreased peroxisome proliferator-activated receptor γ2 expression without any significant changes in lipid accumulation. Stimulation at multiple days during the induction period did not result in changes in lipid accumulation compared to stimulation at a single day. These results suggest that lipid droplet accumulation is effectively decreased by fluid shear stress applied during the cell maturation period. Understanding the cellular response to physical stimulation throughout the entire adipocyte differentiation period may be important in controlling adipogenesis by physical stimulation.

Microenvironmental Control of Adipocyte Fate and Function

The properties of tissue-specific microenvironments vary widely in the human body and demonstrably influence the structure and function of many cell types. Adipocytes are no exception, responding to cues in specialized niches to perform vital metabolic and endocrine functions. The adipose microenvironment is remodeled during tissue expansion to maintain the structural and functional integrity of the tissue and disrupted remodeling in obesity contributes to the progression of metabolic syndrome, breast cancer, and other malignancies. The increasing incidence of these obesity-related diseases and the recent focus on improved in vitro models of human tissue biology underscore growing interest in the regulatory role of adipocyte microenvironments in health and disease.