Paradigms of endothelial stiffening in cardiovascular disease and vascular aging

Endothelial cells, the inner lining of the blood vessels, are well-known to play a critical role in vascular function, while endothelial dysfunction due to different cardiovascular risk factors or accumulation of disruptive mechanisms that arise with aging lead to cardiovascular disease. In this review, we focus on endothelial stiffness, a fundamental biomechanical property that reflects cell resistance to deformation. In the first part of the review, we describe the mechanisms that determine endothelial stiffness, including RhoA-dependent contractile response, actin architecture and crosslinking, as well as the contributions of the intermediate filaments, vimentin and lamin. Then, we review the factors that induce endothelial stiffening, with the emphasis on mechanical signals, such as fluid shear stress, stretch and stiffness of the extracellular matrix, which are well-known to control endothelial biomechanics. We also describe in detail the contribution of lipid factors, particularly oxidized lipids, that were also shown to be crucial in regulation of endothelial stiffness. Furthermore, we discuss the relative contributions of these two mechanisms of endothelial stiffening in vasculature in cardiovascular disease and aging. Finally, we present the current state of knowledge about the role of endothelial stiffening in the disruption of endothelial cell-cell junctions that are responsible for the maintenance of the endothelial barrier.

Liquid-Liquid Phase Separation of DDR1 Counteracts the Hippo Pathway to Orchestrate Arterial Stiffening

BACKGROUND

The Hippo-YAP (yes-associated protein) signaling pathway is modulated in response to various environmental cues. Activation of YAP in vascular smooth muscle cells conveys the extracellular matrix stiffness-induced changes in vascular smooth muscle cells phenotype and behavior. Recent studies have established a mechanoreceptive role of receptor tyrosine kinase DDR1 (discoidin domain receptor 1) in vascular smooth muscle cells.

METHODS

We conduced 5/6 nephrectomy in vascular smooth muscle cells-specific Ddr1-knockout mice, accompanied by pharmacological inhibition of the Hippo pathway kinase LATS1 (large tumor suppressor 1), to investigate DDR1 in YAP activation. We utilized polyacrylamide gels of varying stiffness or the DDR1 ligand, type I collagen, to stimulate the cells. We employed multiple molecular biological techniques to explore the role of DDR1 in controlling the Hippo pathway and to determine the mechanistic basis by which DDR1 exerts this effect.

RESULTS

We identified the requirement for DDR1 in stiffness/collagen-induced YAP activation. We uncovered that DDR1 underwent stiffness/collagen binding-stimulated liquid-liquid phase separation and co-condensed with LATS1 to inactivate LATS1. Mutagenesis experiments revealed that the transmembrane domain is responsible for DDR1 droplet formation. Purified DDR1 N-terminal and transmembrane domain was sufficient to drive its reversible condensation. Depletion of the DDR1 C-terminus led to failure in co-condensation with LATS1. Interaction between the DDR1 C-terminus and LATS1 competitively inhibited binding of MOB1 (Mps one binder 1) to LATS1 and thus the subsequent phosphorylation of LATS1. Introduction of the single-point mutants, histidine-745-proline and histidine-902-proline, to DDR1 on the C-terminus abolished the co-condensation. In mouse models, YAP activity was positively correlated with collagen I expression and arterial stiffness. LATS1 inhibition reactivated the YAP signaling in Ddr1-deficient vessels and abrogated the arterial softening effect of Ddr1 deficiency.

CONCLUSIONS

These findings identify DDR1 as a mediator of YAP activation by mechanical and chemical stimuli and demonstrate that DDR1 regulates LATS1 phosphorylation in an liquid-liquid phase separation-dependent manner.

Invasion and Secondary Site Colonization as a Function of In Vitro Primary Tumor Matrix Stiffness: Breast to Bone Metastasis

Increased breast tissue stiffness is correlated with breast cancer risk and invasive cancer progression. However, its role in promoting bone metastasis, a major cause of mortality, is not yet understood. It is previously identified that the composition and stiffness of alginate-based hydrogels mimicking normal (1-2 kPa) and cancerous (6-10 kPa) breast tissue govern phenotype of breast cancer cells (including MDA-MB-231) in vitro. Here, to understand the causal effect of primary tumor stiffness on metastatic potential, a new breast-to-bone in vitro model is described. Together with alginate-gelatin hydrogels to mimic breast tissue, 3D printed biohybrid poly-caprolactone (PCL)-composite scaffolds, decellularized following bone-ECM deposition through Saos-2 engraftment, are used to mimic the bone tissue. It is reported that higher hydrogel stiffness results in the increased migration and invasion capacity of MDA-MB 231 cells. Interestingly, increased expression of osteolytic factors PTHrP and IL-6 is observed when MDA-MB-231 cells pre-conditioned in stiffer hydrogels (10 kPa, 3% w/v gelatin) colonize the bone/PCL scaffolds. The new breast-to-bone in vitro models herein described are designed with relevant tissue microenvironmental factors and could emerge as future non-animal technological platforms for monitoring metastatic processes and therapeutic efficacy.

In Situ Regulation and Mechanisms of 3D Matrix Stiffness on the Activation and Reversion of Hepatic Stellate Cells

Activated hepatic stellate cells (HSCs) is a key event in the progression of liver fibrosis. HSCs transdifferentiate into myofibroblasts and secrete large amounts of extracellular matrix, resulting in increased liver stiffness. It is difficult for platforms constructed in vitro to simulate the structure, composition, and stiffness of the 3D microenvironment of HSCs in vivo. Here, 3D scaffolds with different stiffness are constructed by decellularizing rat livers at different stages of fibrosis. The effects of matrix stiffness on the proliferation, activation, and reversion of HSCs are studied. The results demonstrate these scaffolds have good cytocompatibility. It is also found that the high stiffness can significantly promote the activation of HSCs, and this process is accompanied by the activation of integrin β1 as well as the nucleation and activation of Yes-associated protein (YAP). Moreover, the low stiffness of the scaffold can promote the reversion of activated HSCs, which is associated with cell apoptosis and accompanied by the inactivation of integrin β1 and YAP. These results suggest that YAP may be a potential therapeutic target for the treatment of liver fibrosis and the theoretical feasibility of inducing activated HSCs reversion to the resting state by regulating matrix stiffness of liver.

Activation of Piezo1 contributes to matrix stiffness-induced angiogenesis in hepatocellular carcinoma

BACKGROUND

Despite integrin being highlighted as a stiffness-sensor molecule in matrix stiffness-driven angiogenesis, other stiffness-sensor molecules and their mechanosensory pathways related to angiogenesis in hepatocellular carcinoma (HCC) remain obscure. Here, we explored the interplay between Piezo1 and integrin β1 in the mechanosensory pathway and their effects on HCC angiogenesis to better understand matrix stiffness-induced angiogenesis.

METHODS

The role of Piezo1 in matrix stiffness-induced angiogenesis was investigated using orthotopic liver cancer SD rat models with high liver stiffness background, and its clinical significance was evaluated in human HCC tissues. Matrix stiffness-mediated Piezo1 upregulation and activation were assayed using an in vitro fibronectin (FN)-coated cell culture system with different stiffness, Western blotting and Ca²⁺ probe. The effects of shPiezo1-conditioned medium (CM) on angiogenesis were examined by tube formation assay, wound healing assay and angiogenesis array. The underlying mechanism by which Piezo1 participated in matrix stiffness-induced angiogenesis was analyzed by microRNA quantitative real-time polymerase chain reaction (qRT-PCR), matrix stiffness measurement, dual-luciferase reporter assay, ubiquitination assay and co-immunoprecipitation.

RESULTS

Increased matrix stiffness significantly upregulated Piezo1 expression at both cellular and tissue levels, and high expression of Piezo1 indicated an unfavorable prognosis. High matrix stiffness also noticeably enhanced the activation level of Piezo1, similar to its expression level. Piezo1 knockdown significantly suppressed tumor growth, angiogenesis, and lung metastasis of HCC rat models with high liver stiffness background. shPiezo1-CM from HCC cells attenuated tube formation and migration abilities of vascular endothelial cells remarkably, and analysis of differentially expressed pro-angiogenic factors revealed that Piezo1 promoted the expression and secretion of vascular endothelial growth factor (VEGF), CXC chemokine ligand 16 (CXCL16) and insulin-like growth factor binding protein 2 (IGFBP2). Matrix stiffness-caused Piezo1 upregulation/activation restrained hypoxia inducible factor-1α (HIF-1α) ubiquitination, subsequently enhanced the expression of downstream pro-angiogenic factors to accelerate HCC angiogenesis. Besides, collagen 1 (COL1)-reinforced tissue stiffening resulted in more expression of Piezo1 via miR-625-5p.

CONCLUSIONS

This study unravels a new mechanism by which the integrin β1/Piezo1 activation/Ca²⁺ influx/HIF-1α ubiquitination/VEGF, CXCL16 and IGFBP2 pathway participates in matrix stiffness-driven HCC angiogenesis. Simultaneously, a positive feedback regulation loop as stiff matrix/integrin β1/miR-625-5p/Piezo1 and COL1/stiffer matrix mediates matrix stiffness-caused Piezo1 upregulation.

Cancer-Associated Fibroblasts in a 3D Engineered Tissue Model Induce Tumor-like Matrix Stiffening and EMT Transition

Simple Summary

The physical properties of a tumor, such as stiffness, are important drivers of tumor progression. However, current in vitro tumor models fail to recapitulate the full range of physical properties observed in solid tumors. Here, we proposed a 3D self-assembly engineered bladder model using cancer-associated fibroblasts in which stromal cells produce their extracellular matrix. We then proceeded to assess how our model recapitulates biological and mechanical features found in tumors. We confirmed that stroma assembled by cancer-associated fibroblasts have increased extracellular matrix content and display increased remodeling and higher stiffness. Moreover, normal urothelial cells seeded on the tumor model displayed a mechanotransduction response, increased cell proliferation, cell infiltration within stroma, and displayed features of the epithelial-to-mesenchymal transition. Altogether, we demonstrated that our cancer-associated fibroblast-derived tumor stroma recapitulates several biological and physical features expected from a tumor-like environment and, thus, provides the basis for more accurate cancer models.

Abstract

A tumor microenvironment is characterized by its altered mechanical properties. However, most models remain unable to faithfully recreate the mechanical properties of a tumor. Engineered models based on the self-assembly method have the potential to better recapitulate the stroma architecture and composition. Here, we used the self-assembly method based on a bladder tissue model to engineer a tumor-like environment. The tissue-engineered tumor models were reconstituted from stroma-derived healthy primary fibroblasts (HFs) induced into cancer-associated fibroblast cells (iCAFs) along with an urothelium overlay. The iCAFs-derived extracellular matrix (ECM) composition was found to be stiffer, with increased ECM deposition and remodeling. The urothelial cells overlaid on the iCAFs-derived ECM were more contractile, as measured by quantitative polarization microscopy, and displayed increased YAP nuclear translocation. We further showed that the proliferation and expression of epithelial-to-mesenchymal transition (EMT) marker in the urothelial cells correlate with the increased stiffness of the iCAFs-derived ECM. Our data showed an increased expression of EMT markers within the urothelium on the iCAFs-derived ECM. Together, our results demonstrate that our tissue-engineered tumor model can achieve stiffness levels comparable to that of a bladder tumor, while triggering a tumor-like response from the urothelium.

Nonswelling and Hydrolytically Stable Hydrogels Uncover Cellular Mechanosensing in 3D

While matrix stiffness regulates cell behavior on 2D substrates, recent studies using synthetic hydrogels have suggested that in 3D environments, cell behavior is primarily impacted by matrix degradability, independent of stiffness. However, these studies did not consider the potential impact of other confounding matrix parameters that typically covary with changes in stiffness, particularly, hydrogel swelling and hydrolytic stability, which may explain the previously observed distinctions in cell response in 2D versus 3D settings. To investigate how cells sense matrix stiffness in 3D environments, a nonswelling, hydrolytically stable, linearly elastic synthetic hydrogel model is developed in which matrix stiffness and degradability can be tuned independently. It is found that matrix degradability regulates cell spreading kinetics, while matrix stiffness dictates the final spread area once cells achieve equilibrium spreading. Importantly, the differentiation of human mesenchymal stromal cells toward adipocytes or osteoblasts is regulated by the spread state of progenitor cells upon initiating differentiation. These studies uncover matrix stiffness as a major regulator of cell function not just in 2D, but also in 3D environments, and identify matrix degradability as a critical microenvironmental feature in 3D that in conjunction with matrix stiffness dictates cell spreading, cytoskeletal state, and stem cell differentiation outcomes.

Mechanomimetic 3D Scaffolds as a Humanized In Vitro Model for Ovarian Cancer

The mechanical homeostasis of tissues can be altered in response to trauma or disease, such as cancer, resulting in altered mechanotransduction pathways that have been shown to impact tumor development, progression, and the efficacy of therapeutic approaches. Specifically, ovarian cancer progression is parallel to an increase in tissue stiffness and fibrosis. With in vivo models proving difficult to study, tying tissue mechanics to altered cellular and molecular properties necessitate advanced, tunable, in vitro 3D models able to mimic normal and tumor mechanic features. First, we characterized normal human ovary and high-grade serous (HGSC) ovarian cancer tissue stiffness to precisely mimic their mechanical features on collagen I-based sponge scaffolds, soft (NS) and stiff (MS), respectively. We utilized three ovarian cancer cell lines (OVCAR-3, Caov-3, and SKOV3) to evaluate changes in viability, morphology, proliferation, and sensitivity to doxorubicin and liposomal doxorubicin treatment in response to a mechanically different microenvironment. High substrate stiffness promoted the proliferation of Caov-3 and SKOV3 cells without changing their morphology, and upregulated mechanosensors YAP/TAZ only in SKOV3 cells. After 7 days in culture, both OVCAR3 and SKOV3 decreased the MS scaffold storage modulus (stiffness), suggesting a link between cell proliferation and the softening of the matrix. Finally, high matrix stiffness resulted in higher OVCAR-3 and SKOV3 cell cytotoxicity in response to doxorubicin. This study demonstrates the promise of biomimetic porous scaffolds for effective inclusion of mechanical parameters in 3D cancer modeling. Furthermore, this work establishes the use of porous scaffolds for studying ovarian cancer cells response to mechanical changes in the microenvironment and as a meaningful platform from which to investigate chemoresistance and drug response.

Matrix Stiffness Potentiates Stemness of Liver Cancer Stem Cells Possibly via the Yes-Associated Protein Signal

A hepatocellular carcinoma tissue has mechanical heterogeneity, where the stiffness gradually increases from the core to the invasion front. Furthermore, there is evidence that stem cells from liver cancer (LCSCs) preferentially enrich the invasion front, exhibiting the stiffest modulus in the tumor. LCSCs have the features of stem/progenitor cells and play a vital part in liver cancer development. However, whether matrix stiffness affects LCSC stemness remains unclear. Here, we established a three-dimensional hydrogel for culturing LCSCs to simulate the stiffness of the core and the invasion front of a liver cancer tissue. The results showed that a stiffer matrix (72.2 ± 0.90 kPa) significantly potentiated LCSC stemness as compared with a soft matrix (7.7 ± 0.41 kPa). Moreover, Yes-associated protein signaling might mediate this promotion. Together, our findings illustrate the relationship between matrix stiffness and LCSC stemness, which may aid the production of novel treatment approaches against liver cancer.

Hydrogel Mechanics Influence the Growth and Development of Embedded Brain Organoids

Brain organoids are three-dimensional, tissue-engineered neural models derived from induced pluripotent stem cells that enable studies of neurodevelopmental and disease processes. Mechanical properties of the microenvironment are known to be critical parameters in tissue engineering, but the mechanical consequences of the encapsulating matrix on brain organoid growth and development remain undefined. Here, Matrigel was modified with an interpenetrating network (IPN) of alginate, to tune the mechanical properties of the encapsulating matrix. Brain organoids grown in IPNs were viable, with the characteristic formation of neuroepithelial buds. However, organoid growth was significantly restricted in the stiffest matrix tested. Moreover, stiffer matrixes skewed cell populations toward mature neuronal phenotypes, with fewer and smaller neural rosettes. These findings demonstrate that the mechanics of the culture environment are important parameters in brain organoid development and show that the self-organizing capacity and subsequent architecture of brain organoids can be modulated by forces arising from growth-induced compression of the surrounding matrix. This study therefore suggests that carefully designing the mechanical properties of organoid encapsulation materials is a potential strategy to direct organoid growth and maturation toward desired structures.

Targeting Mechanosensitive Piezo1 Alleviated Renal Fibrosis Through p38MAPK-YAP Pathway

Renal fibrosis is the most common pathological manifestation of a wide variety of chronic kidney disease. Increased extracellular matrix (ECM) secretion and enhanced microenvironment stiffening aggravate the progression of renal fibrosis. However, the related mechanisms remain unclear. Here, we evaluated the mechanism by which ECM stiffness aggravates renal fibrosis. In the present study, renal mesangial cells (MCs) were cultured on polyacrylamide hydrogels with different stiffness accurately detected by atomic force microscope (AFM), simulating the in vivo growth microenvironment of MCs in normal kidney and renal fibrosis. A series of in vitro knockdown and activation experiments were performed to establish the signaling pathway responsible for mechanics-induced MCs activation. In addition, an animal model of renal fibrosis was established in mice induced by unilateral ureteral obstruction (UUO). Lentiviral particles containing short hairpin RNA (sh RNA) targeting Piezo1 were used to explore the effect of Piezo1 knockdown on matrix stiffness-induced MCs activation and UUO-induced renal fibrosis. An in vitro experiment demonstrated that elevated ECM stiffness triggered the activation of Piezo1, which increased YAP nuclear translocation through the p38MAPK, and consequently led to increased ECM secretion. Furthermore, these consequences have been verified in the animal model of renal fibrosis induced by UUO and Piezo1 knockdown could alleviate UUO-induced fibrosis and improve renal function in vivo. Collectively, our results for the first time demonstrate enhanced matrix stiffness aggravates the progression of renal fibrosis through the Piezo1-p38MAPK-YAP pathway. Targeting mechanosensitive Piezo1 might be a potential therapeutic strategy for delaying the progression of renal fibrosis.

Actin nucleator formins regulate the tension-buffering function of caveolin-1

Both the mechanosensitive actin cytoskeleton and caveolae contribute to active processes such as cell migration, morphogenesis, and vesicular trafficking. Although distinct actin components are well studied, how they contribute to cytoplasmic caveolae, especially in the context of mechano-stress, has remained elusive. Here, we identify two actin-associated mobility stereotypes of caveolin-1 (CAV-1)-marked intracellular vesicles, which are characterized as ‘dwelling’ and ‘go and dwelling’. In order to exploit the reason for their distinct dynamics, elongated actin-associated formin functions are perturbed. We find drastically decreased density, increased clustering, and compromised motility of cytoplasmic CAV-1 vesicles resulting from lacking actin nucleator formins by both chemical treatment and RNA silencing of formin genes. Furthermore, hypo-osmosis-stimulated diminishing of CAV-1 is dramatically intensified upon blocking formins. The clustering of CAV-1 vesicles when cells are cultured on soft substrate is also aggravated under formin inhibition condition. Together, we reveal that actin-associated formins are essential for maintaining the dynamic organization of cytoplasmic CAV-1 and importantly its sensitivity upon mechanical challenge. We conclude that tension-controlled actin formins act as a safety valve dampening excessive tension on CAV-1 and safeguarding CAV-1 against mechanical damage.

Matrix Stiffness Affects Glycocalyx Expression in Cultured Endothelial Cells

Rationale: The endothelial cell glycocalyx (GCX) is a mechanosensor that plays a key role in protecting against vascular diseases. We have previously shown that age/disease mediated matrix stiffness inhibits the glycocalyx glycosaminoglycan heparan sulfate and its core protein Glypican 1 in human umbilical vein endothelial cells, rat fat pad endothelial cells and in a mouse model of age-mediated stiffness. Glypican 1 inhibition resulted in enhanced endothelial cell dysfunction. Endothelial cell culture typically occurs on stiff matrices such as plastic or glass. For the study of the endothelial GCX specifically it is important to culture cells on soft matrices to preserve GCX expression. To test the generality of this statement, we hypothesized that stiff matrices inhibit GCX expression and consequently endothelial cell function in additional cell types: bovine aortic endothelial cells, mouse aortic endothelial cell and mouse brain endothelial cells. Methods and Results: All cell types cultured on glass showed reduced GCX heparan sulfate expression compared to cells cultured on either soft polyacrylamide (PA) gels of a substrate stiffness of 2.5 kPa (mimicking the stiffness of young, healthy arteries) or on either stiff gels 10 kPa (mimicking the stiffness of old, diseased arteries). Specific cell types showed reduced expression of GCX protein Glypican 1 (4 of 5 cell types) and hyaluronic acid (2 of 5 cell types) on glass vs soft gels. Conclusion: Matrix stiffness affects GCX expression in endothelial cells. Therefore, the study of the endothelial glycocalyx on stiff matrices (glass/plastic) is not recommended for specific cell types.

Effects of biomechanical forces on the biological behavior of cancer stem cells

Cancer stem cells (CSCs), dynamic subsets of cancer cells, are responsible for malignant progression. The unique properties of CSCs, including self-renewal, differentiation, and malignancy, closely depend on the tumor microenvironment. Mechanical components in the microenvironment, including matrix stiffness, fluid shear stress, compression and tension stress, affect the fate of CSCs and further influence the cancer process. This paper reviews recent studies of mechanical components and CSCs, and further discusses the intrinsic correlation among them. Regulatory mechanisms of mechanical microenvironment, which act on CSCs, have great potential for clinical application and provide different perspectives to drugs and treatment design.

Matrix stiffness primes cells for future oxidative stress

Focus on metabolic reprogramming has re-emerged in recent years due to the far-reaching consequences of metabolism on nearly all cellular behaviors. In a recent study in Cell Metabolism, Tharp et al. show that adhesion-dependent mechanical signaling induces mitochondrial and metabolic reprogramming to help cells adapt to future oxidative stress.

Mesenchymal Stem Cells Resist Mechanical Confinement through the Activation of the Cortex during Cell Division

The mechanical properties of the natural extracellular matrix (ECM) change extensively, but these specific properties provide a relatively stable environment for resident cells. Although the effect of matrix stiffness on cell functions has been widely studied, the molecular mechanism was still not fully understood. Matrix stiffening is a common phenomenon in tissue damaging processes. To explore the effect of the increase in local matrix stiffness on cell behaviors, a three-dimensional (3D) cell culture system with a tunable modulus but constant other physical parameters was constructed by the alginate hydrogel with different molecular weights and cross-linking degrees. By using this culture system, the transcriptome response of mesenchymal stem cells (MSCs) to matrix stiffness was explored. Furthermore, a finite element model was developed to simulate the interaction between cells and the matrix. Results revealed that the increased matrix stiffness promoted the proliferation-related signaling of MSCs, and this process depended on the increased cortex tension caused by the activation of RAS and myosin II.

Matrix stiffness primes lymphatic tube formation directed by vascular endothelial growth factor-C

Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes. The restoration of dysfunctional lymphatic vessels has been hypothesized as an innovative method to rescue healthy phenotypes in diseased states including neurological conditions, metabolic syndromes, and cardiovascular disease. Compared to the vascular system, little is known about the molecular regulation that controls lymphatic tube morphogenesis. Using synthetic hyaluronic acid (HA) hydrogels as a chemically and mechanically tunable system to preserve lymphatic endothelial cell (LECs) phenotypes, we demonstrate that low matrix elasticity primes lymphatic cord-like structure (CLS) formation directed by a high concentration of vascular endothelial growth factor-C (VEGF-C). Decreasing the substrate stiffness results in the upregulation of key lymphatic markers, including PROX-1, lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1), and VEGFR-3. Consequently, higher levels of VEGFR-3 enable stimulation of LECs with VEGF-C which is required to both activate matrix metalloproteinases (MMPs) and facilitate LEC migration. Both of these steps are critical in establishing CLS formation in vitro. With decreases in substrate elasticity, we observe increased MMP expression and increased cellular elongation, as well as formation of intracellular vacuoles, which can further merge into coalescent vacuoles. RNAi studies demonstrate that MMP-14 is required to enable CLS formation and that LECs sense matrix stiffness through YAP/TAZ mechanosensors leading to the activation of their downstream target genes. Collectively, we show that by tuning both the matrix stiffness and VEGF-C concentration, the signaling pathways of CLS formation can be regulated in a synthetic matrix, resulting in lymphatic networks which will be useful for the study of lymphatic biology and future approaches in tissue regeneration.

Breast tumor stiffness instructs bone metastasis via maintenance of mechanical conditioning

While the immediate and transitory response of breast cancer cells to pathological stiffness in their native microenvironment has been well explored, it remains unclear how stiffness-induced phenotypes are maintained over time after cancer cell dissemination in vivo. Here, we show that fibrotic-like matrix stiffness promotes distinct metastatic phenotypes in cancer cells, which are preserved after transition to softer microenvironments, such as bone marrow. Using differential gene expression analysis of stiffness-responsive breast cancer cells, we establish a multigenic score of mechanical conditioning (MeCo) and find that it is associated with bone metastasis in patients with breast cancer. The maintenance of mechanical conditioning is regulated by RUNX2, an osteogenic transcription factor, established driver of bone metastasis, and mitotic bookmarker that preserves chromatin accessibility at target gene loci. Using genetic and functional approaches, we demonstrate that mechanical conditioning maintenance can be simulated, repressed, or extended, with corresponding changes in bone metastatic potential.

Watson et al. demonstrate that mechanical conditioning by stiff microenvironments in breast tumors is maintained in cancer cells after dissemination to softer microenvironments, including bone marrow. They show that mechanical conditioning promotes invasion and osteolysis and establish a mechanical conditioning (MeCo) score, associated with bone metastasis in patients.

Matrix Stiffness Induces Pericyte-Fibroblast Transition Through YAP Activation

Vascular pericytes, important mural cells that retain progenitor cell properties and protect vascular integrity in healthy tissues, are often associated with tumor development, but their functions in cancer invasion remain elusive. One prominent outcome of tumor occurrence is that the microenvironment of the lesion often stiffens, which could change resident cell behavior. Here, we found pericytes are matrix stiffness-responsive and mechanical stimuli induce pericyte-fibroblast transition (PFT). Soft PA gels that mimic the stiffness of healthy tissues retain the identity and behavior of pericytes, whereas stiff PA gels that reflect the stiffness of tumorous tissues promote PFT and the mobility and invasiveness of the cells. Matrix stiffness-induced PFT depends on the activation of YAP (Yes-associated protein), a transcription factor, which, upon receiving mechanical signals, transfers from cytoplasm to nucleus to mediate cell transcriptional activities. Our result reveals a mechanism through which vascular pericytes convert to fibroblasts and migrate away from vasculatures to help tumor development, and thus targeting matrix stiffness-induced PFT may offer a new perspective to the treatment of cancer metastasis.

Protein Phosphatase 2A Mediates YAP Activation in Endothelial Cells Upon VEGF Stimulation and Matrix Stiffness

Angiogenesis is an essential process during development. Abnormal angiogenesis also contributes to many disease conditions such as tumor and retinal diseases. Previous studies have established the Hippo signaling pathway effector Yes-associated protein (YAP) as a crucial regulator of angiogenesis. In ECs, activated YAP promotes endothelial cell proliferation, migration and sprouting. YAP activity is regulated by vascular endothelial growth factor (VEGF) and mechanical cues such as extracellular matrix (ECM) stiffness. However, it is unclear how VEGF or ECM stiffness signal to YAP, especially how dephosphorylation of YAP occurs in response to VEGF stimulus or ECM stiffening. Here, we show that protein phosphatase 2A (PP2A) is required for this process. Blocking PP2A activity abolishes VEGF or ECM stiffening mediated YAP activation. Systemic administration of a PP2A inhibitor suppresses YAP activity in blood vessels in developmental and pathological angiogenesis mouse models. Consistently, PP2A inhibitor also inhibits sprouting angiogenesis. Mechanistically, PP2A directly interacts with YAP, and this interaction requires proper cytoskeleton dynamics. These findings identify PP2A as a crucial mediator of YAP activation in ECs and hence as an important regulator of angiogenesis.

Stimulation of vascular smooth muscle cell proliferation by stiff matrix via the IKCa channel-dependent Ca2+ signaling

Vascular stiffening, an early and common characteristic of cardiovascular diseases (CVDs), stimulates vascular smooth muscle cell (VSMC) proliferation which reciprocally accelerates the progression of CVDs. However, the mechanisms by which extracellular matrix stiffness accompanying vascular stiffening regulates VSMC proliferation remain largely unknown. In the present study, we examined the role of the intermediate-conductance Ca²⁺ -activated K⁺  (IK_Ca ) channel in the matrix stiffness regulation of VSMC proliferation by growing A7r5 cells on soft and stiff polydimethylsiloxane substrates with stiffness close to these of arteries under physiological and pathological conditions, respectively. Stiff substrates stimulated cell proliferation and upregulated the expression of the IK_Ca channel. Stiff substrate-induced cell proliferation was suppressed by pharmacological inhibition using TRAM34, an IK_Ca channel blocker, or genetic depletion of the IK_Ca channel. In addition, stiff substrate-induced cell proliferation was also suppressed by reducing extracellular Ca²⁺ concentration using EGTA or intracellular Ca²⁺ concentration using BAPTA-AM. Moreover, stiff substrate induced activation of extracellular signal-regulated kinases (ERKs), which was inhibited by treatment with TRAM34 or BAPTA-AM. Stiff substrate-induced cell proliferation was suppressed by treatment with PD98059, an ERK inhibitor. Taken together, these results show that substrates with pathologically relevant stiffness upregulate the IK_Ca channel expression to enhance intracellular Ca²⁺ signaling and subsequent activation of the ERK signal pathway to drive cell proliferation. These findings provide a novel mechanism by which vascular stiffening regulates VSMC function.

Chemotherapy-Enriched THBS2-Deficient Cancer Stem Cells Drive Hepatocarcinogenesis through Matrix Softness Induced Histone H3 Modifications

The physical microenvironment is a critical mediator of tumor behavior. However, detailed biological and mechanistic insight is lacking. The present study reveals the role of chemotherapy-enriched CD133+ liver cancer stem cells (CSCs) with THBS2 deficiency. This subpopulation of cells contributes to a more aggressive cancer and functional stemness phenotype in hepatocellular carcinoma (HCC) by remodeling the extracellular matrix (ECM) through the regulation of matrix metalloproteinase (MMP) activity, collagen degradation, and matrix stiffness. The local soft spots created by these liver CSCs can enhance stemness and drug resistance and provide a route of escape to facilitate HCC metastasis. Interestingly, a positive feed-forward loop is identified where a local soft spot microenvironment in the HCC tumor is enriched with CD133 expressing cells that secrete markedly less ECM-modifying THBS2 upon histone H3 modification at its promoter region, allowing the maintenance of a localized soft spot matrix. Clinically, THBS2 deficiency is also correlated with low HCC survival, where high levels of CSCs with low THBS2 expression in HCC are associated with decreased collagen fiber deposits and an invasive tumor front. The findings have implications for the treatment of cancer stemness and for the prevention of tumor outgrowth through disseminated tumor cells.

Matrix stiffness and shear stresses modulate hepatocyte functions in a fibrotic liver sinusoidal model

Extracellular matrix (ECM) rigidity has important effects on cell behaviors and increases sharply in liver fibrosis and cirrhosis. Hepatic blood flow is essential in maintaining hepatocytes' (HCs) functions. However, it is still unclear how matrix stiffness and shear stresses orchestrate HC phenotype in concert. A fibrotic three-dimensional (3-D) liver sinusoidal model is constructed using a porous membrane sandwiched between two polydimethylsiloxane (PDMS) layers with respective flow channels. The HCs are cultured in collagen gels of various stiffnesses in the lower channel, whereas the upper channel is pre-seeded with liver sinusoidal endothelial cells (LSECs) and accessible to shear flow. The results reveal that HCs cultured within stiffer matrices exhibit reduced albumin production and cytochrome P450 (CYP450) reductase expression. Low shear stresses enhance synthetic and metabolic functions of HC, whereas high shear stresses lead to the loss of HC phenotype. Furthermore, both mechanical factors regulate HC functions by complementing each other. These observations are likely attributed to mechanically induced mass transport or key signaling molecule of hepatocyte nuclear factor 4α (HNF4α). The present study results provide an insight into understanding the mechanisms of HC dysfunction in liver fibrosis and cirrhosis, especially from the viewpoint of matrix stiffness and blood flow.NEW & NOTEWORTHY A fibrotic three-dimensional (3-D) liver sinusoidal model was constructed to mimic different stages of liver fibrosis in vivo and to explore the cooperative effects of matrix stiffness and shear stresses on hepatocyte (HC) functions. Mechanically induced alterations of mass transport mainly contributed to HC functions via typical mechanosensitive signaling.

Pathological matrix stiffness promotes cardiac fibroblast differentiation through the POU2F1 signaling pathway

Cardiac fibroblast (CF) differentiation into myofibroblasts is a crucial cause of cardiac fibrosis, which increases in the extracellular matrix (ECM) stiffness. The increased stiffness further promotes CF differentiation and fibrosis. However, the molecular mechanism is still unclear. We used bioinformatics analysis to find new candidates that regulate the genes involved in stiffness-induced CF differentiation, and found that there were binding sites for the POU-domain transcription factor, POU2F1 (also known as Oct-1), in the promoters of 50 differentially expressed genes (DEGs) in CFs on the stiffer substrate. Immunofluorescent staining and Western blotting revealed that pathological stiffness upregulated POU2F1 expression and increased CF differentiation on polyacrylamide hydrogel substrates and in mouse myocardial infarction tissue. A chromatin immunoprecipitation assay showed that POU2F1 bound to the promoters of fibrosis repressors IL1R2, CD69, and TGIF2. The expression of these fibrosis repressors was inhibited on pathological substrate stiffness. Knockdown of POU2F1 upregulated these repressors and attenuated CF differentiation on pathological substrate stiffness (35 kPa). Whereas, overexpression of POU2F1 downregulated these repressors and enhanced CF differentiation. In conclusion, pathological stiffness upregulates the transcription factor POU2F1 to promote CF differentiation by inhibiting fibrosis repressors. Our work elucidated the crosstalk between CF differentiation and the ECM and provided a potential target for cardiac fibrosis treatment.

Matrix stiffness promotes glioma cell stemness by activating BCL9L/Wnt/β-catenin signaling

Matrix stiffness is a key physical characteristic of the tumor microenvironment and correlates tightly with tumor progression. Here, we explored the association between matrix stiffness and glioma development. Using atomic force microscopy, we observed higher matrix stiffness in highly malignant glioma tissues than in low-grade/innocent tissues. In vitro and in vivo analyses revealed that culturing glioma cells on stiff polyacrylamide hydrogels enhanced their proliferation, tumorigenesis and CD133 expression. Greater matrix stiffness could obviously up-regulated the expression of BCL9L, thereby promoting the activation of Wnt/β-catenin signaling and ultimately increasing the stemness of glioma cells. Inhibiting Wnt/β-catenin signaling using gigantol consistently improved the anticancer effects of chemotherapy and radiotherapy in mice with subcutaneous glioma tumors. These findings demonstrate that a stiffer matrix increases the stemness of glioma cells by activating BCL9L/Wnt/β-catenin signaling. Moreover, we have provided a potential strategy for clinical glioma treatment by demonstrating that gigantol can improve the effectiveness of traditional chemotherapy/radiotherapy by suppressing Wnt/β-catenin signaling.