Introduction

Breast cancer is the most common malignancy for women in the developed world1 and is estimated to result in over 300,000 new cases and 40,000 deaths in the United States in 20242. Metastasis is a well-established hallmark of breast cancer3, accounting for over 90% of breast cancer related deaths4. Breast cancer metastasis occurs when cancer cells disseminate from a primary tumor and successfully complete the individual steps of the metastatic cascade: acquisition of an invasive phenotype, local invasion, intravasation, survival in circulation, extravasation, and secondary tumor formation5,6. Recent studies suggest that fewer than 0.1% of tumor cells successfully complete the metastatic cascade and form secondary tumors7,8,9, hinting at the existence of a subpopulation of tumor cells more fit to metastasize10. Effective treatments for metastatic cancer remain elusive in part due to the severe inter- and intra-tumor heterogeneity found in breast cancer patients1. This heterogeneity is a result of varying molecular subtypes, which reflect the cell type of origin and the acquired mutations, physiological characteristics1, and tumor microenvironment (TME) interactions4.

Cancer progression from the initial tumor formation to the emergence of distant metastases is a complex process driven by a series of genetic and cellular alterations that lead to sustained cell proliferation (sometimes interspersed with extended dormancy) and immune evasion3,7,11. Although much research has focused on the associated genetic and cellular alterations in cancer cells, these changes alone cannot fully explain cancer progression. It is now widely accepted that additional factors in the TME, such as interaction of cancer cells with stromal cells and their physical environment, must be considered12,13. Here, we primarily focus on physical factors, such as mechanical forces and confinement imposed by other cells and the extracellular matrix (ECM), which are increasingly being recognized as playing important roles during all stages of cancer progression14,15. In the primary tumor, solid stress and increased hydrostatic pressure act on the cells within the TME of the primary tumor (see Box 1 for helpful definitions)14. During invasion, intravasation, and extravasation, cells experience extensive mechanical stress as they migrate through constrictions often much smaller than the size of the unconfined cells14,16,17,18; in the vascular system, circulating tumor cells (CTCs) experience substantial fluid shear stress within the vasculature. To successfully complete metastasis, cancer cells must withstand and adapt to these physical challenges.

Cells sense mechanical stimuli through mechanotransduction processes19,20, in which mechanical forces are converted into biochemical signals that lead to mechanoresponses such as increased contractility or changes in gene expression that enable cells to adapt to their physical environment21. To date, significant progress has been made in understanding how mechanosensors at the cell surface and within the cytoplasm, such as integrins and focal adhesion proteins, contribute to mechanotransduction and mechanoresponses during breast cancer progression. In contrast, the role of the nucleus in this process, and particularly the function of specific NE proteins such as lamins, emerin, SUN proteins, and nesprins, which have all been implicated in cellular mechanotransduction22, remain to be fully elucidated.

The nucleus plays a pivotal role in mediating the cellular response to mechanical forces; additionally, as the largest and typically stiffest organelle in the cell, the physical properties of the nucleus directly affect the ability of cells to complete several steps of the metastatic cascade23,24. For example, the nucleus undergoes substantial deformation as cancer cells pass through narrow spaces during invasion, intravasation, and extravasation23,24,25,26. Thus, changes in the expression of lamins and emerin, which control nuclear deformability and stability27,28,29,30,31,32,33,34, could either promote or impair the ability of cells to pass through such confined spaces. Furthermore, extensive nuclear deformation can lead to transient NE rupture, DNA damage, and genomic instability26,35, which could be detrimental to individual tumor cells, but collectively promote cancer progression by increasing cancer cell heterogeneity.

Further supporting a crucial role of NE proteins in breast cancer progression, abnormal nuclear morphology, as well as altered expression of lamin A/C, emerin, nesprins, and SUN proteins often correlate with breast cancer progression and patient outcomes23,25,36,37,38,39,40,41,42. Although the precise contribution of these NE proteins to metastatic progression in breast cancer and the underlying mechanism of action remain unclear, increasing evidence suggests that these proteins can modulate cancer progression both through their mechanoresponsive roles and by modulating the physical properties of the cell nucleus. In this review, we will discuss the interwoven relationship between mechanical stress, NE proteins, and their role in mechanosensing and mechanoresponses at the various stages of metastatic progression. We will primarily focus on NE proteins known to be dysregulated in breast cancer, particularly lamins, emerin, nesprins, and SUN proteins, and discuss their interaction with other pathways known to be perturbed in breast cancer.

Nuclear envelope proteins in breast cancer

The nuclear envelope

The NE separates the nuclear interior from the cytoplasm (Fig. 1) and has essential structural and regulatory functions. The NE includes the inner and outer nuclear membranes (INM and ONM, respectively), the nuclear lamina, and nuclear pores, along with a large number of nuclear membrane proteins. The nuclear lamina is primarily comprised of filamentous lamin proteins and provides resistance to nuclear deformation. It also contributes to the regular spacing of nuclear pores and the organization of peripheral chromatin17,43. Lamin A and lamin C are splice variants encoded by the LMNA gene and are often referred to together as lamin A/C, despite having some unique biological roles44. Lamins A/C are major components of the nuclear lamina and regulate nuclear mechanics and stability31, but also interact with chromatin and numerous transcriptional regulators32. The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex consists of nesprin proteins at the ONM and SUN domain proteins at the INM (Fig. 1, inset) and is crucial to transmit forces between the cytoskeleton and nuclear interior45,46. Nesprins interact with actin filaments, microtubules, and intermediate filaments, either directly or via other proteins, and bind across the nuclear lumen to SUN domain proteins that interact with lamins, emerin, and the nuclear interior. The mechanisms by which cells regulate the interaction of SUN proteins and nesprins, and thus the force transmission across the NE, remains incompletely understood47,48. In addition to providing a physical connection between the cytoplasm and nuclear interior, nesprins and SUN domain proteins can also directly interact with important signaling molecules, such as α- and β-catenins49. Emerin is a nuclear membrane protein that is retained at the INM through its interaction with lamins A/C but can also be found at the ONM50. Lamins, emerin, and LINC complex components are important for maintaining the structure, shape, and function of the nucleus, as well as in transmitting both physical forces and biochemical signals between the cytoplasm and the nuclear interior (Table 1)22,51. Importantly, force transmission and dysregulation of NE proteins such as lamins can alter chromatin organization, including lamin-associated domains (LADs), chromatin accessibility, and epigenetic modifications, along with gene expression and various cellular functions52,53,54,55. We refer the reader to excellent recent reviews by Dupont et al.56, Hoskins et al.54, and Deng et al.57 on the topic of NE proteins, nuclear force transmission, and chromatin organization and function.

Fig. 1: Schematic overview of the NE, including the inner nuclear membrane (INM), outer nuclear membrane (ONM), nuclear pore complexes (NPC), and nuclear lamina.
figure 1

The inset on the right shows the LINC complex, comprised of nesprins, SUN domain proteins, lamin A/C and lamin B, and emerin. The LINC complex physically connects the cytoskeleton, including intermediate filaments, microtubules, and F-actin, to the nuclear lamina and the nuclear interior. Together, these components facilitate force transmission between the cytoskeleton and the nucleus and contribute to nuclear mechanotransduction processes.

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Table 1 Overview of key NE proteins discussed in this review
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Altered expression of nuclear envelope proteins in breast cancer

Characteristic changes in the nuclear morphology of breast cancer cells, such as size, roundness, presence of invaginations, NE smoothness, and chromatin distribution, are routinely used by clinicians for diagnostic and prognostic purposes, as the increased prevalence of nuclear aberrations in breast cancer cells correlates to worse patient outcomes36,40,41. It remains unclear, however, to what extent the observed changes in nuclear morphology result directly from altered expression or function of NE proteins, and if or how such changes mechanistically contribute to breast cancer progression. Supporting a functional involvement, increasing reports indicate that breast cancer tissues exhibit substantial changes in the expression of many NE proteins, and that these changes are associated with disease progression22. For example, changes in the expression of nesprin-1/2 have been reported in many cancers, including breast cancer58,59. Clinically, the loss of nesprin-2 expression is associated with tumorous tissue regions and correlates to breast cancer invasiveness and intrinsic subtype39. Decreased SUN1 and SUN2 expression is observed in tumorous tissue regions, however, their clinical implications are unclear39. Invasive breast cancer subtypes have reduced emerin levels and smaller nuclei29, and overexpression of emerin increases nuclear size, suppresses tumor growth, and reduces lung metastasis in mouse models29. Lamin A/C modulate nuclear structure, genomic stability, cell cycle regulation, and response to DNA damage, so it is unsurprising that alterations in lamin A/C lead to aberrations in nuclear morphology17,33,60, a well-established hallmark to aggressive cancers3. Decreased levels of lamin A/C generally correlate with more metastatic cancer cells and worse prognosis in breast cancer patients, whereas higher expression levels are associated with an early clinical stage and better patient outcomes33,39,40,41,42,60,61.

The causes of altered nuclear envelope protein expression in breast cancer

The altered expression levels of NE proteins could arise through multiple mechanisms. The changes are typically not the result of alterations in the copy number of the corresponding gene(s), but are linked to altered transcriptional and epigenetic regulation62. For example, hyperactivation of the PI3K/AKT pathway, which is frequently altered in breast cancer, can affect both lamin A/C expression and turnover61,63,64. For other NE proteins, the underlying mechanism remains to be determined. It is intriguing to speculate, though, that the altered expression of NE proteins is not only a downstream consequence of oncogenic and tumor suppressor pathways altered in breast cancer cells but might also result from the adaptation of cancer cells to the physical challenges experienced during the metastatic cascade, and/or selection for specific function. In this scenario, the mechanical microenvironment could either activate mechanoresponsive signaling pathways that in turn alter NE protein expression, thereby enabling cancer cells to adapt and enhance their metastatic potential, or the mechanical environment could selectively favor cancer cells with pre-existing elevated or reduced NE protein levels, facilitating their survival through the mechanically challenging environment. Supporting the hypothesis that cells adapt to their microenvironment by altering their NE protein expression, mechanical stimuli can induce numerous changes in the nucleus, affecting nuclear shape, chromatin organization, and expression levels of various NE proteins (reviewed in65 and66). For example, lamin A levels scale with the degree of tissue stiffness67, and mechanical stress can lead to reduced levels of the SUN domain protein SUN268. Furthermore, mechanical force application promotes phosphorylation of NE proteins, such as emerin69 and lamin A/C67,70, affecting their interaction with binding partners and turnover. In the following sections, we will discuss the physical challenges encountered at each stage of the metastatic cascade and explore how changes in the levels of lamins A/C, emerin, and LINC complex components might facilitate cancer cell progression through these stages.

The interplay of nuclear envelope proteins and the mechanical microenvironment during metastatic progression

During tumorigenesis and metastasis, cell behaviors and decision-making processes, such as key cancer hallmarks like migration, proliferation, and apoptosis evasion3, are modulated by the cell’s physical microenvironment15. In each step of the metastatic cascade, including primary tumor formation, acquisition of an invasive phenotype, local invasion, intravasation, circulation, extravasation, and colonization, cancer cells are subject to an array of physical forces (Fig. 2). These forces induce mechanoresponses, mediated in part by the nucleus, which enable further cancer progression, and impose selection pressure on the heterogeneous tumor cell population that results in the emergence of cancer cell subpopulations with specific molecular and physical features.

Fig. 2: Mechanical stress imposed on breast cancer cells from their microenvironment during specific steps of the metastatic cascade.
figure 2

Cells within the primary tumor experience solid and hydrostatic pressure, as well as compressive, tensile and shear stress, imposed by the complex TME. As cell invade the surrounding tissue, they experience compressive and tensile stress. During confined migration events, such as intravasation, and extravasation, cells experience increased compressive stress. In circulation, cell experience increased fluid shear stress imposed by the blood flow.

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Tumorigenesis

Primary tumor formation is associated with increased cell proliferation and inflammation, as well as growth suppressor evasion and extensive ECM remodeling3,11. As a result, cancer cells are subjected to various forces, including compressive and tensile forces, depending on their location within the tumor and the tumor stage14,20. Increased solid pressure, from cell proliferation and physical constraints imposed by surrounding cells’ ECM, and increased hydrostatic pressure lead to compressive forces. At the same time, cell-cell and cell-ECM interactions are often associated with tensile forces, particularly during the transition to tumor invasion, when cancer associated fibroblasts and immune cells might aid the process by pulling on tumor cells71,72. These physical stimuli could promote changes in the expression of NE proteins or conversely lead to selection of cells with altered expression. For example, loss of lamin A/C, and other NE proteins, including nesprin-2, SUN1/2, and emerin, are a signature of tumorous tissue regions39. However, severe lamin A/C depletion impairs breast cancer cell spheroid formation in vitro, potentially due to a reduction in transcription levels of the growth regulator Yap173. Taken together, these data indicate that lamin A/C expression may be required for tumor initiation and formation, but lamin A/C depletion may be favorable to tumor progression – suggesting a change in protein expression of the cell population, whether it be through adaptation to mechanical or biochemical cues, survival bias, or a combination of both. Interestingly, lamin A overexpression decreases breast cancer cell proliferation on two dimensional substrates61, indicating possible competing effects of altered lamin levels during tumorigenesis.

Cell migration: invasion, intravasation, extravasation

A crucial first step in cancer invasion is the acquisition of a migratory phenotype, which can be mediated by an increase in ECM stiffness74. Cell migration, both confined and unconfined, is not driven by a single factor, but rather by a complex interplay of both mechanical and biochemical signals. During the subsequent steps, including invasion through the basement membrane and tissues, intravasation, and extravasation, cancer cells must navigate through interstitial spaces and openings in the endothelial cell lining of blood vessels as small as ≈1 µm in size, which is substantially smaller than the 3–15 µm diameter of the nucleus14,15,22,23,26,61. Such confined migration involves various intra- and extracellular forces, including actomyosin-mediated traction forces on the ECM and surrounding cells, increased intracellular hydrostatic pressure from cell cortex contraction, cytoskeletal forces pulling on the nucleus, and forces exerted by endothelial cells17,75,76,77,78,79,80. Passing through these small openings places severe mechanical stress on the nucleus, which is approximately 2–10 times stiffer than the cytoplasm23,26. As a consequence, the nucleus undergoes large deformation that can lead to transient NE rupture, DNA damage, and increased cell death34,37,81,82. Additionally, in vitro studies using microfluidic devices and porous membranes that mimic tight interstitial spaces revealed that confined migration results in altered chromatin organization, including perturbed 3D configuration, chromatin compartmentalization, and epigenetic modifications53,83,84. Even very transient nuclear deformation during perfusion through tight spaces can alter chromatin organization, gene expression, and cell fate85. However, it remains unclear whether such changes also occur during transendothelial migration or interstitial invasion in vivo.

It is now well recognized that the deformability of the nucleus constitutes a rate-limiting physical factor in the ability of cells to migrate through confined spaces17,24,34,86. Since the deformability of the nucleus is determined by the chromatin in the interior and the composition of the NE, particularly the levels of lamin A/C and emerin27,28,31,87, changes in the expression of these NE proteins can substantially alter the invasive potential of cancer cells. For example, lower lamin A/C levels result in more deformable nuclei and an enhanced ability of breast cancer cells to migrate through confined spaces61. Severe reduction of lamin A/C levels, however, reduces the mechanical stability of the nucleus, resulting in increased NE rupture and cell death in cells migrating through tight spaces34,37,60,81. Supporting this multifaceted role of lamins A/C in cancer progression, a recent study found that nearly complete depletion of lamin A/C enhanced cell migration through small rigid pores in vitro, but did not improve transendothelial migration in vivo, and impaired metastasis to the lungs73. Reduced emerin levels, which have been reported in various breast cancers29,88, can similarly increase nuclear deformability and reduce nuclear size, thereby enhancing the invasive potential of breast and prostate cancer cells and promoting metastasis in multiple breast cancer models29,88,89. Lastly, NE rupture is determined by nuclear membrane tension90. The endoplasmic reticulum (ER) plays a crucial role in regulating and absorbing nuclear membrane tension91, suggesting that proteins in the ER and NE that control the flow of lipid membrane from the ER to the outer nuclear membrane under tension are key determinants for NE rupture and mechanotransduction.

Besides lamin A/C and emerin, other NE proteins can also contribute to nuclear deformability and changes in invasive potential. For example, depletion of nesprin-1/2 in breast cancer cells results in increased nuclear deformation and degradation of lamin A/C92. However, despite the increased nuclear deformability, the cells exhibit a reduced migratory capacity through small pores92. Since nesprin-1/2 is an essential component of the LINC complex, nesprin-1/2 depletion might impair force transmission from the cytoskeleton to the nuclei, which could limit the ability of cells to apply sufficient force to the nucleus to move it through confined spaces76,80. However, nesprins also play important roles in mechanosensing and in signaling, in part through the WNT/catenin pathway93,94,95,96, which could contribute to the detrimental effects of nesprin-depletion. Further highlighting the often unexpected effects of manipulating NE protein levels, depletion of SUN1 and SUN2 in breast cancer cells, characteristic of the lower SUN1/2 levels found many breast cancer tumors39,97, increased cancer cell migration along fibers, without affecting migration on 2-D substrates97. The effect was mediated by the MKL1-SRF pathway, a key mechanoresponsive signaling pathway that responds to changes in actin polymerization97. Another study found that depletion of SUN1, but not SUN2 decreased cell migration through transwell plates with 8-µm-pore filters98. The apparent discrepancy between these two studies suggests that the effect of reduced SUN1/2 levels on breast cancer cell motility is context-dependent, with SUN1/2 depletion enhancing movement along fibrous structures but inhibiting migration through three dimensional barriers. Further studies are needed to gain a full understanding of the role of SUN proteins in the migration of cancer cells through physiological environments, and whether these effects are regulated through altered LINC complex mediated force transmission or through other functions of SUN1/2. A recent study on metastatic melanoma cells, which have a high ability to migrate through confined spaces, suggest that cells can use additional strategies to withstand the high physical stress associated with large nuclear deformation incurred during confined migration99. The authors found that increased levels of the NE protein lamin-associated polypeptide 1 C (LAP1C) facilitate NE blebbing, which could help reduce stress on the NE and thus avoid damage to the cells during confined migration.

Survival in circulation

In the bloodstream, CTCs experience extreme shear forces from the blood as they traverse the body and anchor to the vessel for extravasation14,15,20. Lamin A/C-deficient breast cancer cells, which have more fragile nuclei, show increased cell death under conditions that mimic physiological shear stress levels100, indicating that adequate levels of lamin A/C are required to protect and maintain the structural integrity of the nucleus in CTCs. These findings align with a recent study showing that shear stress promotes lamin A/C protein accumulation, which modulates the YAP pathway101. Given that lamin A/C is a major determinant of nuclear stiffness31, the accumulation of lamin A/C under shear stress may lead to mechanically more stable nuclei, enabling them to better withstand the shear stress in circulation100.

Notably, fluid shear stress increases the proliferation, migration, invasive potential, and chemoresistance of cancer cells, including CTCs101,102,103, as well as the fraction of cells with a cancer stem cell-like phenotype104. These results suggest that cells either adapt and transition into a more invasive phenotype in response to fluid shear stress, or that fluid shear stress acts as a selection pressure, allowing only the most invasive cells to survive. If vulnerabilities in CTCs to survive fluid shear stress can be identified, like a dependence on lamin A/C for survival or the activation of specific pathways, such as YAP, to promote adaptation to shear stress, targeted therapies could be developed to attack these vulnerabilities, hindering CTCs’ survival in circulation and preventing cancer cells from reaching secondary sites.

The complexity and dynamics of nuclear envelope dysregulation in breast cancer metastasis

Reduced lamin A/C levels have been shown to both enhance and inhibit metastasis33,39,40,41,42,60,61,73,101,105. This seemingly paradoxical effect of lamin A/C on breast cancer progression may be explained not only by variations in the location of the tumor samples (e.g., the primary tumor core, the invasive front, or metastatic sites), but also by the diverse functions of lamins, different requirements at each step of the metastatic cascade, and potentially dynamic regulation of lamin levels by metastasizing cancer cells. For example, whereas low lamin A/C levels increase nuclear deformability, which could facilitate invasion, intravasation, and extravasation, complete loss of lamin A/C results in more fragile nuclei that are prone to damage. Additionally, (complete) loss of lamin A/C is expected to also affect other cellular functions modulated by lamins A/C, such as chromatin organization, gene regulation, proliferation etc., which could cause detrimental effects. Thus, cancer cells with an intermediate amount of lamin expression might be best suited to achieve sufficient nuclear deformability to disseminate through the body while ensuring that other functions of lamins critical for metastasis are preserved61,100.

Furthermore, it is likely that specific stages of cancer progression demand different levels of lamin expression, and by extension, of other NE proteins. For instance, higher lamin A/C levels may be required to maintain nuclear integrity when cell are within the primary tumor, where cells are exposed to compressive stress, or during circulation100,101,105, whereas invasion, intravasation, and extravasation may favor lower lamin A/C levels and the associated greater nuclear deformability. Yet different lamin A/C levels might be required at metastatic sites to promote metastatic outgrowth and immune invasion.

This raises the question whether cancer cells dynamically regulate lamin expression in response to mechanical cues from their environment, consistent with the concept that the most aggressive cancer cells are highly plastic and adaptable106,107,108,109. Lamin A/C levels and phosphorylation, which regulates lamin solubility and turnover, respond directly to the mechanical microenvironment67,70. Other NE proteins, such as emerin and LINC complex components, are also responsive to mechanical stimuli68,69,110, raising the intriguing possibility that cancer cells adapt to specific needs during metastatic progression, rather than simply being selected for specific capabilities based on pre-existing properties. In this scenario, mechanical changes of the microenvironment resulting from the uncontrolled cell proliferation within the primary tumor and its interaction with the TME, which lead to increased solid stress, ECM remodeling, and increased stiffness of the TME3,14, could further promote cancer progression and invasion, in part through mechanotransduction pathways in the tumor cells and stromal cells. At subsequent steps in the metastatic cascades, the local mechanical microenvironment and the various mechanical forces acting on the cancer cells could trigger additional mechanoresponsive changes that further promote cancer progression. Alternatively, metastatic progression might create a selection pressure, favoring cells with lamin A/C levels low enough to facilitate migration but high enough to withstand the mechanical stress associated with the various steps of metastatic progression. Although currently no experimental data is available to support or refute this hypothesis, such ‘mechanical selection’ could contribute to the observed low success rate of metastasis7. Likely, both adaptation and selection contribute to cancer metastasis, and additional research will be needed to elucidate the specific contributions and the underlying pathways, which could then be explored for therapeutic interventions.

Nuclear mechanoresponse pathways in cancer

The altered expression of NE proteins during breast cancer progression may be driven not only by transcriptional and epigenetic dysregulation but also by mechanoresponses to the physical TME. Several pathways that are known regulators of cancer progression, including PI3K/AKT, TGF-β, ERK/MAPK, YAP/TAZ, have also been implicated in cellular mechanoresponses. Moreover, DNA damage response (DDR) pathways are increasingly being recognized as being part of the cellular mechanoresponse, either following mechanically induced DNA damage or being directly modulated by mechanical stress35,111,112. Additionally, activation of these pathways can alter the expression of NE proteins and the mechanical properties of the nucleus113,114,115,116,117 which could further promoted cancer progression.

PI3K/AKT

One common driver of tumorigenesis and hallmark of invasive cancer is the activation of the PI3K/AKT pathway, typically through oncogenic mutations118,119. However, mechanical regulation of the PI3K pathway has become an intriguing focus of research, especially in the context of cancer progression and metastasis. Both increased matrix stiffness and uniaxial compression lead to PI3K/AKT pathway activation in cancer cells, promoting migration, proliferation, and enhanced survival118,119, and supporting the presence of a mechanoresponsive component to this pathway. NE proteins, particularly lamins, exhibit a bidirectional regulatory relation to the PI3K/AKT pathway120. Lamin A overexpression or depletion alters several proteins associated with the PI3K pathway in cancer cells, identifying it as a potential downstream target of lamin A61,121,122. Conversely, inhibition of the PI3K/AKT pathway elevates lamin A levels in breast cancer cells, and AKT-positive tumors show reduced lamin A expression61, suggesting an upstream regulatory role on lamin A expression consistent with previous findings in other cell types120,123 and underscoring the connection between NE regulation and mechanoresponsive signaling pathways.

TGF-β

TGF-β plays a multifaceted role in breast cancer progression, functioning not only as a cytokine involved in growth, differentiation, and apoptosis but also as a mechanoresponsive signal. TGF-β is upregulated in environments with increased matrix stiffness, inducing epithelial-mesenchymal transition (EMT) via the PI3K/AKT pathway124,125. Furthermore, TGF-β-induced EMT is dependent on the spatial distribution of mechanical stress, with more mesenchymal cell phenotypes correlating to regions of increased cytoskeletal tension126. Growing evidence suggests that the outcomes of TGF-β stimulation may impact and be impacted by NE proteins – specifically lamin A62,127. Treatment with TGF-β increases lamin A phosphorylation in breast cancer cells127, whereas elevated lamin A expression decreases TGF-β secretion in sarcoma cells62. Deletion of SUN2 results in elevated AKT and MAPK signaling, but show a downregulation or unperturbed expression of TGF-β target genes, likely by altered retention of the NE protein MAN1 at the NE, which is a negative regulator of TGF-β signaling128. Together, these findings suggest that TGF-β may function in concert with NE proteins and cellular mechanotransduction processes, but the precise role of NE proteins in the TGF-β-mediated mechanoresponse remains to be fully elucidated.

Although TGF-β signaling changes in response to mechanical cues, TGF-β itself might not function as a mechanosensor, and alternative explanations exist. For example, PI3K/AKT might act as the primary mechanoresponse pathway, with TGF-β activation arising from a feedback loop involving PI3K/AKT signaling. Alternatively, the mechanoresponsive effects of matrix stiffness could be mediated through other pathways, independent of TGF-β, resulting in cells becoming more responsive to EMT-inducing signals. Supporting this idea, mechanically activated Piezo2 channels trigger calcium influx, leading to AKT activation and nuclear translocation of the key EMT transcription factor SNAIL129. Additionally, inhibition of the NF-κB mechanoresponse pathway suppresses the expression of multiple EMT related transcription factors130. Nonetheless, these findings do not rule out the possibility that TGF-β itself is mechanoresponsive. For a detailed discussion of the interplay between TGF-β and NE proteins, we refer the reader to a previous review (Bell et al.62).

ERK/MAPK

The ERK/MAPK pathway is a critical signaling cascade involved in cell proliferation, differentiation, migration, stress responses, and survival, with several connections to NE proteins and mechanotransduction processes. Lamin A directly interacts with ERK1/2 in vivo and in vitro131 and depletion or mutation of lamin A/C leads to perturbed MAPK signaling cascade by hyperactivating the ERK, JNK, and p38α MAPK pathway components62. Furthermore, some nesprin-2 isoforms act as scaffolding proteins for ERK1/2, tethering it to the NE132, and nesprin-1 mutations can increase ERK pathway activation in vitro and in vivo133. These findings provide a strong link between the ERK/MAPK pathway and NE proteins. The ERK/MAPK pathway is canonically activated by cytokine and biochemical cues134, but can also be rapidly activated by mechanical stimulation135. Increased matrix stiffness induces an invasive phenotype in breast cancer cells, likely through a mechanoresponsive FAK-Rho-ERK signaling network136. Additionally, shear stress induces breast cancer cell proliferation through ERK phosphorylation and nuclear translocation, along with activation of an ERK-YAP signaling cascade137, although the individual mechanosensitive contributions of ERK versus YAP has not been elucidated.

YAP/TAZ

The YAP/TAZ pathway is a critical mechanoresponsive signaling pathway that integrates cues from substrate stiffness, actin dynamics, and various mechanical stimuli138,139. Altered YAP/TAZ signaling has been implicated in many diseases139,140,141, including breast cancer139,141. In breast cancer cells, shear stress facilitates YAP nuclear translocation101,137, which promotes lamin A/C protein accumulation101. Nuclear compression alone is sufficient to induce nuclear YAP accumulation, in part by opening nuclear pores and facilitating nuclear transport139,142. Since lower lamin A/C levels result in more deformable nuclei, reduced lamin A/C expression could also lead to increased YAP nuclear localization in cells subjected to compressive forces. Accordingly, previous studies in Ewing Sarcoma found that lower levels of lamin A levels are associated with increased YAP nuclear localization, enhanced cancer cell migration, and more aggressive disease143, whereas overexpression of lamin A reduces YAP nuclear recruitment and cell invasiveness143. However, whereas lamin A/C increases alongside YAP nuclear translocation in breast cancer cells experiencing shear stress101, Ewing Sarcoma cells show an inverse relationship, with lower lamin A levels correlating with increased YAP nuclear translocation143. These discrepancies may reflect the different mechanical stress applications, cell-type specific differences, or variations in disease-specific mechanisms. A connection between the YAP/TAZ signaling pathway and lamin A/C is supported further by findings in diseases resulting from mutations in the LMNA or related genes. For example, muscle stem cells with LMNA or nesprin-1 mutations that cause increased nuclear deformability show increased YAP nuclear localization144, although another study found that muscle cells with a different LMNA mutation had impaired nuclear translocation of YAP140. Collectively, these findings emphasize the interplay between YAP/TAZ mechanoresponse pathways, NE protein expression, and disease progressions, including breast cancer progression.

DNA damage response pathways

The nuclear lamina plays a crucial role in protecting the contents of the nucleus, maintaining genomic stability, and regulating DDR145. Nuclear deformation and NE rupture associated with confined migration or cell compression can cause DNA damage by allowing influx of cytoplasmic nucleases, efflux or exclusion of DNA repair factors, and increasing replication stress35,146,147, which can be ameliorated at least in part by increased lamin A expression147,148. Conversely, DNA damage can trigger NE rupture via ATR kinase pathway activation, leading to lamin A/C phosphorylation, disruption of lamina assembly, and NE rupture115. These findings suggest a potential positive feedback loop between low lamin A/C levels and DNA damage, where DNA damage destabilizes the nuclear lamina, exacerbating NE rupture and further reducing lamin A expression. Since lamin A is involved in multiple DNA damage repair pathways, including non-homologous end joining (NHEJ) and homologous recombination (HR), by recruiting repair factors like 53BP1 and BRCA1 to damaged sites145, loss of lamin A/C might not only increase the risk of DNA damage, but also impair DNA damage repair, which could further drive genomic instability in cancer cells.

Further linking DNA damage and nuclear mechanobiology, several key DDR pathways were recently shown to be responsive to mechanical cues. Ataxia-telangiectasia mutated (ATM) is activated by mechanical stress and directly binds to the cytoskeleton to regulate cell stiffness and migration potential149. ATM inhibition leads to stress fiber accumulation, disrupting the normal interplay between the nucleus and cytoskeleton149. Interestingly, ATM inhibition also reduces lamin A expression113, thereby increasing nuclear deformability, suggesting that mutations in the ATM pathways could promote cancer cell invasion by increasing nuclear deformability. Besides ATM, Ataxia-telangiectasia and Rad3-related protein (ATR) also responds to mechanical stress, resulting in ATR recruitment to the NE, independent of DNA damage, to modulate NE plasticity and peripheral chromatin association112.

Perspectives

As described in the preceding sections, a growing body of evidence indicates an extensive interplay between the physical microenvironment encountered by cancer cells during metastatic progression, cellular mechanotransduction responses, and altered expression of NE proteins that determines the fate of breast cancer cells. However, several key questions remain.

Foremost, it remains unclear whether changes in the expression of NE proteins are driven by adaptation of cells to the physical environment via mechanoresponses65,66, or if the physical challenges encountered during the individual steps of the metastatic cascade select for a subpopulation of cells from the heterogeneous tumor population that can withstand the increased stress. These possibilities are not mutually exclusive, and it is plausible that a subpopulation of cells is more adaptable to the TME, exhibiting greater plasticity as they traverse the body. These “super-adaptable” cells may further modify their physical and molecular characteristics—such as expression of specific NE protein—in response to the demands of their environment to enhance survival. Alternatively, a subpopulation of tumor cells might exist that is inherently more resistant to mechanical stress within the TME. This “super-fit” population could be selected for as metastasis progresses due to a survival bias based on their NE protein expression. The “super-adaptable” and “super-fit” subpopulations may overlap, forming a subset of cancer cell with the greatest metastatic potential and possibly representing the small fraction of tumor cells that successfully metastasize. Moreover, this “super-adaptable” and “super-fit” subpopulation may overlap with the stem cell-like population frequently associated with disease progression and poor survival. Identifying the specific characteristic signatures of these cells would be highly valuable for prognosis and ultimately for targeting these cells as a therapeutic strategy.

Given the diverse function of NE proteins, it also remains unclear whether the effect of their dysregulation on cancer outcomes is primarily due to their role in cellular mechanosensing and in determining the physical properties of the nucleus, or through more traditional biochemical functions, such as their role in cell signaling or DDR. Similarly, the contribution of genomic alterations versus altered mechanoresponses in driving changes in the expression of NE proteins remains to be fully elucidated, along with the crosstalk between these mechanisms. For example, mechanical stress modulates activation of several mechanoresponse pathways such as YAP and PI3K/AKT101,137, as well as NE protein expression patterns101. Whether these responses are independent of each other or share common upstream pathways will require further investigation. Additionally, when considering perturbed mechanoresponses in cancer cells, it is important to recognize that these changes can arise from at least two separate factors. First, changes in the physical microenvironment, such as stiffening of the stromal environment, can drive pathogenic mechanoresponses that promote a pro-invasive cellular phenotype, even in normal mammary epithelial cells, as demonstrated in a landmark study two decades ago74. Second, oncogenic mutations such as the PTEN mutation in the PI3K pathway, and subsequent changes in the expression of other proteins, including NE proteins, can further modulate key mechanotransduction pathways, thereby altering mechanoresponses in cells. Additionally, changes in the physical environment trigger not only acute mechanoresponses but can also influence cellular behavior even after the mechanical perturbation has been removed. This “mechanical memory” is being increasingly recognized as an important factor in metastasis150, but the precise mechanisms and molecular details underlying mechanical memory formation and retention, which likely involve both epigenetic changes and altered cellular structural organization, remain incompletely understood.

Addressing these questions will require a combination of experimental tools and models to dissect the reciprocal role of mechanical challenges, cellular functions, and NE proteins at each step of the metastatic cascade, going beyond the often still correlative findings shaping our current knowledge. Determining the distinct contributions of cellular adaptation and selection requires time-resolved measurements of specific cellular properties at single-cell resolution, motivating the development of fluorescent reporters for gene expression, chromatin organization, and live-cell biophysical measurements that can be applied in in vivo or ex vivo models mimicking physiological conditions. These experiments should be further aided by more advanced in vitro models that reflect multiple steps of metastatic progression. Current in vitro models typically focus on recapitulating individual steps such as invasion, survival in circulation, or extravasation. Consequently, cells that have been identified as most successful in a particular step, such as confined migration, may ultimately fail in other steps, such as circulation, immune evasion, or metastatic outgrowth. Therefore, developing integrated metastasis models that capture the full metastatic process while preserving the precision and control of in vitro systems could help identify key factors required for successful cancer metastasis. Such models might resolve contradictory findings in the field, such as discrepancies in the contribution of lamin A to breast cancer progression, ultimately providing a more cohesive understanding of NE protein dysregulation in metastasis. Since even the best in vitro models cannot capture all the nuances and complexities of human cancer, it will be crucial to supplement these models with (humanized) in vivo models and analysis of patient tissues, ideally comparing samples from the primary tumor and metastatic sites. These analyses will benefit from spatially resolved -omics approaches to link observed changes to particular physical conditions or subpopulations of cells. Insights gained from these experiments have the potential to improve our understanding of fundamental biological processes, shed light onto the many ways cancer cells adapt to successfully metastasize, and to ultimately uncover novel therapeutic targets to lead to more effective treatment strategies.