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The term "Cell Mechanobiology" refers to the study of role of mechanical forces in cell biology. There are two views of cell mechanobiology; such as (a) the clarification of cell mechanisms of how they sense, translate and respond the mechanical forces and modulate their functions and (b) description of cellular mechanical properties.
All living organisms are composed of one or more cells which are the basic structural and functional units to exhibit the property of life. According to their size and types of internal structures, we can distinguish the cells as two classes; prokaryotic cell and eukaryotic cell.
2.1. Prokaryotic Cell
Figure (1) Prokaryotic Cell Structure
The prokaryotic cells are represented by all of the various forms of bacteria. They have no well-defined nuclei and membrane-bound organelles and are composed of a single DNA circle. Their shapes and sizes are from minute spheres, cylinders and spiral threads to flagellated rods and filamentous chains.
2.2. Eukaryotic Cell
Figure (2) Eukaryotic Cell Structure
All other types of organisms (protists, fungi, plant and animals) are composed of structurally more complex Eukaryotic cells and the size of eukaryotic cells (>10Âµm) are generally much larger than that of prokaryotic cells (<10Âµm). Therefore, they require internal membrane-bound organelles to carry out metabolism and transport mechanism. It is surrounded by a plasma membrane which allows the nutrients to enter and waste products to leave. The plasma membrane is formed from phospholipid bilayer of polar hydrophilic heads and non-polar hydrophobic tails. It serves as a barrier for living and non-living portions of a cell and plays an important role in regulating cellular function .
Figure (3) Phospholipid bilayer
2.3. Load-Sensitive Cells
There are many types of mechanical loading environment of cells and tissues in a human body. Cells experience various mechanical stimuli and particularly important in cardiovascular and musculoskeletal systems. Fibroblasts in the skin, lungs, heart, tendons and ligaments, chondrocytes in cartilage, osteoblasts in bone marrow, endothelial cells in blood vessel and vascular smooth muscle cells are all types of cells subjected to large mechanical forces.
First of all, fibroblasts in the skin are resisted to tension, compression and shear. The fibroblasts which dominate the tendons and ligaments perform many vital functions during development and after maturity . Its function is to organize and maintain the connective tissues during development and repair wounds during wound healing .
The chondrocytes which found in another type of load-bearing tissue, articular cartilage, proliferate and differentiate in multiple stages in creating the extracellular matrix (ECM) . The two major macromolecules such as Type II collagen and large aggregating proteoglycan, aggrecan, are synthesized by chondrocytes at the proliferating stage. The type II collagen fibrils allow the aiticular cartilage in resisting the shear stress and compressive stress .
We know that the bone bears tension, compression and torsion in vivo. The bone cells such as osteoblasts are derived from mesechymal cells found in bone marrow and are subjected to diverse the mechanical forces .
Endothelial cells which form the inner lining of blood vessel are influenced by two distinct haemodynamic loads.
(i). Cyclical strain due to vessel wall distension and
(ii). Shear stress due to frictional forces applied by blood flow.
To maintain the vessel wall and circulatory function, the structural and functional integrity of these cells are very important  and it may contribute to pathogenesis of vascular disease, atherosclerosis .
The smooth muscle cells (SMCs) are another type of vascular cells and are subjected to compression, shear and cyclic stretch due to pulsatile blood pressure. Its proliferation may contribute to pathogenesis of several vascular diseases such as atherosclerosis and hypertension .
Therefore, these cells are applied by corresponding mechanical forces in various types of in vitro systems . The advantage of these systems is that the loading magnitude and frequency can be controlled easily and also the mechanical properties of substrates (stiffness) and their surface chemistry can be modified easily.
The cells can be stretched either uniaxially or biaxially. When stretching the cells in uniaxial direction, the substrate will be lengthened along this direction and will be compressed in its perpendicular direction. This type of stretching is suitable for tendons and ligaments in mechanical loading of cells. When stretching the cells in biaxial directions, the stretching can be either in equibiaxial stretching or non-equibiaxial stretching. The substrate strains are same in all directions in equibiaxial stretching, and are different in non-equibiaxial stretching. This type of stretching is suitable for dermal fibroblasts .
There is limitation in uniaxial stretching system. When the cells are subjected to cyclic uniaxial stretching, they orient away from stretching direction and tend to reorient toward a direction with minimal substrate deformation .
Figure (4) Reorientation of cell in cyclic uniaxial stretching
The microgrooved substrates are used in cell alignment to overcome this cell reorientation problem . Fibroblasts on these microgrooved substrates are aligned with microgrooves and maintain its elongated shape.
Figure (5) Microgrooved substrates
The cells remain align in microgrooves whether it is stretching or not. In an experimental model including collagen gel matrix, the cell deform this matrix because they produce the traction force and are subjected to external mechanical stretching at the same time. It is widely used in functional engineering tissues constructs and in bio-scaffolding materials with high mechanical strength to embed the cells cause of its low mechanical strength for mechanical stretching .
Figure (6) Cells embedded in collagen gel matrix
3. MICROMANIPULATION TECHNIQUES
We investigate how the cells sense and respond to mechanical stress depend on techniques and these investigations are subjected to apply controlled mechanical forces to living cells and to measure the changes in cellular deformation and alteration of molecular events. Micromanipulation techniques manipulate and measure the mechanical properties of cells, nucleus, cell membrane and cytoskeleton using mechanical, optical and magnetic means via a combined use of microscopic intracellular signalling and molecular cell biology techniques.
3.1. Micropipette Aspiration
We can measure the mechanical properties of a single cell by applying a known mechanical force or stress to deform the cell. The cell must be deformed by this force and its deformation must be measured. Micropipette aspiration is a classical technique for measuring the mechanical properties of individual cell such as elastic modulus and viscosity. In this technique, a low magnitude, negative pressure is applied to deform the cell and then the cell is elongated. This elongating portion of the cell is introduced into micropipette.
Figure (7) Micropipette aspiration of a cell
A glass micropipette having internal diameter of 1-5Âµm is used for deformation a cell and the vacuum is applied through the micropipette to the cell. The length of aspiration is varied with applied force. The fine pressure steps measured with a precision pressure sensor are created by an adjustable fluid reservoir. From experimental modelling perspectives, we can classify the cells as solid and liquid according to their response to threshold or critical pressure . For the cells like liquid behaviour (e.g., Neutrophils), if the pressure is applied above threshold or critical level, it can cause the complete cell aspiration into the glass micropipette. However, for the cells like solid behaviour (e.g., fibroblasts, chondrocytes, endothelial cells), they enter only a finite distance into micropipette even if the applied pressure exceeds above the threshold or critical level. The cell can be deformed by applying a sufficient high pressure  .
Experimentally measure the applied negative pressure Î”P and the resulting aspiration length L to characterise the both liquid-like and solid-like cells. To characterise the liquid-like cell, it requires measuring the radius of cell contour outside the micropipette () .
The applied pressures for aspiration are typically on the order of 1pNÂµ=1Pa for soft cells and 1nN Âµ = 1kPa for stiff cells. For deformation, the soft cells required the force on the order of 10-100pN and the stiff cells required several nanonewtons. The key experimental factors which determine the validity of mechanical characterization results are the accuracy of applied pressure, the accuracy of cellular geometrical parameter measurements, the synchronization of applied pressure and resulting geometrical changes of cell .
The elastic model is often used to assess the experimental data and extract material parameters  , equation (1) as
where, Î”P is the applied pressure, K is the area elastic modulus with a unit of pNÂµ, is the original surface area of the entire membrane and Î”A â‰ˆ 2Ï€L (1- ) is the outer surface area change in terms of aspiration length L, is the radius of pipette and is the radius of cell contour.
The analysis for an infinite, homogeneous half-space drawn into a micropipette gives , equation (2) as
Where, E is Young's modulus of the cell and is a constant value of 2.1. The Young's modulus values are 0.66 kPa, 0.96 kPa, 1.14 kPa and 0.047 kPa for chondrodytes, fibroblasts, endothelial cells and neutrophils respectively [16, 17, 20]. Note that the micropipette technique does not take into account the variations of local stiffness. It is shown that a distribution of the elastic modulus values exists across the cell.
We can measure the rate at which a cell flows into a micropipette in response to a stepwise sucking pressure to characterise the viscoelastic properties of liquid-like cells by micropipette aspiration technique. The viscosity of cytoplasm is estimated by equation (3), 
Where, m is a constant value of 6. Underlying linear viscoelastic model consisting of two springs and one damper, the procine aortic endothelial cells and procine aortic valve interstitial cells are quantified by viscoelastic parameters [19, 24].
We can also use the micropipette aspiration technique for time-lapse studies to understand molecular functions. A cell aspirated late in cytokinesis is accumulated green fluorescent protein (GFP)-myosin II to both the pipette end and the furrow .
Figure (8) Micropipette aspiration of amoeboid cells
For testing the strength of specific ligand-receptor bindings, how the two micropipettes are employed was shown in extension of the technique. A micropipette immobilized a microbead coated with a specific antibody which was placed in contact with a cell. The second micropipette was used to pull the cell from the coated microbead by increasing the applied pressure difference. It determines the yield strength of the ligand-receptor interaction .
Figure (9) Test of ligand-receptor binding strength by two micropipettes
3.2. Laser Trapping
Figure (10) Optical tweezers or Laser tweezers
For trapping the small objects within a defined region, there are many types of laser traps to trap different types of particles. The most common type is the optical tweezers or laser tweezers using laser beams. When the local stretching or bending forces is applied to the particle by the cell, the microparticles can be attached to a cell membrane. This force is proportional to the laser power required to constrain the particle. Thus, we can manipulate the cell and measure the stiffness of cell. The laser traps generate the range of forces, 0.1-1 nN.
Optical trapping is a suitable technique for manipulation and mechanical characterization of suspended cells. Various live entities which have been studied by laser tweezers are viruses and bacteria [27, 28], red blood cells [29, 30], natural killer cells  and outer hair cells . The so many articles which have also been investigated by laser tweezers are lateral movements of membrane glycoprotein , neuronal growth cones , adhesiveness of chondrocytes , intracellular elasticity of neutrophils  and intracellular organelle transport in giant amoebas .
The two microbeads coated with adhesive ligands or antibodies are attached to a cell in diametric opposition to each other to bind to specific receptors. The microbeads act as handles or grips for displacing the cell membrane. The surface of a glass slide is fixed with one of the beads and the steady or time-varying stretching force is generated by the relative movement of other bead. The stress applications can be highly selective and localized because the microbeads are coated with ligands or antibodies on its surface.
Although we have published that the laser tweezers is very effective in cell mechanobiology, the cells may be induced unwanted harmful effects by long exposure of cells or using a high powered laser [38, 39]. These unwanted effects have been suggested to result from photochemical and thermal reactions [40, 41]. To minimize the degree of photo damage, the near-infrared laser is commonly used because there is a wavelength dependence of the absorption of laser . However, the cell damage can be still caused by high photon flux density via two photon or multi-photon absorption mechanisms . Therefore, we need to take care to minimize light induced cell damage and to properly interpret experimental results.
Among several variations of laser tweezers, if a weakly focused laser beam is used as an optical channel, guidance and deposition of living cells can be achieved with a high spatial resolution . If the directions of two non-focused laser beams are opposite each other, a cell placed in between these beams would experience surface forces stretching along the axis and the net force would be zero. The stretching force defined by the beams depends on the size and type of the cell, the reflective index, and the laser power. Based on this principle, a device, termed as an optical stretcher, has been used to measure the viscoelastic properties of several cell types . It has proved that a whole cell was stretched by dual optical tweezers . In optical stretcher, the non-focused light beams are used to minimize the potential light induced damage to the cells and the bead attachments are not required.
By physically splitting the original laser beam or by time-sharing the laser beam with a mechano-optical or acousto-optical mechanism to deflect the laser beam, multiple laser traps can be generated simultaneously . In this technique, various modes of stress (e.g., tensile, biaxial and bending) are allowed to be applied on the cell. The arrays of laser emitted from vertical cavity surface have also been applied for optical trapping and active manipulation of multiple cells and microbeads simultaneously .
Improvements continue to refine and expand the capabilities of laser tweezers . For instance, optoelectronic tweezers which utilize direct optical images to create light-addressable electrokinetic forces were demonstrated for massively parallel manipulation of cells . Based on localized surface Plasmon resonance excited by polarized light, researchers have verified a way to manipulate and rotate biological cells . In addition, laser-tracking microrheology (LTM) can measure the mechanical properties within live cells [50, 51]. In laser-tracking microrheology (LTM), low-power laser beam with a high spatiotemporal resolution tracks a probe particle (e.g., a granule). The mechanical properties of the subcellular domain or other complex viscoelastic materials are exposed by Brownian motion of the particle to allow measurements of local changes in cell viscoelasticity. The mechano-activated signalling molecules, such as Src, were visualized and quantified with high temporal and spatial resolutions in combination of fluorescent resonance energy transfer imaging techniques (FRET) .
Figure (11) Laser tweezers traction on the bead
3.3. Magnetic Probes
As the microbeads in laser tweezers, the magnetic probes can be used as the handle for a magnetic trap or tweezers . In the presence of varying magnetic field, the magnetic force exerted by a magnetic particle with a magnetic moment (m) is. If we assume the induced moment is parallel to the magnetic field and the field is large enough that the magnetization of the particle saturates, the force exerted on the magnetic particle can be estimated as, equation (4),
Where, M and V are magnetization and volume of the particle. The magnetic force is very dependent on the material properties and the size of the particle and also on the spatial magnetic field gradient.
Magnetic fields are usually generated by electromagnets which are more easily controlled and permit the generation of time-varying force fields . Single-pole electromagnets with a sharp tip generate a strong electric field gradient near the tip region. It is a function of the distance between the particle and tip of the electromagnet applied by the force to each magnetic bead. To produce a constant magnetic field gradient, a pair of electromagnets can be used. The range of force measurements generated by magnetic tweezers with pole pairs (0.1-10 pN) is lower than the force by laser traps (0.1-1 nN). It has also been reported that the forces up to pN on a 4.5 Âµm particle is in the region (10-100 Âµm) near the tip of a single-pole electromagnet . Multiple pairs of electromagnetic poles are required to control multiple directions and rotations of particles at the same time.
Similar in laser trapping, ligand coated magnetic beads are used to get probing specific cellular components [54, 55, 56]. The modes of working are magnetic gradient  and magnetic twisting cytometry (MTC) . The MTC device can be further utilized to capture and quantify rapid mechanochemical signalling activities in living cells when combined with FRET techniques .
4. MICROELECTROMECHANICAL SYSTEMS (MEMS) TOOLS
MicroElectroMechanical System (MEMS) technology is the integration of mechanical elements, very small actuators and sensors and electronics on a common silicon substrate by using microfabrication technologies. While the electronic devices are fabricated using integrated circuit (e.g., CMOS, BICMOS processes), the micromechanical devices are fabricated using micromachining processes. MEMS technology enables the creation of tiny machines which can work with microelectronics. The sizes of MEMS-based tools are very match with the micrometer scale sizes of most mammalian cells. The size matching gives high accuracy in cell manipulation and high spatial and temporal resolutions in quantitative measurements of cellular responses. Many of the cells sense mechanical forces and convert them from mechanical into biochemical signals. This mechanism is known as mechanotransduction. Therefore, the MEMS techniques have an increasingly strong impact on cell mechanobiology.
4.1. Microcantilever-based Force Sensors
Atomic force microscopy (AFM) is capable of revealing surface structures with high spatial resolution. In AFM, a microscale probe (tip) is attached to the spring-like cantilever with a low spring constant. The cantilever scanned across surface using x-y piezoelectric tube. A laser beam detects the bending motion of lever (changing force) reflected from backside of lever into photodetector. The feedback loop maintains the constant force by moving lever up & down (z-piezo). We can get two types of information from the interaction of cantilever tip and sample surface with AFM such as topographical images and force measurements. In order to measure the force, the sample is vertically aligned by the tip at a fixed position. We can measure the interaction force between a sample surface (e.g., a living cell) and cantilever tip by using a laser to detect lever motion. The microcantilever is used to deform a cell and cell stiffness can be measured from the deflection of cantilever.
Figure (12) Components of Atomic Force Microscopy
The deformation of small cells are normally induced by the major techniques, such as micropipette aspiration, laser trapping, and magnetic twisting cytometry (MTC), mentioned in previous sections, as well as AFM. These techniques also measure the response of their corresponding cell force in the range of 1 pN-10 nN. However, large cell deformation can induce the large cell force response in many physiological conditions (e.g. axonal injury of >50% strain). The new types of microcantilevers or microcantilever-based MEMS devices are advanced in microfabrication and nanofabrication techniques. These are used to probe cell mechanical responses, such as cell indentation force response, cell stretch force response, and in situ observation of the cytoskeletal components during probing, under large deformations in the range of 1 nN to 1 ÂµN, allowing wide applications in studying cell mechanobiology. The traction forces generated by fibroblasts are measured by Galbraith et al using a microfabricated device capable of determining subcellular forces generated by individual adhesive contacts . The forces exerted on adhesive contacts can be continuously monitored by this device. To measure the responses of adherent fibroblasts to stretching forces, Yang and Saif developed a microfabricated force sensor . After developing the polydimethylsiloxane (PDMS) microcantilevers, we can measure the contractile forces of cardiomyocytes in real time . And also measure the large deformations induced by contractile forces of cardiomyocytes because of the low Young's modulus value of PDMS.
A functionalized MEMS force sensor that applied local deformation of a bovine endothelial cell is allowed to measure the force responses of these cells . It is a single-crystal silicon microcantilever beam coated with a thin layer of fibronectin. It forms the adhesion with a cell and deforms the cell locally by a piezoactuator. There is a force transmission from the cell adhesion sites on the substrate to the adhesion site of the cantilever through the cytoskeleton. We can measure the interaction force between the cell and the cantilever from the deformation of cantilever and its calibrated spring constant.
The mass of live single cells in fluids are characterized by an array of functionalized silicon cantilevers without detaching them from the surface. In figure (13), target cells in suspension were captured and immobilized on microcantilevers (top panel). Then the cells were cultured and the mass of a cell on a microcantilever was quantified via microcantilever resonance frequency shifts (bottom panel).
Figure (13) Microcantilever array
4.2. Micropost Arrays
The microfabricated silicone elastomeric post arrays can be used to measure the forces exerted by single adhesion sites of a cell . We can calculate the forces from micropost deflections which quantitatively report the magnitude, direction and location of the cell generated force. The disadvantage of this technique is, for instance, the focal adhesions were strongly affected by the pattern for a height above 1Âµm. This pattern is used for systematic tracking of substrate deformation. It is shown that a constant stress was applied by the cell at its various focal adhesions by the micropost array-based force measurements.
After developing the image-processing techniques, the accuracy and speed of micropost force measurements are improved . The measurements of traction forces exerted by Madin-Darby canine kidney (MDCK) epithelial cells during migration  and measurements of contraction forces of myocytes  are measured by PDMS micropost arrays.
The magnetic microposts containing cobalt nanowires were developed to separately study the cellular response to external forces applied to a cell and the internal forces generated by the cell . The magnetic micropost can apply the force to increase the local focal adhesion size at the site of application but not increase at adhesion sites by nonmagnetic posts.
Figure (14) Microfabricated arrays of magnetic and nonmagnetic posts
4.3. Microelectrode Arrays
We can study the elastic and viscoelastic properties of cells from cell deformation exerted by external electric fields. If we put a cell in an electric field, there may be a dipole to form interfacial polarization on the cell membrane. Depends on the strength of electric field and the effective polarization of the cells, stresses at the interfaces result in a deforming force, this phenomenon is called electrodeformation . If the cell deformation is small, the elastic strain of the cell along the directions of electric field is approximated as, equation (5),
where L is the cell deformation, is the original cell length, is a constant representing the elastic properties of the cell, Ï‰ is the angular frequency of the AC electric field and U(Ï‰) is the complex Clausius-Mossotti factor that depends on the internal structures of the cell and is cell-type specific .
In performing of electrodeformation, the electrode edges are applied by AC voltage to capture the suspended cells. After trapping the cells, the mechanical and electrical properties of the cells can be illustrated by applying the various voltages and frequencies. We can observe the reversible (elastic) deformations of the cells under low voltages and irreversible (plastic) deformation and rupture of cell membrane under high voltages. The relaxation of a deformed cell can be measured by removing the electric field suddenly. Then record and analyze the cell relaxation [69, 70].
The electrodeformation studies have been applied in several cell types and the most extensively studied is red blood cells. Therefore, we can study the deformation and viscoelastic properties at the cellular and molecular level by using microelectrodes.
5.1. Mechanical Phenotype of a Tumor Cell
5.1.1. Tissue assembly and Morphogenesis
Even in normal tissues, the various mechanical forces are constantly encountered and, in turn, the mechanical forces are actively exerted by the cells on their surroundings (Figure 15). The originations of these forces are from neighbouring cells or the extracellular matrix (ECM) and these forces are conducted through specific receptor-ligand interactions to trigger signalling events. Cells are also subjected to non-specific forces applied to the entire tissues, such as interstitial pressure and shear flow. These cell-derived contractile forces are actually essential for moulding the organism during embryogenesis and foetal life. For instance, if the mechanical force is applied to the developing Drosophila embryo, it induces the mechano-sensitive gene Twist expression throughout the embryo. Moreover, the application of compressive force may rescue the developmental deficits in mutants with abnormal Twist expression . Changing of mechanical interactions between cells and their surroundings may contribute to the tissue dysplasia associated with tumour initiation.
Figure (15) Cells experience mechanical forces from their neighbour and ECM
Manipulation of ECM stiffness and contraction of stiffness dependent cell is adequate to induce epithelial transformation in cultured cells. The transformed mammary epithelial cells (MECs) are cultured on collagen gels and affixed to a rigid substrate versus gels which allowed to freely floating . That's why; the intracellular tension conducted through the ECM is a principal indication to regulate tissue assembly and morphogenesis.
5.1.2. Detachment and Invasion
Figure (16) a tumor cell exchange the mechanical force with its behaviour
When a tumor cell detaches from the primary tumor and invades the surrounding parenchyma, it begins to regulate its environment with exchanging mechanical force including tractional forces which are associated with protrusive forces and locomotion. This phenomenon is illustrated by protrusive processes, known as invadopodia . All the processes, including invadopodia extension, cell and nuclear deformation and matrix track formation, require the local, dramatic and highly dynamic changes in cellular mechanics and cytoskeletal organization.
When a tumor is formed in vivo, there may be a progressive stiffness of tissue and ECM. It can be verified that the stiffness of mammary tumor tissue and adjacent tissue stroma are 5-20 times stiffer than that of normal mammary glands . This fact may help in decision making of diagnosis and treatment in cancer. For instance, we can palpate the stiffness of tissue for screening and diagnosis of superficial soft tissue tumor in virtually. The increases of tissue and ECM stiffness make the cells to generate the increase traction forces on their surroundings. It enhances their growth and invasion by supporting the focal adhesion maturation and signalling through the contractility of actomyosin. The increasing contractility of tumor cells and their associated fibroblasts provoke the tension-dependent matrix to support collagen linear reorientation. We have observed that the rapidly migrating transformed mammary epithelial cells on these prominent linear bundles of collagen fibrils nearby the blood vessels .
5.1.3. Interstitial Forces
In the mechanical force journey of a tumor cell, an important component is its ability to survive the non-specific mechanical forces which are arisen from the growth of tumor itself, transport in the lymphatic and blood stream and tissue homeostasis. The expansion of tumor compresses the surrounding extracellular matrix, results in constricting the vascular and lymphatic flow and interstitial space. These compressive stresses are combined with the outward projecting compression force to facilitate tumor cell invasion into the parenchyma when they occur in the setting of tissues such as brain and pancreas . These compressive forces show the initial symptoms of tumors as symptoms of increased intracranial pressure in gliobalstoma multiforme  and as sign of biliary obstructions in pancreatic cancer . These compressive stresses can also shrink the interstitial spaces and concentrate the growth factors and cytokines to enhance the tumor growth.
5.1.4. Shear Forces
Figure (17) a tumor cell reaches the vasculature
If a tumor cell escapes its primary tissue and reaches the vasculature or lymphatic, it must withstand mechanical forces associated with fluid flow and shear. After the tumor is successfully excised, the tumor cells are subjected to significant shear forces or changing patterns of flow by the surgical manipulations of irrigation and suction. These shear forces can induce the adhesion to collagen-based ECM substrates in vitro through the activation of Src.
5.1.5. Diapedesis and Distal Metastasis
Figure (18) a tumor cell undergoes diapedesis
In order for an adherent tumor cell to escape the vasculature and metastasize to a distal tissue, it must undergo diapedesis. Diapedesis is a process in which the cell undergoes the pseudopodial process to penetrate cell-cell junctions in the endothelium. It induces additional mechanical interactions between the endothelial cells and tumor cell and also induces a phenotypic exchange from cell-cell adhesion to cell-ECM adhesion.
As a summary, the tumor cells can absorb and exert mechanical forces on their surroundings in their transformative journey. The highly dynamic changes in cellular mechanical properties are essential in these processes.
5.2. Emerging Opportunities and Challenges
One of the challenges of the role of mechanical phenotype in cancer is clarification of molecular mechanism in which the tumor cells are enabling to modulate the mechanical responses and phenotype and to sense the mechanical properties of ECM. That problem is particularly overwhelming in recent time. It requires a motivation to integrate new advance knowledge about cell mechanics and cell mechanobiology into our existing knowledge of cellular and molecular mechanism of cancer. Now, we briefly discuss the two systems, such as Rho GTPase and focal adhesion kinase (FAK).
5.2.1. Rho GTPase
The small Rho GTPase has been contributed to many steps in cancer progression. These steps are proliferation, evasion of apoptosis, invasion and metastasis . With the ability of Rho of activating Rho-associated kinase (ROCK), Rho can motivate the cellular contractility. Rho acts as a molecular switch with all small GTPases. There, the bound-form of GTP is active and the bound form of GDP is inactive. Rho activation is linked to actomyosin contractility, stress fibre bundles formation and maturation of focal adhesions. Rho GTPases play a major role in pseudopodial protrusion and adhesion formation. Currently, ROCK has been provided as a clinical target. As an example, one of the ROCK inhibitors, fasudil has been provided to delay the progression of lung and breast tumors in human and rat models .
5.2.2. Focal adhesion kinase
The focal adhesions are micron-scale macromolecular complexes of ECM. They serve to anchor the receptors of cell adhesion to the cytoskeleton and to coordinate the mechanotransduction signals. Among various focal adhesion proteins, a few major proteins play a fundamental role in structure organizing and signalling. One of these proteins is focal adhesion kinase (FAK) and it regulates the cell tension in cancer progression . The focal adhesion kinase contains a focal adhesion targeting (FAT) domain which binds with other focal adhesion proteins (e.g., vinculin) and stimulators of Rho GTPase signalling. The FAK plays the central role in regulation of mechanical force journey of tumor cells. But it is still unclear that how the FAK senses and translates mechanical signals.
One of the most increasingly developments in cancer over the past decade is the identification that the tumor growth, invasion to the surroundings, and distal metastasis are fixed to the ability of cancer cell of sense, process and adjust the mechanical forces in their surroundings. In this report, I have conceptualized this process as a "force journey" of a tumor cell in which the tumor cell progressively changes in shape, motility and mechanics in the tumor microenvironment. While this force journey stand for a critical element in the evolution of a tumor, all the lesions of genetic and epigenetic are traditionally associated with cancer. We face up to determine how these two parallel journeys cooperate and which portions are essential to progress tumor. Progress in this field will require a motivation to extend the scope of cancer cell biology to exactly measure the mechanobiological properties of living cells and integrate the exerted mechanical force into traditional experimental point. It will also require the joint-working of bioengineers, biophysicists and trained cancer biologist to manage the experimental problems on time. While creating these connections is far from inconsequential, the paradigms discussed in this report suggest that the benefits to our concepts of mechanobiology of cancer cells more than justify the effort.