Mechanotransduction in tumor progression: The dark side of the force

Abstract

Cancer has been characterized as a genetic disease, associated with mutations that cause pathological alterations of the cell cycle, adhesion, or invasive motility. Recently, the importance of the anomalous mechanical properties of tumor tissues, which activate tumorigenic biochemical pathways, has become apparent. This mechanical induction in tumors appears to consist of the destabilization of adult tissue homeostasis as a result of the reactivation of embryonic developmental mechanosensitive pathways in response to pathological mechanical strains. These strains occur in many forms, for example, hypervascularization in late tumors leads to high static hydrodynamic pressure that can promote malignant progression through hypoxia or anomalous interstitial liquid and blood flow. The high stiffness of tumors directly induces the mechanical activation of biochemical pathways enhancing the cell cycle, epithelial-mesenchymal transition, and cell motility. Furthermore, increases in solid-stress pressure associated with cell hyperproliferation activate tumorigenic pathways in the healthy epithelial cells compressed by the neighboring tumor. The underlying molecular mechanisms of the translation of a mechanical signal into a tumor inducing biochemical signal are based on mechanically induced protein conformational changes that activate classical tumorigenic signaling pathways. Understanding these mechanisms will be important for the development of innovative treatments to target such mechanical anomalies in cancer.

Figure 1.

Latest advances in the interplay of physical and biochemical signaling pathways leading to transcription in the tumor microenvironment. Changes in the mechanical properties of the tumor microenvironment due to tumor initiation (stress-induced hyperproliferation) and late progression (stiffness-induced fibrosis) in the induction of the activation of two distinct pathways involved in tumorigenic transcription: the β-cat pathway and the Yap/Taz pathway, respectively. Hyperproliferative tumor-growth pressure activates the Ret kinase receptor (Tyr1062 phosphorylated), which in turn leads both to the inhibition of the β-cat interaction with E-cadherin through its phosphorylation of the Y654–β-cat and to the inhibition of β-cat degradation through phosphorylation of the Ser473-Akt and subsequent inactivation of the GSK3β (Ser9 phosphorylated). As a consequence, the cytosolic β-cat concentration increases, favoring its translocation into the nucleus and the transcription of tumorigenic target genes, such as c-myc or zeb-1, in vivo. β-cat also induces the expression of microRNAs (miR-18a) that repress the expression of the tumor suppressor phosphatase and Tensin homologue directly or indirectly via HOAX9. ECM high stiffness induced by late tumor development causes conformational changes in the focal adhesion protein Talin, resulting in the recruitment of Vinculin and subsequent dephosphorylation of the transcriptional coactivator YAP at Ser 127. Dephosphorylated YAP/TAZ then shuttle into the nucleus, where they activate TEA domain family members (TEAD)–mediated gene expression. Activation of the membrane-anchored mechanosensor integrin also promotes Twist phosphorylation at Tyr107, releasing it from the anchoring membrane protein G3BP2, thus promoting its nuclear import and downstream expression of targets contributing to EMT, tumor proliferation, and metastasis. The response is increased through cross talk and cooperation between distinct pathways, i.e., integrin-induced AKT activation by focal adhesion kinase (FAK) also contributes to stabilize β-cat by inhibiting GSK3β. Green arrows or red blunted lines indicate activation or inhibition, respectively. Blue arrows indicate nuclear translocation.

Figure 2.

Pressure and stiffness induced tumorigenesis in vivo. (a) Physical mimicking of pressure from hyperproliferative tumor growth in vivo is achieved by injection of ultramagnetic liposomes in the presence of small strong magnets subcutaneously localized in front of the colon that favor liposome extravasation and endocytosis into the mesenchymal cells that constitute the conjunctive tissue of colonic crypts epithelial cells, which generates the 1-kPa mouse tumor-growth pressure, leading to activation of the β-cat tumorigenic pathway through Y654–β-cat and Ser9-GSK3β phosphorylation in healthy epithelial cells tissue (Fernández-Sánchez et al., 2015). Ultramagnetic liposomes (red) colocalize with the intracellular Vimentin of mesenchymal cells (green), leading to the cytoplasmic orange–yellow labeling that surrounds the cell nuclei labeled with Dapi in blue. (b) Mimicking an increase in fibrotic stiffness in vivo is achieved by transgenic Rock activation in the epidermis. This results in activation of Myosin activity (pMypt1 staining), the increase of stiffness measured by AFM, and the inactivation of GSK3 by Ser9 phosphorylation leading to the tumorigenic β-cat pathway enhancement (Samuel et al., 2011).

Figure 3.

Nucleomechanotransduction. (a) The LINC connects the cytoskeleton to the nucleus through SUN proteins anchored in the inner nuclear membrane and Nesprins anchored in the outer nuclear membrane. Mechanical stress transmission (red stars) occurs through integrins, F-actin, Nesprin, and SUN proteins. Under tension, Lamin-A and -C proteins are assembled in the nuclear lamina then dephosphorylated, leading to protein unfolding, a decrease of solubility, and strengthening of the lamina. Cyt, cytoplasm; i.n.m, inner nuclear membrane; o.n.m, outer nuclear membrane; p.m., plasma membrane. (b) Cell and nuclear deformation occurs during migration through narrow constrictions of the ECM. Yellow depicts the actin–myosin network applying contractile forces onto the nucleus. (c) Cell migration through micrometer-sized constrictions provokes mislocalization of DNA-repair factors, causing DNA damage and leading to permanent heterogeneity in chromosome copy numbers, expression levels, cell shape, and migration ability (Irianto et al., 2017).

Figure 4.

3D tumor tissue manufacturing by bioprinting is composed of three steps. The design of the 3D construct pattern, the fabrication of the 3D structure layer, and the maturation of the 3D tissue. The 3D design and fabrication are usually developed in-house with a computer-aided system. The biofabrication can be performed by seeding the different type of cells into a prefabricated biocompatible 3D scaffold by precisely positioning droplets of cells encapsulated within matrix on a substrate, or by coprinting tumor cells and other cells able to produce ECM. The 3D bioprinted construct is matured in a specific bioreactor with conditioned flow parameters and mechanic stimulations that provide a dynamic environment in which cells can proliferate and differentiate.