The TME is characterized not only in terms of chemical composition, but also by physical properties such as stiffness, which influences the morphology, proliferation, and fate of tumor cells. Chen et al. cultured human breast cancer MDA-MB-231 cells on rigid (57 kPa), stiff (38 kPa) or soft (10 kPa) substrates and demonstrated that increasing autophagy levels and autophagic flux in cells cultured on soft substrates partly attenuated soft substrate-induced apoptosis. Mechanistically, this protective autophagy is regulated by intracellular reactive oxygen species (ROS) accumulation, which triggers downstream signals of JNK, Bcl-2, and Beclin-1. More importantly, soft substrate-induced activation of ROS/JNK signaling promotes cell apoptosis through the mitochondrial pathway, whereas it increases protective autophagy by suppressing the interaction between Bcl-2 and Beclin-1 [
55]. These data suggest that JNK is a mediator of soft substrate-induced breast cancer cell apoptosis and autophagy which is likely to be the mechanism that partly attenuates mitochondrial apoptosis. This study provides new insights into the molecular mechanisms by which autophagy plays a protective role against soft substrate-induced apoptosis in human breast cancer cells. Stiffening of the ECM during tumor progression results in increased cancer cell motility. During cell migration, two major isoforms of non-muscle myosin II (NMII), NMIIA and NMIIB, are expressed and assembled into the cytoskeleton. However, the isoform-specific regulatory roles of NMIIA and NMIIB as well as the underlying mechanisms in response to mechanical cues of the ECM remain elusive. Based on polyacrylamide (PAA) gels with tunable elastic moduli, Peng et al. mimicked the mechanical properties of tumor tissue at different stages of breast cancer
in vitro and investigated the distinct roles of NMII isoforms in the regulation of substrate stiffness [
56]. They demonstrated that NMIIA is engaged in establishing cell polarity by facilitating lamellipodia formation, focal adhesion turnover, and actin polymerization at the cell leading edge, whereas NMIIB is recruited to the cell perinuclear region and contributes to traction force generation and polarized distribution, both in a substrate stiffness-dependent manner. They further validated that substrate stiffness modulates the distribution and activation of NMII isoforms via the Rac1/p-PAK1/pS1916-NMIIA and PKCζ/pS1935-NMIIB signaling pathways in a site- and kinase-specific phosphoregulation manner [
56]. This study allowed for an understanding of the mechanotransduction of cancer cells and provides inspiration for molecular targets in antimetastatic therapy. The survival of cancer stem cells is usually limited to a specific TME, which plays a vital role in tumor development. The mechanical properties of the TME differ in different regions of solid tumors. Sun et al. constructed a rat liver pathological model demonstrating sequential progression from hepatitis to liver fibrosis/sclerosis culminating in liver cancer, and analyzed the mechanical properties of liver tissues during the pathological process. Liver cancer tissues showed significant mechanical heterogeneity, and the distribution of liver cancer stem cells (LCSCs) in liver cancer tissues was significantly correlated with the mechanical heterogeneity of liver cancer tissues [
57]. They also used an oxidized sodium alginate hydrogel to establish a three-dimensional cell culture system and studied the effects of substrates of different stiffness on the stemness of LCSCs. It was confirmed that the stemness of LCSCs could respond to changes in the stiffness of substrates through the force-sensitive molecule YAP [
58]. Based on the mechanical properties of the ECM of liver cancer tissue, Chen et al. constructed a ZIF-90 nano-drug delivery system, modified with osmopeptide and simultaneously embedded anti-cancer drug adriamycin and Notch signaling pathway inhibitor of tumor stem cells, and confirmed that the nano-drug could effectively penetrate the liver tumor tissue and kill liver cancer cells and tumor stem cells, thus improving the antitumor effect [
59].