The microenvironment has been increasingly recognized as a critical regulator of cancer progression (reviewed in (1
) ). The ECM, a key component of the microenvironment, is in immediate contact with the tumor cells. The ECM functions as a critical source for growth, survival, motility, and angiogenic factors that significantly affect tumor biology and progression. Additionally, cell adhesion to the ECM triggers intracellular signaling pathways that can regulate cell cycle progression, migration, and differentiation, through integrins and other cell surface receptors.(24
) Thus, integrin-mediated interactions between tumor cells and the ECM are critical modulators of the metastatic potential of tumor cells.
Recently, we presented a study that clarifies potential mechanisms by which the microenvironment may regulate tumor dormancy.(32
) In that study, solitary tumor dormancy and the transition to proliferation were recapitulated in vitro
by utilizing a 3D in vitro
culture system constituted from growth factor-reduced basement membrane, to mimic components of the ECM. Our results revealed that in the 3D culture system, cells with dormant behavior in vivo
remained cell cycle arrested with elevated nuclear expression of p16 and p27. Our findings that the ECM can impose growth inhibitory signals on tumor cells were in concordance with previous reports.(33
) Interestingly, the dormant tumor cells displayed distinct cytoskeletal organization with evidence of only transient adhesion to the ECM.(32
) However, we demonstrated that the switch from quiescence to proliferative metastatic growth was strongly influenced by interactions with the ECM as a result of cytoskeletal reorganization and formation of actin stress fibers (). During the transition the tumor cells formed actin stress fibers via β1 integrin signaling and downstream phosphorylation of myosin light chain by myosin light chain kinase. These findings are consistent with previous work implicating β1 integrins in microenvironmental regulation of cell behavior(35
), and were subsequently confirmed by others,(36
) emphasizing the important role of the full engagement of the dormant tumor cell with the ECM as a mechanism to escape tumor dormancy(32
). These observations are also consistent with previous studies in which downregulation of the urokinase receptor was shown to mediate signaling through the α5β1 integrin, forcing the cells into dormancy.(37
) Furthermore, in transgenic mouse models for mammary or pancreatic beta cell cancer, knockdown of β1 integrin resulted in inhibition of proliferation of the mammary tumor cells and senescence of the pancreatic beta tumor cells.(39
) Thus, multiple lines of evidence indicate that lack of adhesion of the tumor cell to the ECM via integrins can lead a tumor cell to enter a dormant phase.
Cytoskeletal reorganization and formation of actin stress fibers during the switch from dormancy to metastatic growth
A solitary dormant tumor cell that fails to properly adhere to the ECM may initiate, under these stress conditions, mechanisms that lead to its long-term survival. For example, anchorage-independent survival of mammary tumors was shown to be mediated by secretion of laminin-5 by the detached mammary tumor cells. Laminin-5 as a component of the basement membrane induced tumor cell survival via α5β1-mediated NFκB activation. (41
). Recently, it has been shown that detachment of epithelial cells from the ECM may lead to another survival mechanism called autophagy. Autophagy is a highly regulated self-digestion process that produces nutrients and energy for the cell through the breakdown of cytosolic components, and can lead to cell survival under stress conditions (reviewed in (42
)). Evidence in the literature suggests that abrogated adhesion of epithelial cells to the ECM may induce autophagy through growth factor- and nutrient-sensing pathways, energy-sensing pathways, and integrated stress response.(42
) Recently, Lu et al.
reported that controlled induction of the tumor suppressor gene aplasia Ras homolog member I (ARHI) within human ovarian tumor cells induces autophagy and tumor dormancy. Interestingly, the tightly regulated autophagy signaling for survival of the cells was dependent on the presence of components from the tumor microenvironment such as ECM proteins. Absence of such factors led to excessive autophagy and programmed cell death.(43
) Thus, failure of dormant tumor cells to properly engage with the ECM may trigger autophagy and promote long-term survival of the cells. In order to subsequently escape tumor dormancy tumor cells need to fully engage with the ECM components via integrin receptor(s), inducing downstream signaling and leading to cytoskeletal reorganization and proliferation.