Physical Forces and Transient Nuclear Envelope Rupture during Metastasis: The Key for Success?

Abstract

During metastasis, invading tumor cells and circulating tumor cells (CTC) face multiple mechanical challenges during migration through narrow pores and cell squeezing. However, little is known on the importance and consequences of mechanical stress for tumor progression and success in invading a new organ. Recently, several studies have shown that cell constriction can lead to nuclear envelope rupture (NER) during interphase. This loss of proper nuclear compartmentalization has a profound effect on the genome, being a key driver for the genome evolution needed for tumor progression. More than just being a source of genomic alterations, the transient nuclear envelope collapse can also support metastatic growth by several mechanisms involving the innate immune response cGAS/STING pathway. In this review we will describe the importance of the underestimated role of cellular squeezing in the progression of tumorigenesis. We will describe the complexity and difficulty for tumor cells to reach the metastatic site, detail the genomic aberration diversity due to NER, and highlight the importance of the activation of the innate immune pathway on cell survival. Cellular adaptation and nuclear deformation can be the key to the metastasis success in many unsuspected aspects.

Keywords: EMT; SASP; cGAS/STING; chromosomal instability; circulating tumor cells; mechanostress; metastasis; nuclear envelope rupture.

Figure 2

Mechanical challenges during cell migration lead to transient Nuclear Envelope Rupture (NER). (A) Homeostasis in physical forces from the environmental stiffness of the Extra-Cellular Matrix (ECM) (blue arrows), the cytoskeleton (green arrows), and the nuclear rigidity (black arrows—chromatin rigidity and NE composition). At focal adhesions (violet ovals) the mechanical forces are transferred from the ECM to the cytoskeleton, which then transfers them to the nucleus via the LINC complex (blue ovals). (B) During migration, cells can confront diverse ECM structures such as narrow spaces and pores, channels generated by protease action, or by physical forces from the invadopodia from the previous passage of other cells, chemoattractant gradients, and different densities of specific fibers (Figure adapted from Yamada K.M. et al. [41]). (C) A cell passing through a constriction will rotate and position its nucleus to favor the required nuclear deformation. Cytoskeleton forces and extreme nuclear curvation during passage in a constricted area can lead to a transient NER, disrupting the proper nuclear compartmentalization. Unprotected DNA is then in contact with the cytoplasm until NE is repaired, restoring proper nuclear compartmentalization. (D) During cell migration, the cytoskeleton might also apply excessive forces to the nucleus, leading to blebbing and transient NER.

Figure 1

Mechanical challenges affecting tumor cells during migration. (A) Schematic representation of a normal blood vessel within its environment. (B) Intravasation of tumors cells into the circulation. Invading tumor cells cross the basement membrane and migrate through the connective tissue to reach the endothelial wall, which they cross to enter in the blood stream. All these steps involve important squeezing of the cells and their nuclei (hatched circle) imposing mechanical challenges. Tumor cells in the blood stream can be found as single Circulating Tumor Cells (CTC) or as CTC clusters.

Figure 3

Genomic instability associated with nuclear squeezing and nuclear envelope rupture leads to the generation of genomic diversity. Nuclear deformation and nuclear envelope collapse, displaying a transient loss of compartmentalization, lead to replication stress and DNA damage. This DNA damage can initiate a cascade of genomic alterations in the next mitosis with the generation of micronuclei and telomere fusion, provoking a chromatin bridge. Such events are known to initiate a myriad of diverse genomic events such as chromothripsis, extra chromosomal DNA (ecDNA), the hypermutated pattern Kataegis, events of insertions, deletion, and translocation, as well as the mutational signature APOBEC (Adapted from Gauthier BR, et al. [27]).

Figure 4

Modes of metastatic dissemination from the primary tumor. (A) The primary tumor is composed of a multiclonal population with cells competing between each other. Metastatic tumors are formed by clones found in the primary site, as well as by new independent subclones. (B) The phylogenetic tree shows the history of the tumor evolution. Genomic diversity can arise at all steps of tumor progression. Importantly, the metastatic site can be composed of clonal populations found in the primary tumor site or independent subclones.

Figure 5

The activation of cGAS can support metastasis survival. (A) Activation of inflammatory genes through the detection of double strand DNA (ds-DNA) by the enzyme cGAS. Double strand-DNA bound cGAS induces the production of the second messenger cGAMP that in turn activates STING, leading to the transcription of several inflammatory response genes. cGAMP can also be a paracrine signal by being released in the extracellular compartment or transferred to other cells. cGAS pathway is involved in several processes such as alerting the immune cells but is also involved in senescence, autophagy, and surprisingly in favoring metastasis survival. (B) cGAS activation in metastatic cells. (1) cGAMP supports cell own growth as an autocrine signal by the induction of inflammatory genes. (2) To avoid extracellular cGAMP release and activation of immune cell attack, cancer cells express ENPP1 that selectively hydrolyze the extracellular pool of cGAMP. (3) In the context of brain metastasis, cGAMP can transfer to neighbor astrocyte cells by carcinoma–astrocyte gap junctions. This paracrine signal supports the growth of metastatic cells by the astrocytes.