A NUCLEAR SQUEEZE: THE ROLE OF THE NUCLEUS IN CONFINED CANCER CELL MIGRATION

Abstract

Metastasis is the leading cause of breast cancer deaths worldwide. During cancer metastasis, cells must squeeze through small spaces in the basement membrane, interstitium, and endothelium. A major determinant of a cell’s ability to squeeze through confined spaces is nuclear stiffness, which is governed by lamin expression and chromatin condensation. Although the cell cytoplasm is able to squeeze through submicron obstacles, the cell nucleus has difficulty passing through pore sizes smaller than 5-10 µm. As the largest and stiffest organelle in the cell, the nucleus is a rate-limiting step in cell migration. To better understand the role the nucleus plays in confined migration, we used microfluidic devices with constrictions similar in size to interstitial spaces. We first examined whether lamin A/C expression correlates with increased metastatic potential using a large panel of human and mouse breast cancer cell lines. Highly metastatic cancer cell lines had significantly lower levels of lamins A and C than less aggressive cancer cell lines and normal breast epithelial cells, and they were more successful at squeezing through confined microenvironments. This finding suggests that the expression of lamin A/C may play a role in cells’ ability to metastasize. Our results also showed that tight spaces pose a substantial mechanical challenge to the integrity of the nucleus, often resulting in a transient loss of nuclear envelope (NE) integrity—NE rupture. During NE rupture, a temporary exchange of nuclear and cytoplasmic content occurs. The consequences of nuclear deformation and NE rupture include nuclear fragmentation, chromatin herniation, organelle displacement, and DNA damage. To study whether NE rupture is related to metastatic potential, we created an analysis program to automate the detection and recording of NE rupture events in vitro. Using a panel of breast cancer cell lines, we found that NE rupture rate and duration are independent of metastatic potential. Although the incidence and duration of NE rupture did not vary between cell lines, long-term selection studies using isogenic clonal cell lines indicated that the effect of NE rupture might vary between individual clones from the same parent population. This thesis investigates the role of the NE during confined cell migration and the downstream consequences of NE rupture. We postulate that nuclear deformation and rupture during confined cell migration could promote metastatic progression in cancer cells through the accumulation genomic alterations, including DNA damage, aneuploidy, and genomic rearrangements. Confined migration may represent a novel mechanism by which the physical properties of cells and tissues contribute to breast cancer progression.