Nuclear Checkpoints for Melanoma and Breast Cancer Lung Metastasis

Introduction

Metastasis involves cancer cell intravasation and extravasation from blood vessels at target organs. The mechanisms by which the nuclei of metastatic tumor cells squeeze through different vascular and epithelial barriers are poorly understood. The nuclear lamina attaches chromatin domains to the nuclear periphery and controls the mechanical properties of the nucleus and its crosstalk with the cell cytoskeleton. A-type lamins, lamin A and its splice variant lamin C, are key nuclear lamina proteins that control nucleus stiffness and regulate chromatin conformation. Reduced lamin A was reported in some aggressive cancers.

Results and Discussion

Using a new in vitro transendothelial migration (TEM) assay we found that melanoma and breast cancer cells slowly squeeze their nuclei and complete TEM through endothelial junctions irrespective of their relative lamin A/C levels, suggesting that lamin A/C dependent nuclear stiffening is not a rate-limiting step for tumor squeezing through endothelial cells. We currently explore whether reduced lamin A/C expression enhances the emigration of murine melanoma cells and breast cancer cells across lung capillaries and alveolar epithelial barriers of syngeneic mice in vivo. Emigration analysis is performed by new 3D imaging of fluorescently labeled cancer cells with high and low lamin A/C levels. This imaging is based on light sheet microscopy of lipid cleared lungs of recipient mice in situ labeled with anti-vascular CD31 mAb. We have recently found that reduced lamin A/C levels affect the constitutive heterochromatin of melanoma cells and accelerate cell cycle progression and tumor growth in vitro. Hence, we are currently testing if reduced levels of transcriptionally repressed chromatin also affect the growth of tumor cells with variable lamin A/C levels after they cross the lung vasculature and enter the lung parenchyma. Since reduced levels of lamin A/C in the nuclear lamina may also enhance nuclear rupture in tumor cells squeezing through confined and rigid spaces, we also assess if laminA/C silencing in melanoma and breast cancer cells reduce their survival inside the lung parenchyma.

Conclusions

Our results suggest that subsets of tumor cells may benefit from reducing their nuclear content of lamin A/C both at the level of barrier crossing and at the level of post extravasation proliferation. These subsets may, however, need to harness machineries to repair their ruptured nuclear envelopes. How alterations in lamin A/C content contribute to epigenetic tumor heterogeneity and metastatic potential remains an open question.