CANCER:

SUMMARY: Mechanical forces, e.g. by extracellular matrix remodeling, may be important to cancer metastasis.

How does cancer spread throughout the body? Scientists are continuing to gather evidence that common biochemistry isn't the exclusive actor.

Mechanical forces on cancer cells may play a critical role, e.g. by disrupting the cytoskeleton (intracellular "skeleton," comprised of proteins) and enabling organ penetration. If the forces are large enough, they may even kill the cells.

Using model cells and an artificial extracellular matrix (not living animals), Shalini Menon and Karen Beningo (Wayne State University, United States) have found that pulling and releasing of the extracellular matrix (tissue that surrounds, and is not within, a cell) facilitates invasiveness of cancer cells known to be invasive. They have also identified an intracellular protein (cofilin, which contributes to cytoskeleton disassembly) important to this process.

Experimental design.

The scientists' setup was very simple; an illustration is available here. Cancer cells were seeded on top of an artificial extracellular matrix (a one millimeter thick layer of collagen and fibronectin, sprinkled with micrometer-scale magnetic beads bound to the matrix).

A magnet and rotary mixer below the matrix transiently pulls on the beads, by 0.08 to 5.1 micrometers in the xy-plane (i.e. less force than that experienced by cells in living tissue). It's important to note that the beads move back to their original position after removing the magnet, indicating both bead attachment to the matrix and matrix integrity.

Cell percent invasiveness is calculated by the percentage of them pulled into the matrix over 24 hours. The scientists tested two cancer cell lines known to be invasive (HT1080 and B16F10), and one known to be noninvasive (MEF).

Cancer cell invasiveness.

Few of the scientists' HT1080 cancer cells (10%) penetrated the matrix in the absence of magnetic stimulation. A penetration frequency of 23% was observed with the magnet (i.e. doubling invasiveness); mechanical forces indirectly enabled the cells to penetrate into the artificial extracellular matrix.

MEF cells failed to penetrate. This suggests that noninvasive cells might not be capable of becoming invasive, but the scientists should have tested more than one cell type for this conclusion (respectfully, this should have been demanded by the journal editors, if not the journal peer reviewers).

Fibronectin is clearly important to cancer cell invasiveness, in this experimental setup. Disrupting cell-fibronectin interactions largely reduced penetration frequency to baseline (10%) levels.

The invasiveness values reported here (e.g. the doubling previously reported) are rough estimates; for example, it seems to range from a factor of 1.8 to 3.5, and is not a clean function of the magnet rotation speeds. Scientists performing future research should probably compare penetration results over a constant rotation speed, to avoid confusion.

The scientists systematically eliminated other possible explanations for their results. This is described next.

The beads were not remodeling the artificial extracellular matrix on their own, i.e. not enabing the cells to penetrate through a path of "least resistance." Stimulating the matrix with a magnet for 24 hours, and then seeding it without further magnetic stimulation, resulted in only baseline penetration after a further 24 hours (although further stimulation did result in penetration, as expected).

Furthermore, placing the magnet above the matrix, and waiting for seeded cells to rise into it, gave largely similar results to their previous experimental setup. It's clear that cell penetration isn't a simple "gravity" issue.

Are there any biomolecules important to penetration? Using gene silencing, the scientists identified one: cofilin, a protein important to cytoskeleton disassembly.

Implications.

These experiments provide further evidence that mechanical forces, such as those involved in extracellular matrix remodeling (observed in the vicinity of a tumor), may be important to the spread of cancer (i.e. invasiveness, penetration ability). However, the scientists' suggestion that the cancer cells must be invasive to begin with is inconclusive (not enough evidence).

It would be interesting to perform a similar experiment with cell aggregates. Cells do not exist in isolation, and their connectivity (e.g. in a tumor) is surely relevant, e.g. the maximum yield stress that can be reversibly applied to cell aggregates without permanently altering aggregate morphology.

NOTE: The scientists' research was funded by Wayne State University.

Menon, S., & Beningo, K. A. (2011). Cancer Cell Invasion Is Enhanced by Applied Mechanical Stimulation PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0017277