Cell death: ...and a nice Chianti

signaling update home

SOURCE:

Entosis, a form of non-apoptotic cell death in which one cell is completely internalized within another cell, requires activation of the Rho–ROCK pathway and upregulation of myosin II activity in the internalizing cell.

Just like Hannibal Lecter, tumour cells have been associated with cannibalistic tendencies, manifest as a cell-in-cell cytology. Now, Michael Overholtzer, Joan Brugge and colleagues have begun to examine this process and have found the 'victim' to be a more-than-willing participant.


ComStock

While examining the behaviour of MCF10A human mammary epithelial cells under conditions of non-adherence, Overholtzer noticed that a few cells appeared to be internalized within a neighbouring cell. Labelling the cells with red or green fluorescent Cell Tracker dyes or cell-surface biotinylation assays verified that some cells were completely internalized within another cell. The authors investigated whether this was an unusual form of apoptosis; however, standard inhibitors known to disrupt the apoptotic cell death and engulfment pathways had no effect on cell internalization. Similar findings were seen in a variety of non-tumorigenic and tumorigenic epithelial cell lines.

Much of the time-lapse data indicated that the internalized cell was actively involved in the internalization process, so the authors examined whether actin polymerization and/or myosin II contraction were required. Inhibitors of both of these cytoskeletal components prevented internalization, and further experiments showed that the Rho–ROCK pathway, an important regulator of actin and myosin II, is required, but only in the internalizing cell. On the basis of these findings, the authors termed this process entosis, from the Greek word entos, meaning inside, into or within.

The time-lapse data also indicated that the internalizing cell seemed to establish an adherens junction with the outer cell prior to entosis, so the function of the cadherins, which are required to produce these cell–cell junctions, was investigated. Cadherins require calcium to function and entosis was blocked in the presence of calcium chelators or anti-cadherin antibodies.

So what happens to the entotic cells? The most common fate was death by a non-apoptotic mechanism involving lysosomal components, although inhibition of the lysosomal pathway induced apoptosis of the entotic cell. Some internalized cells showed no obvious changes over a period of 20 hours and a few were released from the outer cell.

The authors were able to verify similar behaviour of metastatic breast cancer cells in 7 out of 8 samples obtained from the pleural fluid of patients with breast cancer. Moreover, 11 out of 20 samples from human primary breast cancers and a number of other types of solid tumours also showed evidence of entosis.

So why does entosis occur? Overholtzer, Brugge and colleagues propose that a myosin II-dependent force that is associated with formation of an adherens junction between two cells drives one cell into another because the myosin II-dependent force is unopposed. Normally, the authors argue, the action of myosin II would be balanced by integrin-mediated adherence to the basement membrane. What remains unclear is why, biologically, this process occurs in tumours, although the authors speculate that it might be a tumour-suppressive function that disposes of cells that have lost attachment to a basement membrane but do not undergo anoikis (a specific form of apoptosis) as a result. Further work to explore this and other hypotheses is needed.

Nicola McCarthy

References

Overholtzer, M. et al. A nonapoptotic cell death process, entosis, that occurs by cell-in-cell invasion. Cell 131, 966–979 (2007) | Article | PubMed |