Mechanical Ring Interfaces between Adherens Junction and Contractile Actomyosin to Coordinate Entotic Cell-in-Cell Formation

Entosis is a cell-in-cell (CIC)-mediated death program. Contractile actomyosin (CA) and the adherens junction (AJ) are two core elements essential for entotic CIC formation, but the molecular structures interfacing them remain poorly understood. Here, we report the characterization of a ring-like structure interfacing between the peripheries of invading and engulfing cells. The ring-like structure is a multi-molecular complex consisting of adhesive and cytoskeletal proteins, in which the mechanical sensor vinculin is highly enriched. The vinculin-enriched structure senses mechanical force imposed on cells, as indicated by fluorescence resonance energy transfer (FRET) analysis, and is thus termed the mechanical ring (MR). The MR actively interacts with CA and the AJ to help establish and maintain polarized actomyosin that drives cell internalization. Vinculin depletion leads to compromised MR formation, CA depolarization, and subsequent CIC failure. In summary, we suggest that the vinculin-enriched MR, in addition to CA and AJ, is another core element essential for entosis.

Consecutive entosis stages in human substrate-dependent cultured cells

Entosis, or cell death by invading another cell, is typical for tumor epithelial cells. The formation of cell-in-cell structures is extensively studied in suspension cultures, but remains poorly understood in substrate-dependent cells. Here, we used electron, confocal and time-lapse microscopy in combination with pharmacological inhibition of intracellular components to study the kinetics of entosis using two human substrate-dependent tumor cultures, A431 and MCF7. In total, we identified and characterized five consecutive stages of entosis, which were common for both examined cell lines. We further demonstrated that actin filaments in the entotic as well as invading cells were crucial for entosis. Microtubules and the Golgi apparatus of entotic cells provided membrane expansion required for internalization of the invading cell. Depolymerization of microfilaments and microtubules, and disintegration of the Golgi complex inhibited entosis. We confirmed the presence of adhesive junctions and discovered the formation of desmosomes between the invading and entotic cells. The internalized cell was shown to be degraded due to the lysosomal activation in both cells whereas the disintegration of the Golgi apparatus did not affect the process. Thus, in the substrate-dependent cultures, entosis requires microfilaments, microtubules and the Golgi complex for cell invasion, but not for internalized cell degradation.