Apoptotic and necrotic cells as sentinels of local tissue stress and inflammation: response pathways initiated in nearby viable cells

Abstract

Virtually all cells in the body have the capacity to recognize and respond to dead cells. Viable cells discriminate apo from nec targets via distinct cell surface receptors. Engagement of these receptors induces "recognition-dependent" signaling events in viable responding cells that differ for apo vs. nec targets. Although "engulfment-dependent" signaling events also contribute to the response by viable cells, these events do not differ for apo vs. nec targets. While many signaling events are conserved across diverse cell lineages, other signaling events, especially those involving Akt, demonstrate lineage-specific variation. Whereas apo targets activate Akt in MPhi, they inhibit Akt in kidney epithelial cells. Differences in the responses to dead targets by viable migratory cells, such as MPhi, and viable fixed cells, such as kidney epithelial cells, permit cell-specific adaptations to local environmental change or stress. We propose that dead cells (apo and nec) act as sentinels to alert nearby viable cells to local environmental change or stress.

Figure 1

Dead cells act as sentinels of local environmental change. (A) As depicted, the nature of the response of nearby viable cells to dead target cells is determined by three dichotomies: (1) the form of target cell death (apo vs. nec); (2) the dependence of viable cell signals on receptor-mediated recognition vs. engulfment of the dead targets; and (3) the nature of the responding cell (migratory vs. fixed). (B) The responses of MΦ, a migratory cell, are contrasted with those of fixed and organ-specific kidney epithelial cells.