Apoptosis and autophagy: decoding calcium signals that mediate life or death

Abstract

Calcium is a versatile and dynamic 2nd messenger that is essential for the survival of all higher organisms. In cells that undergo activation or excitation, calcium is released from the endoplasmic/sarcoplasmic reticulum to activate calcium-dependent kinases and phosphatases, thereby regulating numerous cellular processes; for example, apoptosis and autophagy. In the case of apoptosis, endogenous ligands or pharmacological agents induce prolonged cytosolic calcium elevation, which in turn leads to cell death. In contrast, there is now evidence that calcium regulates autophagy by several mechanisms, and these may be important for maintaining cell survival. Here we summarize what is known about how calcium regulates these life and death decisions. We pay particular attention to pathways that have been described in lymphocytes and cardiomyocytes, as these systems provide optimal models for understanding calcium signaling in the context of normal cell physiology.

Figure 1.

Calcium signaling mediated by T-cell activation. In a CD4 positive T cell, activation is induced by antigen binding to the T-cell receptor (TCR) and a co-stimulatory signal mediated by ligation of CD28. Src family kinases Fyn and Lck are activated by autophosphorylation and recruited to the plasma membrane to associate with CD3 (TCR) and CD4, respectively. This in turn leads to the phosphorylation of ZAP-70 and the adaptor protein LAT in order to activate phospholipase C and generate 2nd messengers IP₃ and diacylglycerol (DAG). IP₃ binds to the IP₃R, resulting in ER calcium release. Calcium is released into the cytosol by way of a single calcium transient (left), which is associated with cell death by apoptosis. On the other hand, calcium oscillations activate calcineurin, which dephosphorylates NFAT, thereby sending it to the nucleus to activate transcription of IL-2. The Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA) is responsible for maintaining the appropriate concentration of luminal calcium by actively transporting calcium across the ER membrane.

Figure 2.

Calcium signaling mediated by angiotensin II hormone in a cardiomyocyte. In a cardiomyocyte, calcium signaling is mediated by the influx of calcium through L-type calcium channels (LTCC). The Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA) is responsible for maintaining the appropriate concentration of luminal calcium by actively transporting calcium ion across the SR membrane, while ryanodine receptors and IP₃Rs promote its release into the cytosol. Angiotensin, a peptide hormone, binds to the AT-1 receptor (AT-1R), a G-protein coupled receptor that activates phospholipase C (PLC) following GTP hydrolysis, thereby generating IP₃ and diacylglycerol (DAG). It should be noted that ryanodine receptors are 50- to 100-fold more abundant than IP₃Rs in cardiomyocytes. Therefore, calcium release via ryanodine receptors have shown to be much more robust compared to calcium responses that are mediated by IP₃Rs (Kockskamper et al. 2008). Nevertheless, there is unequivocal evidence for the contribution of IP₃Rs during SR-calcium release, which is likely due to synergy between the two calcium channels in mediating calcium-induced calcium release.

Figure 3.

The Bcl-2-IP₃R interaction inhibits ER-calcium release. Bcl-2 localizes to the ER where it binds IP₃Rs to inhibit calcium transients. In T cells, calcium transients are activated in response to strong T-cell receptor ligation, which results in apoptosis that can be inhibited by Bcl-2. In contrast, calcium oscillations that are associated with cell survival are promoted by Bcl-2 and Bcl-xL. In addition, Bcl-2 regulates the level of ER luminal calcium by increasing membrane permeability or by interacting with the Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA). Bcl-2 also interacts with calcineurin, thereby forming a complex with both calcineurin and IP₃Rs on the ER membrane.

Figure 4.

Electron micrograph of a malignant T cell undergoing autophagy. T cells that ectopically express Bcl-2 readily undergo autophagy in the presence of glucocorticoid hormones. Here, WEHI7.2 T cells stably expressing Bcl-2 (to inhibit apoptosis) were treated with 10⁻⁶ M dexamethasone for 72 hours and visualized by electron microscopy. (A) An electron micrograph of a single cell or (B) a region from within a cell. Examples of autophagosomes are shown next to the arrows.