Death Receptor Stimulation Research Articles

Role of p‐21‐activated kinases in cell survival and apoptotic pathways

A balance between cell survival and cell death is necessary for multicellular organisms to maintain homeostasis. The family of p21-activated kinases (PAKs 1, 2, and 3) are Rho GTPase-regulated serine/threonine kinases implicated in the regulation of a variety of cellular processes. The aim of this review is to describe two mechanisms by which PAKs can influence cellular homeostasis. PAK2 is proteolytically cleaved and activated by caspases after stimulation of death receptors; this activation is implicated in the regulation of biochemical and morphological changes during apoptosis. PAK1 is activated by cell survival factors (for example, interleukin 3) and promotes survival via phosphorylation and inactivation of the pro-apoptotic Bcl-2 family member Bad. Drug Dev. Res. 52:542–548, 2001. © 2001 Wiley-Liss, Inc.

Receptor-mediated control of regulatory volume decrease (RVD) and apoptotic volume decrease (AVD).

A fundamental property of animal cells is the ability to regulate their own cell volume. Even under hypotonic stress imposed by either decreased extracellular or increased intracellular osmolarity, the cells can re-adjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). In most cell types, RVD is accomplished mainly by KCl efflux induced by parallel activation of K+ and Cl- channels. We have studied the molecular mechanism of RVD in a human epithelial cell line (Intestine 407). Osmotic swelling results in a significant increase in the cytosolic Ca2+ concentration and thereby activates intermediate-conductance Ca2+-dependent K+ (IK) channels. Osmotic swelling also induces ATP release from the cells to the extracellular compartment. Released ATP stimulates purinergic ATP (P2Y2) receptors, thereby inducing phospholipase C-mediated Ca2+ mobilization. Thus, RVD is facilitated by stimulation of P2Y2 receptors due to augmentation of IK channels. In contrast, stimulation of another G protein-coupled Ca2+-sensing receptor (CaR) enhances the activity of volume-sensitive outwardly rectifying Cl- channels, thereby facilitating RVD. Therefore, it is possible that Ca2+ efflux stimulated by swelling-induced and P2Y2 receptor-mediated intracellular Ca2+ mobilization activates the CaR, thereby secondarily upregulating the volume-regulatory Cl- conductance. On the other hand, the initial process towards apoptotic cell death is coupled to normotonic cell shrinkage, called apoptotic volume decrease (AVD). Stimulation of death receptors, such as TNF receptor and Fas, induces AVD and thereafter biochemical apoptotic events in human lymphoid (U937), human epithelial (HeLa), mouse neuroblastoma x rat glioma hybrid (NG108-15) and rat phaeochromocytoma (PC12) cells. In those cells exhibiting AVD, facilitation of RVD is always observed. Both AVD induction and RVD facilitation as well as succeeding apoptotic events can be abolished by prior treatment with a blocker of volume-regulatory K+ or Cl- channels, suggesting that AVD is caused by normotonic activation of ion channels that are normally involved in RVD under hypotonic conditions. Therefore, it is likely that G protein-coupled receptors involved in RVD regulation and death receptors triggering AVD may share common downstream signals which should give us key clues to the detailed mechanisms of volume regulation and survival of animal cells. In this Topical Review, we look at the physiological ionic mechanisms of cell volume regulation and cell death-associated volume changes from the facet of receptor-mediated cellular processes.

Role of p‐21‐activated kinases in cell survival and apoptotic pathways

AbstractA balance between cell survival and cell death is necessary for multicellular organisms to maintain homeostasis. The family of p21‐activated kinases (PAKs 1, 2, and 3) are Rho GTPase‐regulated serine/threonine kinases implicated in the regulation of a variety of cellular processes. The aim of this review is to describe two mechanisms by which PAKs can influence cellular homeostasis. PAK2 is proteolytically cleaved and activated by caspases after stimulation of death receptors; this activation is implicated in the regulation of biochemical and morphological changes during apoptosis. PAK1 is activated by cell survival factors (for example, interleukin 3) and promotes survival via phosphorylation and inactivation of the pro‐apoptotic Bcl‐2 family member Bad. Drug Dev. Res. 52:542–548, 2001. © 2001 Wiley‐Liss, Inc.

Hepatic stellate cell behavior during resolution of liver injury.

Acute self-limiting and chronic liver injury are both associated with activation and proliferation of hepatic stellate cells (HSCs). In chronic injury, activated stellate cells are the major source of the collagens that comprise fibrosis and cirrhosis, as well as of the tissue inhibitors of metalloproteinases (TIMPs) which inhibit collagen degradation. Recovery from acute and chronic injury is characterized by apoptosis of activated HSCs, which removes extracellular matrix-producing cells that are also expressing TIMPs, thereby relieving the inhibition of matrix degradation. HSC apoptosis is regulated in progressive injury and counterbalances cell proliferation. Apoptosis probably also represents a default pathway for the HSCs. The survival of activated HSCs in liver injury is dependent on soluble growth factors and cytokines, and on components of the fibrotic matrix. Additionally, stimulation of death receptors expressed on HSCs can precipitate their apoptosis. Our increasing understanding of the process of stellate cell behavior in recovery from injury is likely to be important to the design of antifibrotic therapies.

The ratio of the caspase 3 cleavage products p20: P17 predicts functional caspase activity in human hepatoma cells following stimulation of death receptors

The ratio of the caspase 3 cleavage products p20: P17 predicts functional caspase activity in human hepatoma cells following stimulation of death receptors

ARC, an inhibitor of apoptosis expressed in skeletal muscle and heart that interacts selectively with caspases.

We have identified and characterized ARC, apoptosis repressor with caspase recruitment domain (CARD). Sequence analysis revealed that ARC contains an N-terminal CARD fused to a C-terminal region rich in proline/glutamic acid residues. The CARD domain of ARC exhibited significant homology to the prodomains of apical caspases and the CARDs present in the cell death regulators Apaf-1 and RAIDD. Immunoprecipitation analysis revealed that ARC interacts with caspase-2, -8, and Caenorhabditis elegans CED-3, but not with caspase-1, -3, or -9. ARC inhibited apoptosis induced by caspase-8 and CED-3 but not that mediated by caspase-9. Further analysis showed that the enzymatic activity of caspase-8 was inhibited by ARC in 293T cells. Consistent with the inhibition of caspase-8, ARC attenuated apoptosis induced by FADD and TRADD and that triggered by stimulation of death receptors coupled to caspase-8, including CD95/Fas, tumor necrosis factor-R1, and TRAMP/DR3. Remarkably, the expression of human ARC was primarily restricted to skeletal muscle and cardiac tissue. Thus, ARC represents an inhibitor of apoptosis expressed in muscle that appears to selectively target caspases. Delivery of ARC by gene transfer or enhancement of its endogenous activity may provide a strategy for the treatment of diseases that are characterized by inappropriately increased cell death in muscle tissue.