Old, new and emerging functions of caspases

Abstract

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Figures

Figure 1

Domain structure and functional classification of placental mammalian caspases. Caspase-1, -4, -5, -11 and -12 are inflammatory caspases. Apoptotic caspase-2, -8, -9 and -10 are initiators, while caspase-3, -6 and -7 are key executioner caspases. CARD, caspase recruitment domain; DED, death effector domain; L, large subunit; S, small subunit; S*, short form; L*, long form

Figure 2

Caspases in cell death pathways. Caspases have been implicated in apoptosis, necroptosis and autophagy. Apoptosis can proceed in two ways: intrinsic or extrinsic. (a) Intrinsic apoptosis is initiated by a death-inducing stimulus, which causes activation of p53 and pro-apoptotic Bcl-2 proteins such as Bax and Bak. This leads to MOMP and activation of a death-inducing platform called the apoptosome. Following this caspase-9 is cleaved and activates other effector caspases such as caspase-3. Alternatively, caspase-2 gets activated following DNA damage and causes MOMP by cleaving Bid. (b) Extrinsic or receptor-mediated apoptosis involves complex formation by death-domain-containing proteins (called DISC), activation of caspase-8, which initiates apoptosis by directly activating effector caspases. Caspase-8 also cleaves Bid and this causes cytochrome c release, further amplifying apoptosis via the mitochondrial/intrinsic pathway. Depending on ligands (TNF and Fas) different complexes are formed. TNF stimulates TNFR1 that recruits TRADD, RIP1, TRAFs and IAPs into a complex called TRADD-dependent complex I. Subsequent removal of polyubiquitin on RIP1, dissociates this complex and allows it to interact with TRADD, procaspase-8, FADD and cFLIP as complex IIa/DISC. This results in caspase-8 activation, leading to apoptosis. (c) In certain conditions when caspase activity is blocked, RIP1, FADD and the FLIPL–caspase-8 heterodimer, form a large multiprotein complex called the 'ripoptosome' following genotoxic stress. If caspase activity is lost, RIP1 and RIP3 are stabilized and associate in microfilament-like complexes called necrosomes (complex IIb). Recruitment of MLKL complex at the plasma membrane occurs, leading to pore formation and necroptotic cell death. Alternatively, RIPK1 and RIPK3 are inactivated by active caspase-8 and apoptosis occurs. Necroptosis can also occur following stimulation of TNF receptor or activation of Toll-like receptors (TLRs). (d) Some caspases such as caspase-3, mediate cleavage of crucial autophagic proteins and thus indirectly control autophagy. FADD, FAS-associated death domain; TNFR1, TNF receptor 1; TRADD, TNF receptor-associated death domain; cIAP1, cellular inhibitor of apoptosis protein 1; RIP1, receptor-interacting protein 1; MLKL, mixed lineage kinase domain-like; cFLIP, cellular FLICE-like inhibitory proteins; MOMP, mitochondrial outer membrane permeabilization

Figure 3

Caspase-mediated inflammatory responses. (a) Caspase-8 functions to restrict RIG-1 signaling. Activation of the retinoic-acid-like receptors (RLRs) are critical for anti-viral immunity. Following viral infection the caspase-8 DISC complex comprising FADD, TRADD and ubiquitin-conjugated RIP1 (Ub-RIP1) are recruited to the RIG-1 complex. Although Ub-RIP1 can enhance the phosphorylation of IRF3, it is also cleaved by activated caspase-8. Cleaved RIP1 inhibits IRF3 and NFκB thereby reducing the type I interferon (IFN) and IL-1β-/IL-18-mediated inflammatory responses. (b) Inflammasome complex formation is triggered by specific pathogen-associated molecular patterns (PAMPs) or by damage-associated molecular patterns (DAMPs). Activation of Toll-like receptors (TLRs) following pathogen exposure induces NFκB-mediated transcription of IL-1β, IL-18 and also NLRP3, and this is an important step in inflammasome priming and activation. Caspase-1 activation is mediated by its recruitment to different inflammasome complexes (NLRP3, NLRP1, NLRC4 and AIM2) in an ASC-dependent or independent manner. Activated caspase-1 mediates the cleavage and maturation of pro-inflammatory cytokines IL-1β and IL-18. Following acute infection and an acute inflammatory response, caspase-1 induces cell death by pyroptosis

Figure 4

Caspase function in tissue regeneration and tumor cell proliferation. Activation of caspase-3 and -7 during radiotherapy or during tissue injury causes cleavage and activation of iPLA2. This results in AA synthesis and subsequent PGE2 activation. The secreted PGE2 promotes proliferation in surrounding tumor cells by activation of Wnt/β-catenin signaling. This facilitates tumor cell repopulation and acts as a limiting factor of effective therapy. Shedding of microparticles by apoptosing cells (pancreatic beta cells) stimulates proliferation/differentiation of neighboring cells through activation of regenerating (reg) genes. PGE2 also interacts with Wnt signaling pathway to regulate stem cell/progenitor formation and function. iPLA2, calcium-independent phospholipase A2; AA, arachidonic acid; PGE2, prostaglandin E2; HSC, hematopoietic stem cells

Figure 5

Caspase activation occurs during aging, which is normally accompanied by a general decline in stress tolerance and functional strength. On one hand, this increased caspase activity, accelerates aging by increasing inflammation (e.g., caspase-1). Increased activity of caspases also results in removal of stem cells (e.g., loss of satellite cells by caspase-6), while in conditions such as stroke/ischemia, it causes loss of highly differentiated cells such as neurons and cardiomyocytes (e.g., caspase-9), causing functional decline and reduced regenerative ability. In another alternate scenario, loss of apoptosis may also hasten the aging process by preventing removal of damaged cells, increasing genetic instability and oxidative stress (e.g., caspase-2)