J Med Biochem 2013; 32 (3) DOI: 10.2478/jomb-2013-0010 UDK 577.1 : 61 ISSN 1452-8258 J Med Biochem 32: 207–213, 2013 Review article Pregledni ~lanak APOPTOSIS REGULATION BY INHIBITORS OF PROGRAMMED CELL DEATH REGULACIJA APOPTOZE PREKO INHIBITORA PROGRAMIRANE ]ELIJSKE SMRTI Danica Markovi}, Vidosava \or|evi} Center for Medical Biochemistry, Clinical Center, Ni{, Serbia Summary: Apoptosis is a form of cell death which is Kratak sadr`aj: Apoptoza predstavlja oblik }elijske important in many physiological processes. Four apoptotic smrti i va`na je u mnogim fiziolo{kim procesima. Postoje mechanisms have been identified but two have been well ~etiri oblika }elijske smrti a dva su dobro prou~ena: examined: the intrinsic and the extrinsic mechanism. Due to unutra{nji i spolja{nji. Zahvaljuju}i mnogim pro/antiapop- many pro/antiapoptotic factors, these processes take place toti~kim faktorima, ovaj proces se odvija na fiziolo{ki koris- on a physiologically useful level. In cases of apoptosis nom nivou. U slu~aju disregulacije apoptoze nastaju bolesti dysregu lation, illnesses occur such as neurodegenerative dis- kao {to su neurodegenerativna oboljenja udru`ena sa eases combined with an increased level of cell death or can- povi{enim nivoom }elijske smrti ili karcinogeneza udru`ena cerogenesis associated with uncontrolled cell proliferation. sa nekontrolisanom }elijskom proliferacijom. Apoptozu Apoptosis can be triggered by the activation of the first cas- mo`e po krenuti aktivacija prve kaspaze u nizu i prekinuti pase in a series and stopped by its deactivation, which repre- njena deaktivacija, {to predstavlja novi izazov: odrediti sents a new challenge: determining the »point of no return«. »ta~ku bez povratka«. Pored antiapoptoti~nih proteina (Bcl Besides the antiapoptotic proteins (Bcl 2, Bcl XL), a family of 2, Bcl XL), familija proteina nazvanih inhibitori apoptoze proteins called the Inhibitors of Apoptosis Proteins (IAPs) play (eng. Inhibitors of Apoptosis Proteins, IAPs) igra klju~nu a key role in the regulation of apoptosis. Members of the IAP ulogu u procesu regulacije. Pripadnici familije IAP su: family are: cIAP1, cIAP2, XIAP, Survivin, Livin and TsIAP. cIAP1, cIAP2, XIAP, survivin, livin i TsIAP. Domen BIR je Domain BIR is the most important in the IAP structure since naj zna~ajniji u strukturi IAP budu}i da odre|uje it determines their specificity for caspases. The interaction of specifi~nost ka kaspazama. Interakcija IAP sa kaspazama je IAPs with caspases is complex and not completely under- kompleksna i nedovoljno istra`ena, me|utim, smatra se da stood, however, IAPs are considered to be important target IAPs pred stavljaju va`ne ciljne proteine u terapiji tumora i proteins in the therapy of tumor and autoimmune diseases. auto imunih oboljenja. Keywords: IAP protein (apoptosis), caspases, pro- Klju~ne re~i: IAP protein (apoptoza), kaspaze, programi- grammed cell death type I rana }elijska smrt I tipa List of abbreviations: IAP, Inhibitors of Apoptosis Proteins; Address for correspondence: NAIP, Neuronal Apoptosis Inhibitory Protein; RZF, RING zinc- Danica Markovi} finger domain; cIAP, cellular IAP1; XIAP, X chromosome Vizantijski bulevar 124/11 linked IAP; TsIAP, Testis specific IAP; TNFR2, Tumor Necrosis 18000 Ni{, Serbia Factor Receptor 2; NFkB, Nuclear Factor kB; CARD, Caspase Mob.: 069/1983411 Recruitment Domain; NOD, Nucleotide-binding and Oligo- e-mail: bobiªptt.rs merisation domain; XAF1, XIAP-associated factor 1. 208 Markovi}, \or|evi}: Apoptosis and IAPs Introduction and B lymphocyte proliferation and erythrocyte, monocyte and epidermal cell differentiation and mat - Apoptosis is a special form of cell death which uration. The research has, in fact, proved that caspas- normally occurs during the growth and aging and as es could be regulated, and that the Inhibitor of a homeostatic mechanism maintains cell population Apoptosis Proteins (IAP) family members are the in tissues. It is considered to be a vital component of main controllers of this process. The mechanism of various processes (1). Some authors refer to apopto- IAP action is well known, however, there are certain sis simply as a form of cell death used by an organ- variations in the pathways of caspase inhibition and in ism to eliminate unwanted or harmful cells (2). It can the parts of specific proteins’ structure that still need be a consequence of weakening/absence of positive to be explored. The fact is that IAPs have a seeming- signals necessary for cell survival or receiving negative ly simple structure and a very complex function, and signals. Apoptosis can be activated by external and internal stimuli (3). Recent approaches are presented this makes them central molecules in the future ther- with the aim to analyse mechanistic relationships apy research (7–9). between human diseases, and dysregulated apoptosis seems to be connected with the occurrence of certain diseases. Thus, suppression of cellular apoptosis may Function and Inhibition of Caspases lead to the occurrence of tumor or autoimmune dis- Caspases represent the central components of ease, while its increased activation contributes to neu- the apoptosis initiation machinery, and other proteins rological diseases pathogenesis (3–6). (caspases, IAP, Smac/DIABLO, etc.) are responsible The central dogma of the cell apoptosis is the for the regulation of their activity. Caspases involved so-called »point of no return« determination, and for in apoptosis are divided in two groups: the initiator a long period of time it was believed that it was caspases (caspases 2, 8, 9 and 10) and the effector impossible to stop the apoptotic process after the first caspases (caspases 3, 6 and 7). After their activation, caspase activation. Also, there were claims that the the initiation caspases cause cascade activation of caspase activation and apoptosis are the same downstream caspases by proteolysis and this step is process and that caspases are the only proteins difficult to block. Thereby, a complex of specific mol- involved in apoptosis. Later, however, it became clear ecules is needed for the activation of initial caspases that caspases also participate in processes such as: T and those molecules join only if there are extracellu- Figure 1 Mechanism of apoptosis and the way IAPs can inhibit this process (author Danica Markovi}). J Med Biochem 2013; 32 (3) 209 lar and intracellular signals that induce the apoptotic saving of cells that have already entered apoptosis (3, process. After the activation of effector caspases, they 7, 8, 12, 13). However, there is a new question: degrade structural proteins of the cell and its vital ele- Should we »rescue« a cell which is damaged to such ments (actin, proteins that build nuclear lamine, reg- an extent that it started the apoptotic process? ulator proteins, deoxyribonuclease inhibitors (DFF45, ICAD) as well as other proapoptotic proteins and cas- The gene that encodes IAP was firstly identified pases). Antiapoptotic control relies on antiapoptotic in the baculovirus genome and its transcriptome func- proteins activation (Bcl 2, Bcl XL) and IAPs (c-IAP-1, tion was to protect the infected cell from death and SURVIVIN, LIVIN, XIAP, etc.) (4, 10). There are cer- thereby maintain virus replication inside the organ- tain proteins in the cell that inhibit initiator caspases, ism. Later studies in the field of biology have shown but IAPs are the only known endogenous proteins that IAP genes are found in cells of many organisms that regulate the activity of both initiator and effector at different evolution stages. All IAPs contain one to caspases (11). The specific position and function of three BIR (Baillovirus IAP Repeats) domains, each of IAPs in apoptosis can be seen in Figure 1. which consists of approximately 70–80 amino acid residues (3, 9, 13). The first identified IAP homologue in mammals IAP Family was Neuronal Apoptosis Inhibitory Protein (NAIP), IAPs include a family of proteins which contain which contains three BIR domains and a carboxyter- one or more BIR domain in their structure and have minal RING zinc-finger domain (RZF). The NAIP the function of an intrinsic regulator of the caspase encoding gene was isolated during the study of spinal cascade (Ta b l e I ). In modern medicine, IAPs are con- muscular atrophy etiopathogenesis and the genes sidered to be po tential target molecules for the thera- that cause this disease. Although it has not been py of many human diseases. Their potential lies in the proven as directly responsible, it appears that NAIP fact that if they are overexpressed, a cell may become still has an impact on the development of the disease. resistant to apoptotic signals (therapy of neurodege - After the NAIP discovery, other family members were neration), and if they are overinhibited, an increased identified and are represented in the Table I: cellular activation of cell death (therapy of tumors) will appear. IAP1 (cIAP1), cIAP2, X chromosome linked IAP They are also considered to be the only defence (XIAP), Survivin, Livin, Testis specific IAP (TsIAP) (3, against activated caspases and the only factors in the 7, 14). After their identification, cell culture experi- ments showed that IAPs suppress cell death by bind- Table I List of all the IAPs along with all the abbreviations ing to caspases and that they prevent apoptosis used in scientific literature and the specific caspases they caused by both external and internal signals (7). inhibit. Later, it was shown that the biological activity of these proteins in the cell is far more complex and includes: Inhibitor of Caspases they Synonyms binding to caspases and apoptosis inhibition, cell Apoptosis Protein bind to and inhibit cycle regulation and signal transduction modulation API3; BIRC4; Caspase 3 mediated by re ceptors such as TNFR 2 (Tumor Ne - XIAP IAP-3; ILP1; MIHA; Caspase 7 cro sis Factor Re ceptor 2) and NFkB (Nuclear Factor XLP2; hIAP-3 Caspase 9 kB) (3, 12, 15). BIRC2; API1; Caspase 9 cIAP-1 hIAP-2; MIHB; RNF48 IAP Structure BIRC3; AIP1; API2; Caspase 9 There are four domains in the IAP structure: HAIP1; HIAP1; BIR, RZF, CARD (Caspase Recruitment Domain) and cIAP-2 MALT2; MIHC; NOD (Nucleotide-binding and Oligomerisation do- RNF49 main) (8).