Caspase-9 as a therapeutic target for treating cancer.

Review

Caspase-9 as a therapeutic target for treating cancer Bonglee Kim, Sanjay K Srivastava & Sung-Hoon Kim† †

1.

Introduction

2.

Regulators of caspase-9

3.

Caspase-9 activation by natural

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compounds 4.

Clinical trials for caspase-9

5.

Conclusion

6.

Expert opinion

Kyunghee University, College of Korean Medicine, Cancer Preventive Material Development Research Center, Seoul, South Korea

Introduction: Caspase-9 is the apoptotic initiator protease of the intrinsic or mitochondrial apoptotic pathway, which is activated at multi-protein activation platforms. Its activation is believed to involve homo-dimerization of the monomeric zymogens. It binds to the apoptosome to retain substantial catalytic activity. Variety of apoptotic stimuli can regulate caspase-9. However, the mechanism of action of various regulators of caspase-9 has not been summarized and compared yet. In this article, we elucidate the regulators of caspase-9 including microRNAs, natural compounds that are related to caspase-9 and ongoing clinical trials with caspase-9 to better understand the caspase-9 in suppressing cancer. Areas covered: In this study, the basic mechanism of apoptosis pathways, regulators of caspase-9 and the development of drugs to regulate caspase-9 are reviewed. Also, ongoing clinical trials for caspase-9 are discussed. Expert opinion: Apoptosis has crucial role in cancer, brain disease, aging and heart disease to name a few. Since caspase-9 is an initiator caspase of apoptosis, it is an important therapeutic target of various diseases related to apoptosis. Therefore, a deep understanding on the roles as well as regulators of caspase-9 is required to find more effective ways to conquer apoptosis-related diseases especially cancer. Keywords: apoptosis, apoptosis-activating factor-1, apoptosome, cancer, caspase-3, caspase-9 Expert Opin. Ther. Targets (2015) 19(1):113-127

1.

Introduction

Apoptosis is a programmed cell death, which plays critical roles in development, immune system and homeostasis of a multicellular organism [1]. Failure of this process can cause serious diseases such as cancer, Alzheimer’s disease, stroke, viral infection or autoimmune diseases [2-6]. The proteins responsible for inducing apoptosis could be targeted therapeutically to overcome resistance to apoptosis. Therefore, understanding the mechanism of apoptosis is critical for a successful therapy. Caspases are cysteine-dependent aspartate-specific proteases, involved in the process of apoptosis. Caspases can be divided into two groups; initiator caspases, such as caspase-2, -8, -9 and -10, and effector caspases, such as caspase-3 and -7 [1,7]. Caspase-9 activity is enhanced by phosphorylation at Tyr153 [8] and inhibited by phosphorylation at Thr125 [9-12], Ser196 [13], Ser144 [14] and Ser348 [15]. Unlike effector caspases, caspase-9 does not need to be cleaved, but just dimerized to be activated. Two different pathways exist in apoptosis process, extrinsic and intrinsic pathway. The binding of death receptors with their respective ligands results in the sequential activation of caspase-8 and -3 in extrinsic apoptosis. This pathway is downregulated by cFLIP and human X-chromosome-linked inhibitor of apoptosis protein (XIAP). DNA-damaging agents activate mitochondrial or intrinsic pathway by inducing the release of the cytochrome c. Cytochrome c, a peripheral protein of 10.1517/14728222.2014.961425 © 2015 Informa UK, Ltd. ISSN 1472-8222, e-ISSN 1744-7631 All rights reserved: reproduction in whole or in part not permitted

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Article highlights. . .


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Caspase-9 participates in apoptosis process as initiator caspase. In the process of intrinsic apoptosis pathway, cytochrome c is released in cytosol from mitochondria, forms apoptosome with apoptosis-activating factor-1 and caspase-9, activating caspase-9. Caspase-9 is regulated by several signal pathways such as hepatocellular carcinoma antigen 66, histone H1.2, putative HLA-DR-associated protein, nucleotide-binding domain and caspase recruitment domain, death effector filament-forming Ced-4-like apoptosis protein, Nucling, Akt, Apaf-1-interacting protein, c-Abl (an ubiquitous nonreceptor tyrosine kinase), CDK1/cyclin B1, casein kinase 2, dual-specificity tyrosine-phosphorylationregulated protein kinase, MAPK, hepatitis B X-interacting protein, heat shock protein 70, protein kinase A, PKC, tumor-upregulated caspase-associated recruitment domain (CARD)-containing antagonist of caspase nine, human X-chromosome-linked inhibitor of apoptosis protein and microRNAs. A large number of natural compounds were reported to enhance caspase-9 and suppress cancer by inducing apoptosis. Two clinical trials with caspase-9 as a suicide gene for T cells against leukemia are undergoing. Future research should focus on finding safer and more effective drugs, based on a better understanding of caspase-9.

This box summarizes key points contained in the article.

recruitment domain (NAC), death effector filament-forming Ced-4-like apoptosis protein (DEFCAP), Nucling and inhibited by Akt, Apaf-1-interacting protein (APIP), c-Abl (an ubiquitous nonreceptor tyrosine kinase), cyclin-dependent kinases 1(CDK1)-cyclin B1, casein kinase 2 (CK2), dualspecificity tyrosine-phosphorylation-regulated protein kinase (DYRK1A), extracellular signal-regulated kinase (ERK)2, hepatitis B X-interacting protein (HBXIP), heat shock protein 70 (Hsp70), protein kinase A (PKA), protein kinase C (PKC), p38a, tumor-upregulated caspase-associated recruitment domain (CARD)-containing antagonist of caspase nine (TUCAN), XIAP and miRNA-24a, -133, -585, -23a (Table 1), (Figure 1). HCA66 Microdeletions of HCA66 around NF1 gene are frequently associated with a severe form of neurofibromatosis type I microdeletion syndrome in patients. The patients usually show a more severe clinical phenotype, including frequent tumor incidence than other NF1 patients [34]. HCA66 mRNA is expressed in most human tissues but expressed highly in heart, liver and placenta [35]. Piddubnyak et al. reported that HCA66 significantly increased the interaction of caspase-9 with Apaf-1 in 293 T cells. On the other hand, depletion of HCA66 inhibited recruitment of caspase-9 by Apaf-1.The N-terminus of HCA66 showed inhibitory effect on caspase9-related cell death [35]. 2.1

Histone H1.2 Mass spectrometry analysis showed that histone H1, H2A and H2b have regulatory effects on caspases. Ruiz-Vela et al. developed a novel nonhypotonic cell-free system based on the zwitterionic detergent CHAPS to further study its interaction. As a result, pro-apoptotic histone H1.2 but not H1.0, H1.3, H1.4 or H1.5 induced activation of caspase-3,-7 and -9. Also, histone H1.2 interacts with Apaf-1, caspase-9, caspase-3 and cytochrome c after UV irradiation [36]. 2.2

the mitochondrial inner membrane, functions as an electron shuttle [16]. Cytochrome c once released in the cytosol forms apoptosome with apoptosis-activating factor-1 (Apaf-1) and procaspase-9 [17]. Procaspase-9 is activated by apoptosome, which in turn activates procaspase-3 to caspase-3 leading to apoptosis. Caspase-9 is a key caspase in intrinsic apoptosis pathway. Mutation in caspase-9 gene results in embryonic lethality and impaired brain development [18]. Like other initiator caspases, procaspase-9 is an inactive monomer (zymogen) at physiological conditions [19]. Apoptosome binds to multiple procaspase-9 and promotes the dimerization of caspase-9 resulting in its activation [20]. In other words, apoptosome serves as a recruitment platform that promotes procaspase-9 activation [7]. A number of epidemiological and experimental studies have reported caspase-9-mediated apoptosis by natural compounds, including isothiocyanates [21-23], emodin [24,25], tanshinone IIA [26-29], ursolic acid [30] and decursin [31-33].

Putative HLA-DR-associated protein A small molecule, PETCM (a-(trichloromethyl)-4-pyridineethanol) is known as apoptosome formation enhancer and caspase-3 activator. Jiang et al. showed that there are Q-ft, Q30 and Q100 proteins in HeLa cell extracts. These three proteins were identified by chromatography from Q100, PHAPI, PHAPI2a and PHAPIII. PHAP did not affect the efficiency of apoptosome formation but enhance caspase-9 activation after apoptosome formation in mammalian cells [37]. 2.3

Nucleotide-binding domain and caspase recruitment domain

2.4

2.

Regulators of caspase-9

Caspase-9 is regulated by many factors. It has been reported that caspase-9 is activated by hepatocellular carcinoma antigen 66 (HCA66), histone H1.2, putative HLA-DR-associated protein (PHAP), nucleotide-binding domain and caspase 114

CED4 family proteins play role in programmed cell death [38]. CED3 binds to oligomerized CED4, exerting protease activation [39]. According to Chu et al., NAC is expressed in kidney, brain and epidermis. Overexpressed NAC showed synergistic effect with Apaf-1 and pro-caspase-9 in the activation of

Expert Opin. Ther. Targets (2015) 19(1)

Role of caspase-9 in apoptosis

Table 1. Regulators of caspase-9.


Regulator

Efficacy

Mechanism

System

HCA66

Activation or inhibition

In vitro

HEK293T, HeLa, MCF-7

[35]

Histone H1.2

Activation

In vitro

MEFs

[36]

PHAP

Activation

In vitro

HeLa

[37]

NAC

Activation

In vitro

HEK293T, human tissue

[40]

DEFCAP Akt

Activation Inhibition

In vitro In vitro

HEK293, K562, MCF-7 HEK293T, 267B, DLD-1

[41] [13]

APIP c-Abl

Inhibition Inhibition

In vitro In vitro

C2C12, HEK293 U-937

[51] [8]

CDK1/ Cyclin B1 CK2

Inhibition

In vitro

HeLa

[9]

In vitro

MEFs, FL5.12

[15]

DYRK1A

Inhibition

In vitro

HeLa, U2OS

[12,64]

ERK


In vitro, In vivo In vitro

HeLa, U2OS, Xenopus egg extract Jurkat

[10,11,154]

HBXIP Hsp70

Inhibition

In vitro

Jurkat, HEK293T

[71,72]

PKA

Inhibition

In vitro

U2OS, HeLa, HEK293

[76]

PKC

Inhibition

In vitro

U2OS, HeLa, HEK293

[14]

TUCAN

Inhibition

In vitro

Jurkat, HEK293T, MCF7

[79]

XIAP

Inhibition

Increases casp-9 recruitment to the apoptosome or blocks casp-9 recruitment Enhances association of casp-9 and apoptosome Enhances casp-9 activity after apoptosome formation Increases casp-9 recruitment to the apoptosome Enhances casp-9 activity Inhibits casp-9 by phosphorylation at Ser196 Inhibits casp-9 binding to Apaf-1 Inhibits casp-9 by phosphorylation at Tyr153 Inhibits casp-9 by phosphorylation at Thr125 Inhibits casp-9 by phosphorylation at Ser348 Inhibits casp-9 by phosphorylation at Thr125 Inhibits casp-9 by phosphorylation at Thr125 Binds to survivin, inhibiting recruitment of casp-9 to Apaf-1 Prevents oligomerization of Apaf-1 or recruitment of casp-9 to Apaf-1 Prevents recruitment of casp-9 to Apaf-1 Inhibits casp-9 by phosphorylation at Ser144 Binds to procasp-9 and suppresses its activation Inhibits casp-9 activity by blocking homo-dimerization

In vitro

Escherichia coli Bl21, HEK293, Jurkat

[83] [84]

Inhibition

Cell lines

Ref.

[69]

DEFCAP: Death effector filament-forming Ced-4-like apoptosis protein; ERK: Extracellular signal-regulated kinase; HBXIP: Hepatitis B X-interacting protein; Hsp70: Heat shock protein 70; NAC: Nucleotide-binding domain and caspase recruitment domain; PHAP: Putative HLA-DR associated protein-1; PKA: Protein kinase A; PKC: Protein kinase C; TUCAN: Tumor-up-regulated CARD-containing antagonist of caspase nine; XIAP: Human X-chromosome-linked inhibitor of apoptosis protein.

caspases and induction of apoptosis. Association of NAC and Apaf-1increased recruitment and proteolysis process of pro-caspase-9 after cytochrome c stimulation [40]. Death effector filament-forming Ced-4-like apoptosis protein

2.5

DEFCAP is a member of the mammalian Ced-4 family of apoptotic proteins. The mammalian Ced-4 proteins (Apaf-1, Nod1 and DEFCAP) contain CARD. Human DEFCAP mRNA is expressed in liver, spleen and K562, a chronic myelogenous leukemia cell line. DEFCAP has two isoforms, DEFCAP-L and DEFCAP-S. Overexpression of DEFCAPL, but not DEFCAPS-S, induced apoptosis in MCF-7, a breast cancer cell line. Hlaing et al. indicated that DEFCAP may play a role in targeting caspase-9 but the mechanism remains unclear [41].

Nucling Nucling is a highly expressed protein during cardiac muscle differentiation and isolated from murine embryonal carcinoma cells [42]. Nucling has inhibitory effects on galectin-3, which blocks apoptosis process [43]. A bovine homolog of Nucling, b CAP73 is b-actin-specific binding protein, regulates b-actin assembly [44]. Sakai et al. reported that Nucling was expressed in an Apaf-1/pro-caspase-9 complex after UV irradiation. Nucling has role in apoptosis, specifically, stabilizes apoptosome and is required for the translocation of Apaf-1 to the nucleus [45]. 2.6

Akt Akt, a serine threonine protein kinase B, plays a critical role in malignant transformation and subsequent processes of growth, proliferation and metastases [46]. It is reported that 2.7


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P Ser196

P Tyr153 P Thr125 P Ser348 P Thr125 P Thr125

HCA66 Histone H1.2


PHAP NAC DEFCAP

Akt APIP

HBXIP

C-Abl

Hsp70

CDK1/Cyclin B1

PKA

CK2

PKC

DYRK1A

TUCAN

ERK

XIAP

P Ser144 miR-133 miR-24a miR-582-5p miR-23a

Caspase-9

Cytochrome c Apaf-1

Caspase-3

Apoptosis

Figure 1. Regulation of the caspase-9. APIP: Apaf-1-interacting protein; DEFCAP: Death effector filament-forming Ced-4-like apoptosis protein; ERK: Extracellular signal-regulated kinase; HBXIP: Hepatitis B X-interacting protein; Hsp70: Heat shock protein 70; NAC: Nucleotide-binding domain and caspase recruitment domain; PKA: Protein kinase A; PKC: Protein kinase C; PHAP: Putative HLA-DR-associated protein; TUCAN: Tumor-upregulated caspase-associated recruitment domain (CARD)-containing antagonist of caspase nine; XIAP: Human X-chromosome-linked inhibitor of apoptosis protein.

activation of Akt especially if PTEN expression is lost, is associated with a poor 5-year survival in several cancers [47]. Akt inhibits cytochrome c induced cleavage of pro-caspase-9. Akt phosphorylates pro-caspase-9 at Ser196 and inactivates it in vitro and in vivo but not any other caspases such as caspase-3 or caspase-8 [13].

phosphorylates caspase-9 at Tyr-153, attenuating the activity of caspase-9-induced apoptosis. Conversely, when c-Abl is inhibited by STI571, the process of caspase-9 phosphorylation is attenuated, indicating that caspase-9 is downregulated by c-Abl. [8]. CDK1/cyclin B1 The cell cycle is one of the most significant mechanism in growth and development, and its deregulation in many human disorders. Researchers have greatly expanded knowledge on the cell cycle and have contributed to the universally accepted view of how the basic cell cycle machinery is regulated [56]. CDK1 is one of the family of enzymes essential for the progression of the cells. Moreover, checkpoint control and DNA repair, requires the phosphorylation of a wide variety of target substrates by CDK [57]. Studies have shown that caspase-9 is regulated by CDK1/cyclin B1 by phosphorylation at Thr125, a single major inhibitory site during mitosis [9]. 2.10

2.8

Apaf-1-interacting protein

APIP has been reported to inhibit two main types of programmed cell death, caspase-1-dependent pyroptosis and caspase-9-dependent apoptosis [48,49]. Also, APIP induces the activation of Akt and ERK1/2 [50]. Cho et al. revealed that APIP acts as a negative regulator of ischemic injury. APIP binds competitively to CARD of Apaf-1 with caspase-9. It also inhibits apoptosis process by suppressing the downstream signal of cytochrome c [51]. c-Abl c-Abl plays role in cell proliferation, differentiation, apoptosis and cell adhesion [52,53]. In response to DNA damage, c-Abl is activated and phosphorylates caspase-9 at Tyr-153 [54,55]. According to Raina et al., c-Abl binds directly to caspase-9, 2.9

116

Casein kinase 2 Casein kinase 2 (protein kinase, CK2) is a eukaryotic serine/ threonine kinase with multiple substrates and affects several 2.11




signaling pathways in differentiation, proliferation, stress response, DNA damage, circadian rhythm and apoptosis [58,59]. It is overexpressed in many cancer cells and protects cancer cells from apoptosis [60]. Unlike other protein kinases, constitutively active CK2 has two catalytic subunits, a and a¢, and two regulatory b units [61]. McDonnell et al. revealed that CK2 phosphorylates murine caspase-9 at Ser348 in vitro and interferes with its cleavage at Asp353 by caspase-8 during intrinsic apoptosis [15]. 2.12 Dual-specificity tyrosine-phosphorylationregulated protein kinase 1A

The dual-specificity tyrosine-phosphorylation-regulated protein kinase (DYRK) family comprises at least seven mammalian isoforms (DYRK1A, DYRK1B, DYRK1C, DYRK2, DYRK3, DYRK4A and DYRK4B), the yeast homolog Yak1p and the Drosophila kinase minibreak [62]. The human gene for DYRK1A is a candidate gene for mental retardation in Down’s syndrome [63]. Seifert et al. revealed that DYRKS1A plays role in inhibitory phosphorylation of caspase-9 at Thr125 and involves its co-localization in the nucleus [12,64]. MAPK All eukaryotic cells have multiple MAPK pathways, which coordinately regulate diverse cellular activities running the gamut from gene expression, mitosis and metabolism to motility, survival, apoptosis and differentiation. It is reported that five distinct groups of MAPKs have been characterized in mammals: extracellular signal-regulated kinases (ERKs) 1 and 2 (ERK1/2), JNKs-1, 2 and 3, p38 isoforms a, b, g, and d, ERKs 3 and 4, and ERK5 [65]. The ERKs are most commonly activated by mitogenes, whereas JNK and p38MAPKs are responsive to stress and inflammatory signals [66]. Allan et al. and Martin et al. showed that inhibition of caspase-9 activity due to phosphorylation at Thr125 was mediated by ERK1/2 and not JNK or p38a/b [10,11]. Also, activated purified wildtype ERK protein showed inhibition of post-cytochrome c induced apoptosis in meiotic X. Laevis egg extracts. 2.13

2.14

Hepatitis B X-interacting protein

HBXIP was found as interactor of HBX protein of hepatitis B virus [67]. HBX is a candidate of oncogenic protein implicated in apoptosis [68]. Survivin, anti-apoptotic protein, is highly expressed in many cancers [3]. Survivin and HBXIP make a complex, bind to pro-caspase-9 and inhibit its recruitment to Apaf-1. Hence HBXIP acts as cofactor of survivin in suppression of apoptosis process [69]. Heat shock protein 70 The high expression of inducible Hsp70 is known to correlate with poor prognosis in many cancers. Hsp70 plays role in survival as well as resistance to chemotherapeutic agents and promotes tumor cell invasion [70]. According to Saleh et al., Hsp70 interacts with Apaf-1, preventing its oligomerization

and association of Apaf-1 with procaspase-9 [71]. On the other hand, Beere et al. showed that Hsp70 does not block Apaf-1 oligomerization but prevents caspase activation mediated by cytochrome c/dATP, binding to Apaf-1 and blocking the recruitment of apoptosome [72]. Protein kinase A It has been reported that cAMP inhibits apoptosis [73,74]. cAMP binds to PKA, dissociates the holoenzyme and releases the free catalytic subunits. In other words, cAMP regulates apoptosis by the activation of PKA [75]. Martin et al. showed that PKA phosphorylates caspase-9 at Ser99, 183 and 195. But this event is not necessary for inhibition of caspase-9. Instead, PKA inhibits caspase-9/3 activation through prevention of apoptosome assembly [76]. 2.16

Protein kinase C Another protein kinase, PKC is also a modulator of apoptosis. The PKC family, which consists of at least 10 isoforms with distinct methods of regulation, has been shown to have both inhibitory and activating effect on apoptosis. PKC a, b, " and atypical isoforms are anti-apoptotic, whereas the d and q isoforms are involved in the stimulation of apoptosis [77]. Interestingly, caspase-9 is phosphorylated at Ser144 by PKC z and inhibits apoptosis [14]. 2.17

2.18 Tumor-upregulated CARD-containing antagonist of caspase nine

The CARD protein interaction motif is found in apoptosis regulatory proteins [78]. CARD-containing pro-caspases are -1, -2, -4, -5, -9, -11, -12 and -13. Also, there are several noncaspase CARD-containing proteins such as Apaf-1, NAC, cIAP1, cIAP2 etc. According to Pathan et al., CARD-carrying protein, TUCAN, selectively binds to procaspase-9, inhibiting its apoptotic activity [79]. 2.19 Human X-chromosome-linked inhibitor of apoptosis protein

Inhibitor of apoptosis protein (IAP) family proteins are characterized by the baculoviral IAP repeat (BIR), the name of which was derived from the original discovery of these apoptosis suppressors in the genomes of baculoviruses by Birnbaum et al. [80]. XIAP has been reported to suppress apoptosis by inhibition of active caspase-9 through its BIR3 domain [81,82]. XIAP binds to the amino terminus of the linker peptide on the procaspase-9 on Asp315, suppressing the activity of caspase-9 [83]. Shiozaki et al. revealed that XIAP blocks caspase-9 homo-dimerization thus inhibiting its activity [84].

2.15

microRNAs Several studies have shown that microRNAs, small noncoding RNA, play critical role in cancer development, invasion and metastasis [85-91]. Thousands of microRNAs were discovered up to date. These control about one-third of all human 2.20


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Table 2. miRNAs regulating caspase-9. Regulator

Efficacy

Mechanism

System

miRNA-133

Inhibition

In vitro

H9C2, HEK293

[94]

miRNA-24a miRNA-582-5p


Inhibits casp-9 expression at both protein and mRNA levels Inhibits casp-9 and Apaf-1 activities Inhibits casp-9 expression at both protein and mRNA levels

In vivo In vitro

[95] [99]

miRNA-23a

Inhibition

X. Laevis eggs GBM stem cell lines (0308, 0822, 1288), U373, A172 HCT116, HT29

Inhibits casp-9 activity

mRNAs. Single microRNAs targets whole complex of mRNAs, which have same signal pathways and same target [92,93]. microRNA-133, preferentially expressed in cardiac and skeletal muscles, is known as regulator of differentiation and proliferation. microRNA-133 repressed caspase-9 expression at both the protein and mRNA levels [94]. microRNA-24a is also believed to have inhibitory effects on caspase-9. microRNA-24a is expressed in the neural retina and is required for correct eye morphogenesis in Xenopus. Primary microRNA RT-PCR from stages 7 to 31 showed that microRNA-24a expression began at stage 19 and was expressed throughout maturation. The controlling of eye size in xenopus was regulated by microRNA-24a overexpression as shown in loss-of-function experiments. microRNA-24a inhibited caspase-9 activity [95]. Glioblastoma multiforme is a brain cancer, its morbidity in the US is around 17,600 [96]. After surgical resection, glioblastoma multiforme recur shortly, indicating possible existence of glioblastoma stem cells, which may have resisted chemotherapy [97,98]. According to Floyd et al., microRNA-582-5p was predicted to target caspase-9 and that was confirmed by Western blotting, qPCR. microRNA-582-5p effectively inhibited caspase-9 protein and mRNA expressions [99]. Shang et al., showed that microRNA-23a (miR-23a)-regulated apoptosis in response to the 5-fluorouracil-induced intrinsic apoptotic pathway. 5-fluorouracil treatment increased the expression of miR-23a and decreased the level of Apaf-1 in colon cancer cells. Apaf-1, as a target gene of miR-23a, was identified and miR-23a antisense activated caspase-9 (Table 2) [100].

In vitro

Caspase-9 activation by natural compounds Accumulating literature suggests that natural compounds play significant inhibitory role in cancer development, invasion and drug resistance. Natural compounds often modify all phases of cancer and several of these targets at the same time [101]. It is no wonder that around 75% of the approved chemotherapeutic agents are of natural origin [102]. During the past decades, to reduce the side effects and enhance the therapeutic effects, botanical compounds play important role in the development of drugs. To suppress cancer growth, most of the natural compounds induce apoptosis via activation of caspase-9. Some of the natural compounds, such as emodin, phenethyl isothiocyanate and 118

Ref.

[100]

fucosterol, are reported to regulate caspase-9 and enhance the apoptosis process in several cancers (Table 3) [24,101,103-139]. 4.

Clinical trials for caspase-9

Two clinical trials targeting caspase-9 were recently initiated by Brenner et al. [140]. The Phase I trial, ‘CASPALLO: Allodepleted T cells transduced with inducible casepase-9 suicide gene’ is being currently conducted against acute lymphoblastic leukemia, non-Hodgkin’s lymphoma, myelodysplastic syndrome and chronic myeloid leukemia. This trial is currently active but not recruiting any patients. Patients will be receiving a stem cell transplant and will be given chemotherapy, to supposedly kill their stem cells. It is hard to find a perfect match for the patients, so find a close relative whose stem cells can be used, so-called ‘allogenic.’ To avoid graft-versushost disease (GvHD), allogenic cells will be grafted in the patients. Specialized cells are made to carry inducible caspase-9 (iCasp9). These cells can be killed when they encounter a specific drug called AP1903. This drug is being tested for GvHD [140]. Another ongoing clinical trial on “Administration of donor T cells with the caspase-9 suicide gene” is in recruiting stage currently. The base concept is the same as former one, but to find a safe and effective dose of iCasp9 and those specialized T cells, so can recover patient’s immune system faster [141]. 5.

3.

Cell lines

Conclusion

Caspase-9, initiator caspase, induces death signals by activating other caspases, and its expression triggers the apoptosis. Caspase-9 has a pivotal role in apoptosis in multiple cancer cells types. The positive regulators such as HCA66, histone H1.2, PHAP and NAC and negative regulators such as Akt, APIP, c-Abl and microRNAs have been reported. The regulation of caspase-9 could be phosphorylation at different sites, recruitment of caspase-9 to apoptosome or inhibition of caspase-9 expression. Activation of caspase-9 by natural compounds has been studied exhaustively. Development of new drugs against cancer from natural resources is important because resistance to conventional drugs is a big issue in cancer treatment. There are ongoing clinical trials in USA using caspase-9 as a suicide gene in T cells. These efforts are



Green tea

Ellagitannins

Rheum palmatum L.

Polytrichum commune L.exHedw Algae

Glycine max

EGCG

Ellagic acid

Emodin

Ethyl acetate

Genistein

Efficacy

Apoptosis

Apoptosis, cell cycle arrest

Apoptosis

Apoptosis

Apoptosis

Apoptosis

Apoptosis

Apoptosis, cell cycle arrest, sensitization of drug resistance Apoptosis, anti-angiogenesis, chemo-sensitization, anti-metastasis

Apoptosis

Apoptosis

ERK: Extracellular signal-regulated kinase; N/A: Not available.

Fucosterol

Clematis ganpiniana

D Rhamnose b-Hederin

Curcumin

Curcuma longa

Berberis amurensis

Berbamine

Cudraxanthone I

Erythrina suberosa

Origin

Alpinumisoflavone

Name

Table 3. Natural compounds regulating caspase-9.

Cytochrome c, casp-3, -8, -9, Fas, FasL, FADD " MMP # FasL, cytochrome c, Bid, caspase-3, -9, miR-574-3p " PI3K, Akt, MMP, MDM2, EGFR #

Bax, casp-3, -8, -9, PARP, p53, cytochrome c, Apaf-1, FasL, HSP " Bcl-2, Bcl-xl, Mcl-1, b-catenin, MMP-2, -9, VEGF-1, HDAC, NF-kB # ERK, cytochrome c release, casp-3, -9 " PI3K/Akt # Casp-3, -4, -9, p38, Bax, cytochrome c, ROS " MMP # Casp-3, -9 " MMP # Casp-3, -8, -9, Apaf-1, Fas, FasL, FADD " Pro-casp-3, -8, -9, PCNA, cFLIP # ROS, Casp-9 " MMP #

Cytochrome c, Bax, casp-3, -9 and PARP " Casp-3, -9, PARP, JNK, c-Jun " Cyclin D1, D2 # Casp-3/7, -8, -9 " MMP #

Mechanism

54

N/A

299.7

518

0.17 ± 0.02

39.6 ± 14.2

N/A

484 ± 74

N/A

N/A

N/A

Bioavailability (mg·h/l)

In vitro, in vivo

In vitro

In vitro

In vitro

In vitro

In vitro, in vivo

In vitro

In vitro, in vivo

In vitro

In vitro

In vitro

System

Cell lines

MCF-7, MCF-10a, HeLa, PC3, DU145, RWPE-1, xenograft mouse

HL-60

L1210

HeLa HBZY-1

MCF-7, MDA-MB-231, BT474, SUM1315, MCF-10A TSGH-8301, T98G, U87MG, SMMC7721, xenograft mouse SY5Y

A549, HepG2, A2780CP, MCF-7, mice, C57 BL/6EAE mice

CCRF-CEM, HepG2, MDA-MB-231, HCT116, U87MG, AML12

G292, KHOS, MG-63

HL-60


[136-139]

[109]

[107]

[24] [156]

[104]

[133-135]

[106]

[129] [155] [130-132] [127,128]

[111]

[123]

[101]

Ref.


119

120

Juglans mandshurica

Chromohalobacter salexigens Olea europaea Sophora family

Juglanthraquinone C

K30


Sargassum fusiforme

Citrus peel oil

Tannins

SFPS-B2

Tangeretin

Tannic acid

Apoptosis

Apoptosis

Apoptosis, cell cycle arrest Apoptosis

Apoptosis

Apoptosis Apoptosis, cell cycle arrest Apoptosis

Apoptosis


Apoptosis, cell cycle arrest Apoptosis


ROS, casp-3, -9, p53, p21, Bax " MMP # Casp-3, -9, Bax/Bcl-2 ratio " Ki67, cyclin A, E, CDK2, Cip1/p21 # Casp-3/7, -9, PARP-1 " MMP # JNK, casp-9 " p21, p53, Casp-9 " Bcl-2/BID ratio # Casp-3, -9, sub-G1 " MMP, SOD, GSH, Bcl-2/Bax ratio # Bax, casp-3, -9 " MMP, Bcl-xl # Bax/Bcl-2 ratio, Casp-3, -9, PARP, ROS, P53 " MPTP, Casp-3, -9, PARP, Bax, cytochrome c " MMP, Bcl-2 # Casp-3, -9, PARP, Bax, Bid, tBid, p53, p-21/cip1, Fas " Casp-3/7, -9 "

DR4, FasL, Bax/Bcl-2 ratio, casp-3, -8, -9, Bid " Bax, P21, casp-3, -9 " Bcl-2, cycline A, E, CDK2 # ROS, casp-3, -9 "

Apoptosis

N/A

N/A

N/A

N/A

88.7 ± 2.2

N/A

N/A N/A

N/A

N/A

N/A

N/A

N/A

N/A

40.0 ± 15.7

Casp-3, -8, -9 "

Apoptosis

Bioavailability (mg·h/l) N/A

Mechanism Sub-G1, casp-9 "

Efficacy Apoptosis


Phenethyl isothiocyanate PAP-3

Entada phaseoloides MERR. Cruciferous vegetables P. abalonus

Phaseoloideside E

Masilinic acid Matrine

Isodon rubescens

Thujopsis dolabrata Siebold and Zucc. Var. hondai Makino Crataegus Laevigata

Goniothalamus macrophyllus Panax quinquefolius Allium nigrum

Origin

Jaridonin

Hawthorn fruit

Ginsenoside 20(s)-Rg3 Hexane extract of aged black garlic Hinokitiol

Goniothalamin

Name

Table 3. Natural compounds regulating caspase-9 (continued).

vitro vitro, vivo vitro

In vitro

In vitro

In vitro

In vitro

In vitro

In In in In

In vitro

In vitro

In vitro

In vitro

In vitro, in vivo

In vitro

In vitro

In vitro

System

Cell lines

MCF10A, MCF-7, MDA-MB-231

AGS, A549, H460, and H1299

SGC-7901

MCF-7

CCA, KKU-100

HeLa, MCF-7, DU125 Caco-2, HT-29 Eca-109, Xenograft model Ec-109, L-O2, 3T3-L1

HepG2

EC9706

HCG-116, SW-620, Xenograft model MCF-7

U937

AGS

HeLa


[122]

[108] [157]

[109]

[120]

[121]

[21,115]

[118] [122]

[119]

[124]

[114]

[113]

[112]

[116]

[117]

[103]

Ref.

B. Kim et al.

necessary to better understand caspase-9-related apoptosis to lead in the identification of novel therapeutic targets.

[101]

[126]

[30]

[30]

[105]

Ref.


HepG2, Kunming mice HL-60 In vitro, in vivo In vitro N/A

1007.1

N/A

K562, Xenograft model PC-3, LNCaP, DU145 In vitro, in vivo In vivo N/A

EC-1, ECa-109 In vitro 346.98 ± 32.748

Apoptosis, cell cycle arrest Apoptosis Juglans cathayensis

Erythrina suberosa

2-Methoxyjuglone

4’-Methoxy licoflavanone


Apoptosis Ursolic acid

Marsdenia tenacissima Tenacissoside C

Apoptosis

Bax/Bcl-2 ratio, caspase-3, -9, p53, p21 " Akt1, cyclin B1, CDC2 # Bax, Bak, casp-3, -9 " Bcl-2, Bcl-xl # Caspase-3,-9, Bax, PARP, GSK3b " Bcl-XL, Bcl-2, Mcl-1, Wnt5a/b and b-catenin # Cytochrome c, casp-3, -9, PARP " Cytochrome c release, BAX, activation of casp-3, -9 and PARP cleavage Apoptosis Salviae miltiorrhizaew Tanshinone IIA

System Bioavailability (mg·h/l) Mechanism Efficacy Origin Name

Table 3. Natural compounds regulating caspase-9 (continued).


Cell lines

6.

Expert opinion

Apoptosis is a program of cell suicide and plays role in embryonic development and homeostasis of adult tissues [142,143]. The basic concept of apoptosis was established in 1972 by Kerr et al. [144]. Apoptosis is characterized by condensation and fragmentation of nuclear chromatin, compaction of cytoplasmic organelles, dilatation of the endoplasmic reticulum and phagocytosis of apoptotic cells [145]. Inappropriate or resistance to apoptosis is related to many human diseases such as neurodegenerative diseases like Alzheimer’s disease, autoimmune diseases and cancer [146]. Researchers have reported hundreds of genes that control the initiation, execution and regulation of apoptosis [142]. Caspases play pivotal role in the apoptosis process. Caspase-9 is involved in apoptosis pathways as an initiator caspase, activated by apoptosome that consist of Apaf-1, cytochrome c and dATP. Caspase-9 bears high similarity to caspase-3, the major difference between caspase-9 and other CED-3 subfamily is the active-site pentapeptide QACGG, Gly takes place instead of the usual Arg [147]. In this pathway, series of steps are tightly regulated, for example, cytochrome c is released from mitochondria, binding Apaf-1, forming apoptosome by oligomerization of Apaf-1, eventually activating caspase-9 [148]. Activated caspase-9 cleaves and activates the effector caspase-3. Much of the fundamental process of caspase-9 activation has been studied and now known at atomic level, but there are remaining challenges in viewing the entire process. For example, exact arrangement of Apaf1 with caspase-9 is not clear. There are significant difference in the predicted size of apoptosome and the experimentally observed size of apoptosome [149]. Also, it is not clear why affinity of caspase-9 is reduced during apoptosis process. Several studies reported that caspase-9 is directly targeted for activation or inhibition by several molecules, including protein kinases, heat shock protein and microRNAs (Tables 1 and 2). Similarly, several agents whether natural or synthetic have been shown to activate caspase-9 leading to apoptosis (Table 3). These molecules/agents can be candidate mediators for development of new drugs, regulating apoptosis in cancer cells. The benefits of targeting caspase-9 in cancer treatment are that cancer cells are more susceptible to certain treatment than normal cells. According to Warburg effect, cancer cells rely on glycolysis to meet high metabolic demands, producing energy at much higher rate through glycolysis followed by lactic acid fermentation in the cytosol, rather than by a relatively lower rate of glycolysis followed by oxidation of pyruvate in the mitochondria of normal cells. Even in the presence of sufficient oxygen, glucose metabolism is upregulated in cancer cells. There are increasing number of studies that have recently shown that caspases play a role in multiple cellular processes


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including immunity, cell survival, cell proliferation, cell cycle regulation and cell differentiation other than apoptosis purpose [150-152]. How and why this process takes place is not clear. Also, few caspases like caspase-1 and -5 are involved in inflammation and are called inflammatory caspases. Like apoptosome formation during apoptosis, inflammasome is formed during inflammation and is composed of caspase-1, NALP3 and ASC [153]. In-depth studies are required to show the activation of caspase-9 and its role in inducing apoptosis in numerous cancer cells. Also, more clinical trials are demanded to establish the role of caspase-9 in suppressing cancer. As an apoptosis-initiator, caspase-9 remains important in the field of apoptosis. Bibliography Papers of special note have been highlighted as either of interest () or of considerable interest () to readers. 1.

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Affiliation Bonglee Kim1,2 MD (KMD) PhD, Sanjay K Srivastava1,3 PhD & Sung-Hoon Kim†4 MD (KMD) PhD † Author for correspondence 1 Kyunghee University, College of Korean Medicine, Cancer Preventive Material Development Research Center, 1 Hoegi-dong, Dongdaemun-ku, Seoul 131-701, South Korea 2 Postdoctoral Research Fellow, Texas Tech University Health Sciences Center, Department of Biomedical Sciences and Cancer Biology Center, Amarillo, TX 79106, USA 3 Professor, Texas Tech University Health Sciences Center, Department of Biomedical Sciences and Cancer Biology Center, Amarillo, TX 79106, USA 4 Professor, Kyunghee University, College of Korean Medicine, Cancer Preventive Material Development Research Center, 1 Hoegi-dong, Dongdaemun-ku, Seoul 131-701, South Korea Tel: +82 2 964 1074; Fax: +82 2 964 1064; E-mail: [email protected]


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Ror2 as a therapeutic target in cancer.

Apoptotic pathways as a therapeutic target for colorectal cancer treatment.

IL-11 signaling as a therapeutic target for cancer.

Transcription inhibition as a therapeutic target for cancer.

HER2 as a novel therapeutic target for cervical cancer.

E2F8 as a Novel Therapeutic Target for Lung Cancer.

The Inositide Signaling Pathway As a Target for Treating Gastric Cancer and Colorectal Cancer.

Heme as a Target for Therapeutic Interventions.

LAMC2 as a therapeutic target for cancers.

Mast cells as therapeutic target in cancer.

Macrophage Metabolism As Therapeutic Target for Cancer, Atherosclerosis, and Obesity.

Aurora-A Kinase as a Promising Therapeutic Target in Cancer.

GSK-3 as potential target for therapeutic intervention in cancer.

PAK1: A Therapeutic Target for Cancer Treatment.

KRAS as a Therapeutic Target.

PPARγ as a Novel Therapeutic Target in Lung Cancer.

miRNA-21 as a novel therapeutic target in lung cancer.

Self-renewal as a therapeutic target in human colorectal cancer.

Nucleo-cytoplasmic transport as a therapeutic target of cancer.

Overactivated neddylation pathway as a therapeutic target in lung cancer.

The potential of heparanase as a therapeutic target in cancer.

Activation of the Tumor Suppressor PP2A Emerges as a Potential Therapeutic Strategy for Treating Prostate Cancer.

Targeting CD44 as a novel therapeutic approach for treating pancreatic cancer recurrence.

Roles of NOTCH1 as a Therapeutic Target and a Biomarker for Lung Cancer: Controversies and Perspectives.