Importance Of Bcl2 Family To Tumourigenesis

Published Date: 02 Nov 2017

1.4.2 Classification of Bcl-2 family.

The family members are subdivided into 3 major groups based on their structures and functions: pro-apoptotic Bcl-2 effector proteins, pro-apoptotic Bcl-2 proteins and pro-apoptotic BH3-only proteins (Kang & Reynolds, 2009).

Pro-apoptotic Bcl-2 effector proteins comprise all four BH domains together with a transmembrane tail at the C-terminus for targeting themselves to the outer mitochondrial membrane. The BH3 domain is considered to be the death-promoting domain as it is the ligand domain that binds to the receptor domain in anti-apoptotic members (Kelekar & Thompson, 1998; Adams & Cory, 1998). Two pro-apoptotic Bcl-2 effector proteins have been well-characterized: Bcl-2 associated X protein (BAX) and Bcl-2 antagonist killer 1 (BAK). They share similar structures and functional redundancy, but they have different subcellular localization. Both of them can be found on the mitochondrial outer membrane, but BAK is inserted into the membrane while BAX is loosely associated with the mitochondria. Only BAX also appears as soluble monomers in the cytosol (Dejean et al., 2006). The main function of BAX and BAK is to disrupt the OMM integrity. They exist as monomer species under normal unstressed circumstance. Once the cell is under stress or receives sufficient apoptotic stimuli, BAX and BAK are activated and undergo conformational changes. Then, they form homo-oligomers at the OMM that leads to MOMP. Channels formed by the homo-oligomers of BAX or BAK at the OMM are mitochondrial apoptosis-induced channel (MAC) that enables the release of pro-apoptotic proteins such as cytochrome C, AIF from the intermembrane space of mitochondria into the cytosol. The release of these pro-apoptotic proteins will activate the subsequent intrinsic pathway of programmed cell death (Elkholi et al., 2011).

The pro-apoptotic effectors of the Bcl-2 family are regulated by the anti-apoptotic members. Similarly, anti-apoptotic Bcl-2 proteins also contain all four highly conserved BH domains as well as the transmembrane region. The first three BH domains form the key structure ��hydrophobic groove�� that acts as the receptor domain for the BH3 domain of their pro-apoptotic relateives (Kelekar & Thompson, 1998; Feng et al.,2003). Under normal unstressed condition, the basal levels of anti-apoptotic Bcl-2 proteins prevent the counteracting effector proteins from being activated (Zhang, Ming & Yu, 2007). The members of this subfamily maintain the integrity of OMM by binding to their pro-apoptotic counterparts. This protein-protein interaction prevents the activation of the pro-apoptotic members and neutralizes their apoptosis promoting activities. These events inhibit the induction of MOMP together with the release of pro-apoptotic mitochondrial proteins. Eventually, the execution phase of the intrinsic apoptotic pathway will not be activated. Well-known examples of this subfamily are B-cell lymphoma 2 (Bcl-2), the founding member of the family, and B-cell lymphoma-extra large (Bcl-xL) (Llambi & Green, 2011; Elkholi et al., 2011).

Pro-apoptotic BH3-only proteins is a subclass of the pro-apoptotic members of the Bcl-2 family. They generally localize in cytosol and most of them exist as unstructured proteins or peptides that share the BH3 domain as the only structural similarity with one another (Kelekar & Thompson, 1998). These BH3-only proteins promote programmed cell death either by directly activating the pro-apoptotic Bcl-2 effector proteins (direct activators) or by competitively binding to the anti-apoptotic Bcl-2 proteins to prevent their interaction with pro-apoptotic effector proteins (sensitizers and de-repressors) (Adams & Cory, 2007; Llambi & Green, 2011; Elkholi et al., 2011).

1.4.3 Importance of Bcl-2 family to tumourigenesis.

Balance between counteracting members in the Bcl-2 family acts as the life and death switch of the intrinsic apoptotic pathway. Dysregulation of the Bcl-family that favors cell survival such as underexpression of BAX or overexpression of Bcl-2 can be oncogenic. It helps cells establish resistant to cell death and significantly affects the cell population homeostasis that leads to tumour formation. Previous studies showed that overexpression of Bcl-2 prevented thyroid cancer cells from apoptosis induced by chemotherapeutic agents. Therefore, Bcl-2 family is one of the common targets for apoptosis-based cancer therapeutic researches (Reed & Pellecchia, 2005; Adams & Cory, 2007; Mitsiades et al., 2007; Llambi & Green, 2011).

?

1.5 Heterogeneity and cancer stem cells (CSCs) in tumour tissues.

The concept of heterogeneity in cancer tumour was first introduced in late 1990s when the study by Lapidot and co-workers demonstrated that a subpopulation of acute myeloid leukemia (AML) could transfer the malignancy from human to immune-deficient mice (Lapidot et al., 1994; Bonnet and Dick, 1997). Many studies further investigated the small stem cell-like subset in different tumours such as AML, breast cancer and central nervous system malignancy (Spillane & Henderson, 2007). Study findings showed that the whole tumour tissue consists of a heterogeneous population of cells according to their morphology and function. Most of the cells are more differentiated while the minor subset consists of undifferentiated cells that can self-renew. Due to their similarities to normal stem cells, this small group of tumour cells is frequently called ��Cancer Stem Cells�� (CSCs). CSCs have the ability to self-renew by duplicating themselves. They are pluripotent and able to differentiate into any cell types found in the tumour tissue (Garvalov & Acker, 2011). They are also capable to form multicellular spheroid that facilitates metastasis (Vinogradov & Wei, 2012). It is suggested that CSCs population is one of the major source for resistance to chemo- and radiotherapy and the cancer relapse after treatment. Therefore, CSCs become one of the major targets in novel cancer target therapy (Vinogradov & Wei, 2012; Zhao et al., 2012).

1.6 Hypothesis and Aim of Study.

Phosphorylation activates STAT3 through the JAK/STAT signaling pathway. The dimer of active STAT3 mediates expression of genes related to cell proliferation and survival including those encoded for anti-apoptotic protein Bcl-2 and Bcl-xL. These two members of the Bcl-2 family are the key molecules at the upstream of intrinsic pathway of apoptosis. They bind to the BH3 death-promoting domain in their counteracting relatives such as BAX and BAK to protect cells against programmed cell death. Previous studies on NSC 74859 showed that the STAT3 inhibitor can disrupt the STAT3 dimerization and DNA binding activity to dysregulate STAT3 targets genes related to cell cycle progression and pro-survival function (Al Zaid Siddiquee et al., 2007) while those worked on BH3I-2�� demonstrated that the BH3 mimetic compound binds to the hydrophobic groove of anti-apoptotic Bcl-2 proteins to sequester the protective effect against apoptosis (Degterev et al., 2001). Therefore, the current study hypothesize that the STAT3 inhibitor NSC 74859 and BH3 mimetic compound BH3I-2�� can induce cell death in NPC cells via apoptosis. This study aims to examine and compare the antitumour activity of NSC 74859 and BH3I-2�� in NPC cells.