Apoptosis Regulator BAX
Bax is a proapoptotic protein encoded by Bax gene belonging to B-cell lymphoma 2 (Bcl-2) protein family. Bcl-2 family is classified into two groups: antiapoptotic proteins (e.g., Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1) and proapoptotic proteins according to their different functions. proapoptotic proteins are comprised of multidomain proteins (e.g., Bax and Bak) and BH3-only proteins (e.g., Bid, Bim, Puma, Bad, Noxa, Bik, Bmf, and Hrk) based on the presence of Bcl-2 homology domains (BH1–4 domains) (Liu et al. 2016a, b; Han et al. 2015). Interactions between Bcl-2 protein members regulate the mitochondrial signaling which is critical for normal cellular homeostasis and participates in the pathogenesis of different diseases including cancer, diabetes, obesity, and neurodegenerative disorders.
Bax was originally identified as a heterodimer with Bcl-2 in 1993 (Oltvai et al. 1993). Overexpressed Bax neutralizes the death repressor activity of Bcl-2 and accelerates apoptotic cell death. Bax acts as a regulator of apoptosis in various cell types given it is expressed in essentially all organs. Bax-deficient mice display selective expansion of cell population, selective hyperplasias, and resistance to certain apoptotic stimuli (Knudson et al. 1995). Three-dimensional structure of Bax was resolved by nuclear magnetic resonance (NMR) in 2000 (Suzuki et al. 2000). Bax was initially considered to be composed of transmembrane domain, BH1, BH2, and BH3 domain without BH4. The presence or absence of BH4 domain is taken as a distinguishing feature of the anti- and proapoptotic family members. Until 2008, the presence of BH4 motif of Bax was illustrated (Kvansakul et al. 2008). Crystal structures of C-terminally truncated BaxΔC21 with detergents and Bid BH3 peptides were determined in 2013, which defined critical Bax transitions toward apoptosis (Czabotar et al. 2013).
Bax is predominantly found as an inactive monomer in the cytosol of healthy cells or loosely attached to the mitochondrial, nuclear, or endoplastic reticulum membrane and is commonly believed to translocate to mitochondria following an apoptotic stimulus. Once activated, it will induce mitochondrial targeting and outer-membrane permeabilization, promote the formation of pores, and enable the release of cytochrome c and Smac/DIABLO from the intermembrane space into cytosol. There are mainly three different models of Bax activation-mediated mitochondrial outer membrane permeabilization (MOMP) including direct activation models, displacement models, and unified models (Liu et al. 2016a). Numerous anticancer agents including both chemotherapeutic and chemopreventive agents take effects via Bax activation indirectly (Zhang et al. 2000). Bax activators represent a new class of apoptotic sensitizers to combat pathologic cell survivals (e.g., cancers), while Bax inhibitors may serve as novel methods of removing unwanted or premature cell death (e.g., autoimmune disorders).
Structure of Bax
Functions of Bax in Apoptotic Signaling Pathways
Bax also participates in the extrinsic signaling which is mediated by transmembrane death receptors. It reinforces the extrinsic signaling when caspase 8 cleaves Bid to generate the activated tBid. Moreover, it was reported that Bax or Bak is required for normal fusion of mitochondria into elongated tubules, indicating that Bcl-2 family proteins may regulate apoptosis via organelle morphogenesis machineries (Karbowski et al. 2006). In addition, Bax is involved in the endoplasmic reticulum (ER) signaling pathway. Mouse embryonic fibroblasts without Bax and Bak were found to be resistant to proapoptotic agents that induce unfolded protein response (UPR) and show a defect in steady-state ER calcium homeostasis under nonapoptotic conditions (Scorrano et al. 2003).
Bax interacts with many non-Bcl-2 proteins such as tumor suppressor p53 and inositol-requiring 1α (IRE1α) as well as Bcl-2 family members. The protein p53, a key tumor suppressor, is frequently inactivated in human cancers. In 1995, tumor suppressor p53 was found to be a direct transcriptional activator of the human bax gene (Miyashita and Reed 1995). p53 can activate Bax directly without other proteins to permeabilize mitochondria and engage the apoptotic program. It was proposed that when p53 accumulates in cytosol, it acts like the BH3-only subset of proapoptotic Bcl-2 members to activate Bax and trigger apoptosis (Chipuk et al. 2004).
Small Molecular Direct Bax Activators
Bax as a critical proapoptotic member of Bcl-2 family proteins suppresses tumorigenesis and stimulates apoptosis. Interactions between Bcl-2 members control the fates of normal and cancer cells. Bax is not only the unique point for the intrinsic apoptotic pathway, but also participates in the extrinsic signaling. While a number of anticancer drugs in clinic induce Bax activation to facilitate apoptosis of cancer cells indirectly, it is promising to directly activate Bax with small molecules for cancer therapy. Currently, several classes of direct Bax activator have been identified to effectively induce Bax-mediate apoptosis in vitro and in vivo. It is believed that identification of more novel chemical entities directly targeting Bax is feasible facilitated by the assistance of modern drug discovery technologies and multidisciplinary approaches. The Bax activators alone or as combination therapies with other chemotherapeutic drugs are of great potential to benefit cancer patients in the years to come.