Apoptosis

Caspases are a family of cysteine-aspartic acid proteases that cleave caspase proenzymes as well as other protein substrates. Caspases are well known for their role in apoptosis, but they also play a significant role in other cellular processes including inflammation (1). Apoptotic caspases include Caspases-3, -6, -7, -8, and -9. Meanwhile, human inflammatory caspases include Caspases-1, -4, -5, and -12.

c-Myc is a protein of the Myc family of transcription factors (c-Myc, B-Myc, L-Myc, N-Myc, and s-Myc) encoded by the MYC proto-oncogene. c-Myc was first discovered as the cellular homolog of the retroviral v-Myc oncogene. c-Myc is a transcription factor for genes involved in cell growth, proliferation, differentiation, and apoptosis. c-Myc contains a basic helix-loop-helix domain and a leucine zipper domain that allow for its heterodimerization with its binding partner Max. Myc/Max complexes are able to activate genes via the Myc transactivation domain (1).

Caspases are a family of cysteine-aspartic acid proteases that are responsible for the initiation and execution of apoptosis. Caspase-8 is a 55 kDa protein expressed as an inactive procaspase that resides in the cytosol. Activation of Caspase-8 requires cleavage into its large (17-21 kDa) and small (10-13 kDa) catalytic subunits. Caspase-8 has been shown to play a role in the induction of apoptosis by both death receptor mediated and non-receptor mediated mechanisms (1). Caspase-8 signals to effector Caspase-3 to execute apoptosis.

During cellular stress the protein folding capacity of the ER is diminished. In order to maintain homeostasis and ensure proper protein folding cells activate the unfolded protein response (UPR), a signaling network consisting of sensors and effectors to enhance the chaperone activity of the cell, increase degradation of accumulated proteins, and/or trigger apoptosis.  Inositol-requiring enzyme 1 (IRE1), an ER-transmembrane protein, is an essential component of the UPR pathway important for sensing and responding to ER stress.

Integrins are a large family of trasmembrane proteins involved in cell adhesion and form a link between the intracellular cyskeletal proteins and extracellular matrix proteins. Integrins exist as heterodimers consisting of alpha and beta subunits. In addition to cell adhesion these integrin complexes play key roles in diverse processes such as signal transduction, cell migration, proliferation, differentiation, and apoptosis. Integrins consist of three domains: the extracellular domain, the transmembrane domain, and the cytoplasmic tail.

B-cell lymphoma 2 (Bcl-2) protein is an oncogene that normally acts as an apoptotic inhibitor and localizes to the mitochondrial membrane where it prevents the release of cytochrome c. The Bcl-2 protein family consists of over 20 proteins each containing at least one Bcl-2 homology (BH) domains and have either proapoptic or antiapoptotic activities.

Survivin is an anti-apoptotic protein which is the smallest protein within a large family of proteins including X-linked IAP, c-IAP1 and 2, IAP-like protein-2, melanoma IAP, NAIP, and Livin. Survivin is responsible for a wide range of basic cellular functions that include the cell cycle regulation, fetal development, cell migration, and tumor progression.

The inhibitor of apoptosis proteins (IAPs) are important regulators of cell death and inflammation. The cellular inhibitor of apoptosis protein 2 (cIAP2) contains three Baculovirus IAP repeat (BIR) domains, a Ubiquitin associated (UBA) domain, and a RING domain with E3 ligase activity. cIAP2 inhibits apoptosis through direct inhibition of the pro-apoptotic caspase-3. cIAP2 also regulates cell survival through its role in the tumor necrosis factor alpha (TNF-alpha) signaling pathway.

Bcl-2 nineteen-kilodalton interacting protein 3 (BNIP3) is a pro-apoptotic BH3-only protein. BNIP3 localizes to the mitochondrial membrane where it plays a key role in mitochondrial autophagy and cell death pathways. Similar to other Bcl-2 family members, BNIP3 binds to Bcl-2 and can activate the downstream effectors of Bax/Bak.