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. Caspase-8 expression and activation is regulated on many levels. The prodomain of Procaspase-8 plays an important role in Caspase-8 activation. The prodomain interacts with FADD (Fas-Associated protein with Death Domain) to initiate apoptosis via the Fas/FasL pathway. The protein FLIP inhibits Caspase-8 activation by binding to FADD and preventing Procaspase-8 cleavage (1). Meanwhile, proteins of the inhibitors of apoptosis family (Survivin, XIAP, CIAP, Livin) inhibit Caspase-8 and the other pro-apoptotic caspases.
Apoptosis is a hot topic in cancer research. Measuring the levels of apoptosis is a valuable way to measure the efficacy of new cytotoxic therapies. Cheng et. al. studied 39-hydroxypterostilbene (HPSB), a natural pterostilbene analogue, and its potential as a novel colon cancer therapy (2). The group used the Caspase-8 antibody to look at the levels of both active (cleaved) and inactive (full length) Caspase-8 in the presence of increasing levels of their drug. Immunoblotting with the Caspase-8 antibody showed a slight induction of cleaved Caspase-8 in the presence of increasing doses of HPSB. Of note, they also used the Caspase-9 antibody and saw a much stronger induction of cleaved Caspase-9 with HPSB treatment. This evidence showed a clear induction of apoptosis by HPSB, which warrants further studies of HPSB as a colon cancer therapeutic.
Caspases and apoptosis play a much larger role than their involvement in cancer. Hermel et. al. sought to elucidate the involvement of specific caspases in Huntington’s Disease, a rare and progressive neurodegenerative disorder. Caspase cleavage of the protein Huntingtin (Htt) produces toxic byproducts that result in progressive neuronal death. This group used the active/cleaved Caspase-8 antibody for immunoprecipitation and western blot to see if Caspase-8 formed complexes with Htt. They also used the active/cleaved Caspase-8 antibody for staining murine brain cortex and striatum samples to localize its expression and to correlate this with the localization of neuron death seen in Huntington’s. Meanwhile, El-Hawary et. al. characterized a specialized apoptotic process called apocytolysis involved in a rare disorder known as Epidermolysis Bullosa (4). Apocytolysis results in extensive blistering of the skin due to a mutation in keratin. The group used the Caspase-8 antibody to stain 10 subepidermal tissue samples to assess Caspase-8 expression and localization. The data showed a strong positive correlation between Caspase-8 levels and the extent of disease.
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