The functional domains for Bax∆2 aggregate-mediated caspase 8-dependent cell death

Abstract

BaxΔ2 is a functional pro-apoptotic Bax isoform having alterations in its N-terminus, but sharing the rest of its sequence with Baxα. BaxΔ2 is unable to target mitochondria due to the loss of helix α1. Instead, it forms cytosolic aggregates and activates caspase 8. However, the functional domain(s) responsible for BaxΔ2 behavior have remained elusive. Here we show that disruption of helix α1 makes Baxα mimic the behavior of BaxΔ2. However, the other alterations in the BaxΔ2 N-terminus have no significant impact on aggregation or cell death. We found that the hallmark BH3 domain is necessary but not sufficient for aggregation-mediated cell death. We also noted that the core region shared by Baxα and BaxΔ2 is required for the formation of large aggregates, which is essential for BaxΔ2 cytotoxicity. However, aggregation by itself is unable to trigger cell death without the C-terminus. Interestingly, the C-terminal helical conformation, not its primary sequence, appears to be critical for caspase 8 recruitment and activation. As BaxΔ2 shares core and C-terminal sequences with most Bax isoforms, our results not only reveal a structural basis for BaxΔ2-induced cell death, but also imply an intrinsic potential for aggregate-mediated caspase 8-dependent cell death in other Bax family members.

BaxΔ2 sensitizes colorectal cancer cells to proteasome inhibitor-induced cell death

“…Furthermore, bortezomib-induced cell death in BaxΔ2(+) cells is predominantly dependent on the caspase 8/3 pathway ( Fig. 4 ), consistent with our previous findings [ 20 , 23 , 28 ]. BaxΔ2-mediated sensitization of colon cancer cells is not cell line-specific, as a similar behavior was also observed in other colorectal cancer cell lines ( Fig.…”

“…We have previously shown that BaxΔ2 induces cell death through activation of caspase 8 [ 23 , 28 ]. To determine whether the proteasome inhibitor-induced cell death could also be through BaxΔ2-meditated activation of caspase 8, BaxΔ2(+) cells were treated with proteasome inhibitors for 24 h, and analyzed for BaxΔ2 expression and caspase 8 activation by immunoblotting.…”

“…As with Baxα, BaxΔ2 is pro-apoptotic, forms homodimers, and interacts with Bcl2 [ 20 ]. However, the pro-death potency of BaxΔ2 is stronger than that of the parental Baxα, and unlike the majority of Bax isoforms, BaxΔ2 fails to target mitochondria due to the lack of exon 2, which encodes for the mitochondrial targeting signal [ 23 – 27 ]. Instead, BaxΔ2 induces cell death by formation of large aggregates in the cytosol and activation of caspase 8 [ 20 , 23 , 28 ].…”

“…However, the pro-death potency of BaxΔ2 is stronger than that of the parental Baxα, and unlike the majority of Bax isoforms, BaxΔ2 fails to target mitochondria due to the lack of exon 2, which encodes for the mitochondrial targeting signal [ 23 – 27 ]. Instead, BaxΔ2 induces cell death by formation of large aggregates in the cytosol and activation of caspase 8 [ 20 , 23 , 28 ]. Interestingly, soluble cytosolic BaxΔ2 monomers are very unstable and susceptible to degradation, which can be blocked by a common proteasome inhibitor, MG132 [ 28 ].…”

Detection of Pro-apoptotic BaxΔ2 Proteins in the Human Cerebellum

“…It is generally believed that generation of Bax∆2 requires a microsatellite frameshift mutation in combination with an alternative splicing event that restores the reading frame. Like Baxα, Bax∆2 is pro-apoptotic and has similar characteristics, such as binding with Bcl-2; however, Bax∆2 does not target mitochondria, and instead activates a caspase 8-dependent death pathway (Haferkamp et al 2012;Zhang et al 2014;Mañas et al 2017). In the process of analyzing the expression and distribution of Bax∆2 throughout the human body, we examined several tissue sections of human brain.…”