BCL-2 family member BOK promotes apoptosis in response to endoplasmic reticulum stress

Abstract

B-cell lymphoma 2 (BCL-2) ovarian killer (BOK) is a BCL-2 family protein with high homology to the multidomain proapoptotic proteins BAX and BAK, yet Bok−/− and even Bax−/−Bok−/− and Bak−/−Bok−/− mice were reported to have no overt phenotype or apoptotic defects in response to a host of classical stress stimuli. These surprising findings were interpreted to reflect functional compensation among the BAX, BAK, and BOK proteins. However, BOK cannot compensate for the severe apoptotic defects of Bax−/−Bak−/− mice despite its widespread expression. Here, we independently developed Bok−/− mice and found that Bok−/− cells are selectively defective in their response to endoplasmic reticulum (ER) stress stimuli, consistent with the predominant subcellular localization of BOK at the ER. Whereas Bok−/− mouse embryonic fibroblasts exposed to thapsigargin, A23187, brefeldin A, DTT, geldanamycin, or bortezomib manifested reduced activation of the mitochondrial apoptotic pathway, the death response to other stimuli such as etoposide, staurosporine, or UV remained fully intact. Multiple organs in Bok−/− mice exhibited resistance to thapsigargin-induced apoptosis in vivo. Although the ER stress agents activated the unfolded protein response, both ATF4 and CHOP activation were diminished in Bok−/− cells and mice. Importantly, BAX and BAK were unable to compensate for the defective apoptotic response to ER stress observed in SV40-transformed and primary Bok−/− cells, and in vivo. These findings support a selective and distinguishing role for BOK in regulating the apoptotic response to ER stress, revealing—to our knowledge—the first bona fide apoptotic defect linked to Bok deletion.

Inactivation of prosurvival Bcl-2 proteins activates Bax/Bak through the outer mitochondrial membrane

“…However, this hypothesis was not supported by the overwhelmingly strong apoptotic phenotype of Bax/Bak DKO cells (Wei et al 2001). Recent data from Bok knockout and Bax/ Bak/Bok triple-knockout cells further demonstrated that Bok's apoptotic activity is Bax/Bak-dependent and that it does not play a role as an effector protein during MOMP (Echeverry et al 2013;Ke et al 2013;Carpio et al 2015). Interestingly, a new study provided evidence to suggest that Bok is a noncanonical effector of MOMP in Bax/Bak DKO cells when its degradation is blocked by proteasome inhibition (Llambi et al 2016).…”

“…Based on Bok's Bax/Bak-dependent apoptotic activity, it has also been suggested that Bok may function as a BH3-only protein (Echeverry et al 2013;Carpio et al 2015;). However, this proposition is in conflict with two important findings about Bok.…”

Bok is a genuine multi-BH-domain protein that triggers apoptosis in the absence of Bax and Bak

“…For instance, through its own conserved BH3 domain, Bok might act itself like a BH3-only protein and directly activate Bax and Bak or neutralize anti-apoptotic Bcl-2 proteins, leading to BH3-only protein release, and Bax and Bak activation. In addition, Bok might form heterodimers with Bax and/or Bak, although previous studies failed to demonstrate a direct interaction of Bok with other multidomain proteins (Carpio et al, 2015).…”

“…In line with this report, we also detected a certain portion of Bok not only at mitochondria, but also at the ER, as revealed by co-localization with the ER marker calnexin. It has been recently suggested that Bok might exert a selective role in ERstress-induced Bax and Bak activation, and promotion of mitochondrial apoptosis, as Bok −/− cells have been found to be defective in their response to ER stress stimuli (Carpio et al, 2015). The contribution of Bok to ER-stress-induced apoptosis, however, remains controversial, as other studies have detected no differences of Bok-deficient cells in their sensitivity to ER stress or have even reported that brefeldin-A-treated cells died faster in the absence of Bok (Fernandez-Marrero et al, 2016;Echeverry et al, 2013).…”

“…In addition to mitochondria, a fraction of Bok is localized to the Golgi and ER, implying a potential role of Bok in ER-stress-induced apoptosis (Echeverry et al, 2013;Carpio et al, 2015;Llambi et al, 2016). Furthermore, in several human cancers, including ovarian and breast carcinoma, the genomic locus encoding Bok is frequently deleted, suggesting a potential role of Bok as a tumor suppressor, similar to Bax and Bak (Beroukhim et al, 2010).…”