THE BCL-2 FAMILY MEMBER BOK PROMOTES APOPTOSIS IN RESPONSE TO ENDOPLASMIC RETICULUM STRESS

MARCOS A. CARPIO; JESSICA SU; MICHAEL MICHAUD; WENPING ZHOU; JILL K. FISHER; LOREN D. WALENSKY; SAMUEL G. KATZ

Praga

Workshop; IMPLEMENTATION OF KNOWLEDGE OF CELL DEATH; 2015

INTERNATIONAL CELL DEATH SOCIETY

BCL-2 Ovarian Killer (BOK) is a member of the Bcl-2 protein family, which is widely expressed and controls intrinsic apoptosis. Bok is most closely related to the pro-apoptotic proteins Bak and Bax, but in contrast to Bak and Bax, very little is known about its cellular role and it has been an enigma since its discovery. Interestingly, Bok-/- and even Bax-/-Bok-/- and Bak-/-Bok-/- mice have no readily discernible abnormalities; this may indicate that BOK does not have a crucial pro-apoptotic function overlapping with BAX or BAK. In order to determine the function of BOK, we examined the apoptotic response in Bok-/- mouse embryonic fibroblasts and diverse tissues and investigated the consequences of its loss in Bok (-/-) mice. Apoptosis, referred as programmed cell death, follows a controlled, predictable routines among them include the unfolded protein response (UPR), which initially improves the folding and degradation of unfolded proteins, but if the UPR is overcome, apoptosis could be initiated. We demonstrate by XTT, caspase-3/7, clonogenic, and annexin V assays that Bok-/- cells are selectively deficient in the apoptotic response to ER stress (consistent with the predominant subcellular localization of BOK at the ER), but not under conditions such as staurosporin (kinase inhibition), etoposide (DNA damage), or UV irradiation. We found that loss of BOK results in decreased UPR-induced expression of UPR protein. Moreover, multiple organs in Bok-/- mice exhibited resistance to ER stress-induced apoptosis in vivo. 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 the first bona fide apoptotic defect linked to Bok deletion.