Novel post-transcriptional and post-translational regulation of pro-apoptotic protein BOK and anti-apoptotic protein Mcl-1 determine the fate of breast cancer cells to survive or die

Abstract

Deregulation of apoptosis is central to cancer progression and a major obstacle to effective treatment. The Bcl-2 gene family members play important roles in the regulation of apoptosis and are frequently altered in cancers. One such member is pro-apoptotic protein Bcl-2-related Ovarian Killer (BOK). Despite its critical role in apoptosis, the regulation of BOK expression is poorly understood in cancers. Here, we discovered that miR-296-5p regulates BOK expression by binding to its 3’-UTR in breast cancers. Interestingly, miR-296-5p also regulates the expression of anti-apoptotic protein myeloid cell leukemia 1 (Mcl-1), which is highly expressed in breast cancers. Our results reveal that Mcl-1 and BOK constitute a regulatory feedback loop as ectopic BOK expression induces Mcl-1, whereas silencing of Mcl-1 results in reduced BOK levels in breast cancer cells. In addition, we show that silencing of Mcl-1 but not BOK reduced the long-term growth of breast cancer cells. Silencing of both Mcl-1 and BOK rescued the effect of Mcl-1 silencing on breast cancer cell growth, suggesting that BOK is important for attenuating cell growth in the absence of Mcl-1. Depletion of BOK suppressed caspase-3 activation in the presence of paclitaxel and in turn protected cells from paclitaxel-induced apoptosis. Furthermore, we demonstrate that glycogen synthase kinase (GSK3) α/β interacts with BOK and regulates its level post-translationally in breast cancer cells. Taken together, our results suggest that fine tuning of the levels of pro-apoptotic protein BOK and anti-apoptotic protein Mcl-1 may decide the fate of cancer cells to either undergo apoptosis or proliferation.

The Multifaceted Roles of the BCL-2 Family Member BOK

“…This study also describes an miR-296-5p-mediated regulatory feedback loop between MCL-1 and BOK levels that determines whether the breast cancer cells die by apoptosis or survive. Like in NSCLC, BOK protein levels were again found to be significantly lower in tumor tissue compared to matched normal tissue, and higher BOK expression positively correlated with overall as well as with relapse-free survival of breast cancer patients (Onyeagucha et al, 2017).…”

“…In those cancers, high BOK and low TRIM28 mRNA levels correlated with increased survival, and low BOK and high TRIM28 levels with decreased survival (Fernandez-Marrero et al, 2018). Translational silencing of BOK mRNA by miRNA miR-296-5p was reported by Onyeagucha et al (2017) in human breast cancer cell lines. This study also describes an miR-296-5p-mediated regulatory feedback loop between MCL-1 and BOK levels that determines whether the breast cancer cells die by apoptosis or survive.…”

“…At the level of mRNA stability and translation, BOK was found to be downregulated by binding of the miRNA miR-296-5p to its 3' UTR in breast cancer cells (Onyeagucha et al, 2017). Furthermore, destabilizing (AU/U)-rich elements [one AU-rich element (ARE) site in mouse Bok, two U-rich element (URE) sites in human BOK] were recently described in the 3' UTR of BOK (Fernandez-Marrero et al, 2018).…”

The BCL-2 selective inhibitor ABT-199 sensitizes soft tissue sarcomas to proteasome inhibition by a concerted mechanism requiring BAX and NOXA

“…Although interaction of BOK with anti-apoptotic BCL-2 proteins is debated, stabilization of BOK and MCL-1 might even be caused upstream of the proteasome by interaction of BOK with MCL-1. Indeed, BOK and MCL-1 have been proposed to interact at least functionally and constitute a regulatory feedback 52 . Non-proteasomal regulation of BOK expression by mRNA stability, as proposed by Oyneagucha et al 52 , was also published by Fernandez-Marrero et al 53 .…”

Podophyllotoxin Isolated from Podophyllum peltatum Induces G2/M Phase Arrest and Mitochondrial-Mediated Apoptosis in Esophageal Squamous Cell Carcinoma Cells

“…During sustained ER stress, the C/EBP homologous protein (CHOP), one of the death-related transcription factors, triggers the transcription of apoptosis-related genes, including Bcl-2 family proteins, death receptor (DR) 4, DR5, and Puma [13]. Anti-apoptotic proteins, such as Bcl-2 and Mcl-1, are crucial in maintaining mitochondrial functions, while pro-apoptotic proteins, such as bcl-2-like protein 4 (Bax), Bak, and Bid, promote the release of cytochrome c (cyto c) [14]. Reactive oxygen species (ROS), such as superoxide anion radicals and hydroxyl radicals, are chemically reactive molecules produced by common metabolic processes.…”