Hypoxia-inducible regulation of placental BOK expression

Abstract

BOK (BCL-2-related ovarian killer) is a member of the pro-apoptotic BCL-2 family that is highly expressed in the human placenta. BOK excess causes increased trophoblast autophagy and apoptosis in pre-eclampsia, a pathological condition of hypoxia and oxidative stress. In the present study, we identified an HRE (hypoxia-response element) at the junction of exon-1 and intron-1 (+229 to +279) in the human BOK gene, as well as an antisense transcript driven by a promoter located in intron-2. The isolated BOK-HRE bound hypoxia-inducible HIF (hypoxia-inducible factor) proteins in vitro as well as in trophoblastic JEG3 cells and was functional in its natural position as well as in front of a heterologous promoter. Being a reverted repeat, the BOK-HRE functioned in both orientations. This directionless feature of the BOK-HRE facilitates hypoxia regulation via HIF of both BOK and its antisense transcript as demonstrated by RNAi knockdown of the HIF system. Although the antisense transcript was expressed in several human carcinoma cell lines, including choriocarcinoma-derived JEG3 cells, no antisense-regulated mechanism for BOK expression was noted. Taken together, these findings indicate that hypoxia-induced expression of BOK in placental cells is regulated via HIF and is not affected by its antisense transcript.

Characteristics of Antisense Transcript Promoters and the Regulation of Their Activity

“…Some ASPs can also be located in the second intron, such as those of human BCL-2-related ovarian killer ( BOK ) [ 47 ], a human natural killer cell (NK) I type MHC receptor gene (killer cell immunoglobulin-like receptor surface, KIR ) [ 48 ] and the murine c-myc gene [ 49 ].…”

The Multifaceted Roles of the BCL-2 Family Member BOK

“…Regulation of BOK expression in health and disease has been described at transcriptional, epigenetic, and posttranslational levels, but is overall only incompletely understood (Figure 3). Within the promoter of human and mouse BOK, a conserved E2F-binding site has been described (Rodriguez et al, 2006;Luo et al, 2014). Consequently, Rodriguez et al (2006) described BOK to be cell cycle regulated by the transcription factor E2F1, which upon binding to the promoter activates transcription.…”

“…But, while apoptosis induction in the mouse mammary gland was described as a physiological process after weaning, the increased induction of apoptosis in H1299 cells was observed only in response to an extrinsic stressor ( Ha et al, 2001 ; Rodriguez et al, 2006 ). Within the BOK promoter region, Luo et al (2014) described the presence of hypoxia response elements (HREs). Hypoxia-inducible factors play an important role in adapting the cellular metabolism in response to cellular stress caused by hypoxia ( Vanderhaeghen et al, 2020 ) and were shown to bind to HRE in the promoter region of BOK, leading to an induction of BOK expression and increase in mitochondrial apoptosis ( Soleymanlou et al, 2005 , 2007 ; Luo et al, 2014 ).…”

“…Within the BOK promoter region, Luo et al (2014) described the presence of hypoxia response elements (HREs). Hypoxia-inducible factors play an important role in adapting the cellular metabolism in response to cellular stress caused by hypoxia ( Vanderhaeghen et al, 2020 ) and were shown to bind to HRE in the promoter region of BOK, leading to an induction of BOK expression and increase in mitochondrial apoptosis ( Soleymanlou et al, 2005 , 2007 ; Luo et al, 2014 ). Furthermore, the expression of BOK, as well as of BAX, has been reported to be controlled by Wnt signaling in intestinal cancer ( Zeilstra et al, 2011 ).…”

The Mysteries around the BCL-2 Family Member BOK

“…On the transcription level, BOK was shown to be cell cycle-regulated via the binding of the transcription factor E2F1 to a conserved E2F-binding site in the BOK promoter region [39]. Furthermore, the promoter region of the BOK gene contains a hypoxia response element that binds to hypoxia-inducible factor proteins leading to an induction of BOK expression upon placental hypoxia or oxidative stress [40]. It was also reported that BOK expression is post-transcriptionally downregulated by a mechanism that involves conserved (AU/U)-rich elements in its 3 Untranslated Region (UTR) [41].…”