A Novel Role for Proline- and Acid-rich Basic Region Leucine Zipper (PAR bZIP) Proteins in the Transcriptional Regulation of a BH3-only Proapoptotic Gene

Abstract

Proline-and acid-rich (PAR) basic region leucine zipper (bZIP) proteins thyrotroph embryonic factor (TEF), D-sitebinding protein (DBP), and hepatic leukemia factor have been involved in neurotransmitter homeostasis and amino acid metabolism. Here we demonstrate a novel role for these proteins in the transcriptional control of a BH3-only gene. PAR bZIP proteins are able to transactivate the promoter of bcl-gS. This promoter is particularly responsive to TEF activation and is silenced by NFIL3, a repressor that shares the consensus binding site with PAR bZIP proteins. Consistently, transfection of TEF induces the expression of endogenous bcl-gS in cancer cells, and this induction is independent of p53. A naturally occurring variant of DBP (tDBP), lacking the transactivation domain, has been identified and shown to impede the formation of active TEF dimers in a competitive manner and to reduce the TEF-dependent induction of bcl-gS. Of note, treatment of cancer cells with etoposide induces TEF activation and promotes the expression of bcl-gS. Furthermore, blockade of bcl-gS or TEF expression by a small interfering RNA strategy or transfection with tDBP significantly reduces the etoposide-mediated apoptotic cell death. These findings represent the first described role for PAR bZIP proteins in the regulation of a gene involved in the execution of apoptosis.Two genes, cell death specification protein 1 (ces-1) and ces-2, control the decisions of neuro-secretory motor sister cells in Caenorhabditis elegans to undergo apoptosis. A genetic approach showed that these factors regulate the genes required for apoptosis and that a gain of ces-1 function or a reduction of ces-2 function prevents these cells from dying (1). The pro-apoptotic CES-2 protein negatively regulates CES-1, which prevents the death of the neuro-secretory motor sister cells by transcriptional silencing of Egl-1, a BH3-only protein required for apoptosis in C. elegans (2, 3). CES-2 is similar to members of the proline-and acid-rich (PAR) 4 subfamily of basic region leucine zipper (bZIP) transcription factors, and both share the DNA binding specificity (4).Mammalian homologs of CES-2 include thyrotroph embryonic factor (TEF), albumin D-site-binding protein (DBP), and hepatic leukemia factor (HLF) (5-7). These PAR bZIP proteins have recently been shown to be involved in amino acid and neurotransmitter metabolism through transcriptome profiling analyses in both liver and brain (8). Thus, despite the pro-apoptotic activity described for CES-2 promoting the induction of Egl-1 through an indirect pathway, none of its human homologs have been associated with the transcriptional regulation of BH3-only genes or other executors of apoptosis.BH3-only proteins are a pro-apoptotic subgroup of the Bcl-2 family of apoptosis regulators, which share only the short BH3 region with the rest of the family (9). Genetic experiments have shown that these proteins are essential initiators of programmed cell death in species as distantly related as mice and C. elegans. They are r...

PAR bZIP-bik is a novel transcriptional pathway that mediates oxidative stress-induced apoptosis in fibroblasts

“…Other PAR bZIP proteins were not as efficient as TEF in activating transcription, as we described earlier. 6 However, mutagenesis of the PAR bZIP site abrogated the transactivation capacity of the bik promoter in response to TEF. A consensus site for p53 was also present between the PAR bZIP sequence and the transcription start site of the bik promoter, but as shown in Figure 3b, the luciferase activity increased very modestly in response to p53.…”

“…We have shown earlier that PAR bZIP proteins modulate the expression of the BH3-only gene bcl-gS in human tumor cells. 6 As this was the first described role for PAR bZIP proteins in the regulation of a gene involved in the execution of apoptosis, we tried to extend this functional association to other members of the BH3-only subfamily. For this purpose, we compared the expression of different BH3-only genes in fibroblasts obtained from the tail of wild-type and PAR bZIP knockout mice, which are defective of TEF, DBP, and HLF.…”

“…26 We recently showed for the first time that PAR bZIP proteins are involved in the regulation of apoptosis through the transcriptional control of the BH3-only gene, bcl-gS. 6 Blockade of the bcl-gS A significant fraction of cellular Bik localizes to the endoplasmic reticulum membranes, where it can promote the release of Ca 2 þ that leads to the activation of effector caspases. 27 Oxidative stress triggers the endoplasmic reticulum Ca 2 þ release and therefore, the presence of Bik may be critical to set the apoptotic cascade in motion after exposure to oxidant agents.…”

“…This transcriptional pathway can be activated by chemotherapeutic agents in cancer cells, thus contributing to the apoptotic response to chemotherapy. 6 Bcl-gS is a member of the pro-apoptotic subgroup of the Bcl-2 family of apoptosis regulators, which share only the short BH3 region with the rest of the family. 7 Genetic experiments have shown that these proteins are essential initiators of programmed cell death in species as distantly related as mice and C. elegans.…”

BCL2L14 (BCL2-like 14 (apoptosis facilitator))

“…The BCL2L14 promoter contains a cAMP responsive element, an interferon-γ-activated site (GAS) and an interferon regulatory factor element (IRF-E); interferon-α and γ co-operatively hyperactivate BCL2L14 expression in hepatoma cells (Zhang et al, 2006). A consensus site for certain proline-and acid rich basic region leucine zipper proteins is also present; binding of thyrotroph embryonic factor (TEF) or albumin Dsite-binding protein (DBP) enhances promoter activity, whereas binding of nuclear factor, interleukin-3 regulated (NFIL3) is repressive (Benito et al, 2006). Note that exons only are drawn to scale and that only those transcript variants for which reference sequences exist are shown.…”