Zhangfei/CREB-ZF – A Potential Regulator of the Unfolded Protein Response

Abstract

Cells respond to perturbations in the microenvironment of the endoplasmic reticulum (ER), and to the overloading of its capacity to process secretory and membrane-associate proteins, by activating the Unfolded Protein Response (UPR). Genes that mediate the UPR are regulated by three basic leucine-zipper (bLZip) motif-containing transcription factors – Xbp1s, ATF4 and ATF6. A failure of the UPR to achieve homeostasis and its continued stimulation leads to apoptosis. Mechanisms must therefore exist to turn off the UPR if it successfully restores normalcy. The bLZip protein Zhangfei/CREBZF/SMILE is known to suppress the ability of several, seemingly structurally unrelated, transcription factors. These targets include Luman/CREB3 and CREBH, ER-resident bLZip proteins known to activate the UPR in some cell types. Here we show that Zhangfei had a suppressive effect on most UPR genes activated by the calcium ionophore thapsigargin. This effect was at least partially due to the interaction of Zhangfei with Xbp1s. The leucine zipper of Zhangfei was required for this interaction, which led to the subsequent proteasomal degradation of Xbp1s. Zhangfei suppressed the ability of Xbp1s to activate transcription from a promoter containing unfolded protein response elements and significantly reduced the ability to Xbp1s to activate the UPR as measured by RNA and protein levels of UPR-related genes. Finally, specific suppression of endogenous Zhangfei in thapsigargin-treated primary rat sensory neurons with siRNA directed to Zhangfei transcripts, led to a significant increase in transcripts and proteins of UPR genes, suggesting a potential role for Zhangfei in modulating the UPR.

The SMILE transcriptional corepressor inhibits cAMP response element–binding protein (CREB)–mediated transactivation of gluconeogenic genes

“…Moreover, both transcriptional activity and DNA recruitment of CREB coactivator CRTC2 on CRE were significantly diminished by SMILE, but not by leucine zipper-mutated SMILE. Consistent with our observation, the short form of SMILE (Zhangfei) has been shown to cause the proteasomal degradation of the basic leucine zipper motif-containing transcription factor Xbp1s in a leucine zipper-dependent interaction (20). We also found that the short form of SMILE inhibited CREB/CRTC2 transcriptional activity (Fig.…”

Effects of cyclic AMP response element binding protein–Zhangfei (CREBZF) on the unfolded protein response and cell growth are exerted through the tumor suppressor p53

“…While our results clearly identify the leucine zipper of Zhangfei as important for its interaction with p53 and subsequent p53-mediated suppression of the UPR, we have also shown that Zhangfei interacts with the bLZip UPR transcription factor Xbp1s, targeting it for proteosomal degradation, 11 and that the leucine zipper of Zhangfei is also important for this interaction. Hence, modulation of UPR by Zhangfei may require a combination of several coordinated events, including direct interaction with UPR mediators and indirect mediation through p53.…”

“…10 Zhangfei also has a profoundly suppressive effect on the unfolded protein response (UPR), at least partly because it targets Xbp1s, an important UPR mediator, for proteasomal degradation. 11 We have detected Zhangfei protein in differentiated neurons, but not in developing neurons or cells of neuronal tumors, 10 nor in osteosarcoma cell lines. The ectopic expression of Zhangfei in several tumor cells lines derived from medulloblastomas (ONS-76, UW228 12 ) and osteosarcomas (D-17 13 ) causes the cells to stop growing and display markers of apoptosis.…”

“…13 We have also shown that Zhangfei requires its leucine zipper to suppress the UPR in some cells. 11 To better characterize the molecular mechanism by which Zhangfei suppresses the growth of these cells, we attempted to generate clones of D-17 cells in which Zhangfei could be induced. Six clones that produced stable and detectable Zhangfei only in the presence of the inducer, doxycycline ( [MI]) the induction of Zhangfei had little or no effect on most of the clones (Fig.…”

Proteomic and transcriptomic analyses to explain the pleiotropic effects of Ankaferd blood stopper

“… 45 CREBZF also regulates the unfolded protein response to protect against excessive protein synthesis during endoplasmic reticulum stress. 46 …”