Cooperative interaction of Zhangfei and ATF4 in transactivation of the cyclic AMP response element

MiR-425-5p suppression of Crebzf regulates oocyte aging via chromatin modification

Female infertility due to declining oocyte quality with age remains a significant challenge for patients and physicians, despite extensive research efforts. Recent studies suggest that microRNAs (miRNAs), which respond to various stressors in the aging process, may provide a promising solution. With the approval of small RNA drugs for clinical use, miRNA-based treatment of oocyte aging appears to be a viable option. Through high-throughput sequencing, miR-425-5p was identified as the only miRNA elevated under natural aging and oxidative stress. Microinjection of inhibitors to inhibit miR-425-5p effectively improved compromised phenotypes of old oocytes in vitro. Further investigation revealed that Crebzf acts as a mediator of miR-425-5p's age-related functions in old oocytes. In vivo treatment with miR-425-5p antagomirs significantly improved impaired oocyte development in reproductively old females by targeting Crebzf. Single-cell RNA sequencing revealed that Crebzf plays a vital role in regulating mRNAs targeting histone H3, trimethylated lysine 4 (H3K4me3), a crucial marker for transcriptional silencing. Overexpression of miR-425-5p could hinder oocyte maturation by downregulating Crebzf expression and disrupting transcriptional regulation. Our findings provide new insights into the potential of miR-425-5p antagomirs as a treatment for female infertility and highlight an elegant mechanism by which miR-425-5p inhibition of Crebzf inhibits a developmental switch in GV oocytes by regulating a group of histone methyltransferase mRNAs.

An extensive β1-adrenergic receptor gene signaling network regulates molecular remodeling in dilated cardiomyopathies

We investigated the extent, biologic characterization, phenotypic specificity, and possible regulation of a β1-adrenergic receptor-linked (β1-AR-linked) gene signaling network (β1-GSN) involved in left ventricular (LV) eccentric pathologic remodeling. A 430-member β1-GSN was identified by mRNA expression in transgenic mice overexpressing human β1-ARs or from literature curation, which exhibited opposite directional behavior in interventricular septum endomyocardial biopsies taken from patients with beta-blocker-treated, reverse remodeled dilated cardiomyopathies. With reverse remodeling, the major biologic categories and percentage of the dominant directional change were as follows: metabolic (19.3%, 81% upregulated); gene regulation (14.9%, 78% upregulated); extracellular matrix/fibrosis (9.1%, 92% downregulated); and cell homeostasis (13.3%, 60% upregulated). Regarding the comparison of β1-GSN categories with expression from 19,243 nonnetwork genes, phenotypic selection for major β1-GSN categories was exhibited for LV end systolic volume (contractility measure), ejection fraction (remodeling index), and pulmonary wedge pressure (wall tension surrogate), beginning at 3 months and persisting to study completion at 12 months. In addition, 121 lncRNAs were identified as possibly involved in cis-acting regulation of β1-GSN members. We conclude that an extensive 430-member gene network downstream from the β1-AR is involved in pathologic ventricular remodeling, with metabolic genes as the most prevalent category.

CREBZF mRNA nanoparticles suppress breast cancer progression through a positive feedback loop boosted by circPAPD4

BACKGROUND

Breast cancer (BC) negatively impacts the health of women worldwide. Circular RNAs (circRNAs) are a group of endogenous RNAs considered essential regulatory factor in BC tumorigenesis and progression. However, the underlying molecular mechanisms of circRNAs remain unclear.

METHODS

Expression levels of circPAPD4, miR-1269a, CREBZF, and ADAR1 in BC cell lines and tissues were measured using bioinformatics analysis, RT-qPCR, ISH, and IHC. Cell proliferation and apoptosis were measured using CCK8, EdU staining, flow cytometry, and TUNEL assays. Pearson correlation analysis, RNA pull-down, dual-luciferase reporter, and co-immunoprecipitation assays were used to explore the correlation among circPAPD4, miR-1269a, CREBZF, STAT3, and ADAR1. Effects of circPAPD4 overexpression on tumor progression were investigated using in vivo assays. Moreover, CREBZF mRNA delivered by polymeric nanoparticles (CREBZF-mRNA-NPs) was used to examine application value of our findings.

RESULTS

CircPAPD4 expression was low in BC tissues and cells. Functionally, circPAPD4 inhibited proliferation and promoted apoptosis in vitro and in vivo. Mechanistically, circPAPD4 biogenesis was regulated by ADAR1. And circPAPD4 promoted CREBZF expression by competitively binding to miR-1269a. More importantly, CREBZF promoted circPAPD4 expression by suppressing STAT3 dimerization and ADAR1 expression, revealing a novel positive feedback loop that curbed BC progression. Systematic delivery of CREBZF-mRNA-NPs effectively induced CREBZF expression and activated the positive feedback loop of circPAPD4/miR-1269a/CREBZF/STAT3/ADAR1, which might suppress BC progression in vitro and in vivo.

CONCLUSION

Our findings firstly illustrated that circPAPD4/miR-1269a/CREBZF/STAT3/ADAR1 positive feedback loop mediated BC progression, and delivering CREBZF mRNA nanoparticles suppressed BC progression in vitro and in vivo, which might provide novel insights into therapeutic strategies for breast cancer.

A stay of execution: ATF4 regulation and potential outcomes for the integrated stress response

ATF4 is a cellular stress induced bZIP transcription factor that is a hallmark effector of the integrated stress response. The integrated stress response is triggered by phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 complex that can be carried out by the cellular stress responsive kinases; GCN2, PERK, PKR, and HRI. eIF2α phosphorylation downregulates mRNA translation initiation en masse, however ATF4 translation is upregulated. The integrated stress response can output two contradicting outcomes in cells; pro-survival or apoptosis. The mechanism for choice between these outcomes is unknown, however combinations of ATF4 heterodimerisation partners and post-translational modifications have been linked to this regulation. This semi-systematic review article covers ATF4 target genes, heterodimerisation partners and post-translational modifications. Together, this review aims to be a useful resource to elucidate the mechanisms controlling the effects of the integrated stress response. Additional putative roles of the ATF4 protein in cell division and synaptic plasticity are outlined.

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

CREBZF regulates testosterone production in mouse Leydig cells

CREBZF, including the two isoforms SMILE (long isoform of CREBZF) and Zhangfei (short isoform of CREBZF), has been identified as a novel transcriptional coregulator of a variety of nuclear receptors. Our previous studies found that SMILE is expressed in the mouse uterine luminal and glandular epithelium and is upregulated by estrogen. In the present study, CREBZF was age-dependently and -specifically expressed in mouse interstitial Leydig cells during sexual maturation. The expression pattern of CREBZF exhibited an age-related increase, and SMILE was the dominant isoform in the mouse testis. Although hCG did not affect CREBZF expression, CREBZF silencing significantly inhibited hCG-stimulated testosterone production in primary Leydig cells and MLTC-1 cells. Meanwhile, the serum concentration of testosterone was significantly decreased after microinjection of lentiviral-mediated shRNA-CREBZF into the mature mouse testis. In addition, CREBZF silencing markedly decreased P450c17, 17β-HSD, and 3β-HSD expression following hCG stimulation in primary Leydig cells, and this inhibitory effect was obviously reversed by overexpression of CREBZF. Furthermore, CREBZF significantly upregulated the mRNA levels of Nr4a1 and Nr5a1, which are the essential orphan nuclear receptors for steroidogenic gene expression. Together our data indicate that CREBZF promotes hCG-induced testosterone production in mouse Leydig cells by affecting Nr4a1 and Nr5a1 expression levels and subsequently increasing the expression of steroidogenic genes such as 3β-HSD, 17β-HSD, and P450c17, suggesting a potential important role of CREBZF in testicular testosterone synthesis.

Activation of CREBZF Increases Cell Apoptosis in Mouse Ovarian Granulosa Cells by Regulating the ERK1/2 and mTOR Signaling Pathways

CREBZF, a multifunction transcriptional regulator, participates in the regulation of numerous cellular functions. The aims of the present study were to detect the localization of CREBZF expression in the ovary and explore the role of CREBZF and related mechanisms in the apoptosis of ovarian granulosa cells. We found by immunohistochemistry that CREBZF was mainly located in granulosa cells and oocytes during the estrous cycle. Western blot analysis showed that SMILE was the main isoform of CREBZF in the ovary. The relationship between apoptosis and CREBZF was assessed via CREBZF overexpression and knockdown. Flow cytometry analysis showed that CREBZF induced cell apoptosis in granulosa cells. Western bolt analysis showed that overexpression of CREBZF upregulated BAX and cleaved Caspase-3, while it downregulated BCL-2. Furthermore, overexpression of CREBZF inhibited the ERK1/2 and mTOR signaling pathways through the phosphorylation of intracellular-regulated kinases 1/2 (ERK1/2) and p70 S6 kinase (S6K1). Moreover, we found that CREBZF also activated autophagy by increasing LC3-II. In summary, these results suggest that CREBZF might play a proapoptotic role in cell apoptosis in granulosa cells, possibly by regulating the ERK1/2 and mTOR signaling pathways.

Hepatic CREBZF couples insulin to lipogenesis by inhibiting insig activity and contributes to hepatic steatosis in diet-induced insulin-resistant mice

Insulin is critical for the regulation of de novo fatty acid synthesis, which converts glucose to lipid in the liver. However, how insulin signals are transduced into the cell and then regulate lipogenesis remains to be fully understood. Here, we identified CREB/ATF bZIP transcription factor (CREBZF) of the activating transcription factor/cAMP response element-binding protein (ATF/CREB) gene family as a key regulator for lipogenesis through insulin-Akt signaling. Insulin-induced gene 2a (Insig-2a) decreases during refeeding, allowing sterol regulatory element binding protein 1c to be processed to promote lipogenesis; but the mechanism of reduction is unknown. We show that Insig-2a inhibition is mediated by insulin-induced CREBZF. CREBZF directly inhibits transcription of Insig-2a through association with activating transcription factor 4. Liver-specific knockout of CREBZF causes an induction of Insig-2a and Insig-1 and resulted in repressed lipogenic program in the liver of mice during refeeding or upon treatment with streptozotocin and insulin. Moreover, hepatic CREBZF deficiency attenuates hepatic steatosis in high-fat, high-sucrose diet-fed mice. Importantly, expression levels of CREBZF are increased in livers of diet-induced insulin resistance or genetically obese ob/ob mice and humans with hepatic steatosis, which may underscore the potential role of CREBZF in the development of sustained lipogenesis in the liver under selective insulin resistance conditions.

CONCLUSION

These findings uncover an unexpected mechanism that couples changes in extracellular hormonal signals to hepatic lipid homeostasis; disrupting CREBZF function may have the therapeutic potential for treating fatty liver disease and insulin resistance. (Hepatology 2018).

Regulation of Cyclic AMP-Response Element Binding Protein Zhangfei (CREBZF) Expression by Estrogen in Mouse Uterus

CREBZF (cAMP-response element binding protein zhangfei) is a member of ATF/CREB family, and which regulates various cellular functions by suppressing major factors with direct interaction. In this study, we have examined the expression of CREBZF on mouse endometrium during uterus estrous cycles and estrogen (E2) treatment. In uterus, CREBZF mRNA expression was higher than other organs and mRNA and protein of CREBZF was increased in proestrus phase and decreased in estrus phase. The expression of CREBZF in 3-weeks old mouse uterus was reduced by E2 injection in endometrium. In addition, the expression of progesterone receptor, a marker of E2 in ovariectomized mice was found to be strongly expressed in stroma, while CREBZF was only expressed in epithelium. Also, we conformed that E2-suppressed CREBZF was restored by co-injection of ICI 182,780, an estrogen receptor antagonist. Overall, these results suggest that CREBZF is regulated by estrogen and involved in ER signaling pathway in mouse uterus.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

Insulin-Inducible SMILE Inhibits Hepatic Gluconeogenesis

The role of a glucagon/cAMP-dependent protein kinase-inducible coactivator PGC-1α signaling pathway is well characterized in hepatic gluconeogenesis. However, an opposing protein kinase B (PKB)/Akt-inducible corepressor signaling pathway is unknown. A previous report has demonstrated that small heterodimer partner-interacting leucine zipper protein (SMILE) regulates the nuclear receptors and transcriptional factors that control hepatic gluconeogenesis. Here, we show that hepatic SMILE expression was induced by feeding in normal mice but not in db/db and high-fat diet (HFD)-fed mice. Interestingly, SMILE expression was induced by insulin in mouse primary hepatocyte and liver. Hepatic SMILE expression was not altered by refeeding in liver-specific insulin receptor knockout (LIRKO) or PKB β-deficient (PKBβ(-/-)) mice. At the molecular level, SMILE inhibited hepatocyte nuclear factor 4-mediated transcriptional activity via direct competition with PGC-1α. Moreover, ablation of SMILE augmented gluconeogenesis and increased blood glucose levels in mice. Conversely, overexpression of SMILE reduced hepatic gluconeogenic gene expression and ameliorated hyperglycemia and glucose intolerance in db/db and HFD-fed mice. Therefore, SMILE is an insulin-inducible corepressor that suppresses hepatic gluconeogenesis. Small molecules that enhance SMILE expression would have potential for treating hyperglycemia in diabetes.

Novel role for PINX1 as a coregulator of nuclear hormone receptors

Estrogen receptor alpha (ERα) has an established role in breast cancer biology. Transcriptional activation by ERα is a multistep process influenced by coactivator and corepressor proteins. This work shows that Pin2 interacting protein 1 (PINX1) interacts with the N-terminal domain of ERα and functions as a corepressor of ERα. Furthermore, it represses both AF-1 and AF-2 transcriptional activities. Chromatin immunoprecipitation assays verified that the interaction between ERα and PINX1 occurs on E2 regulated promoters and enhanced expression of PINX1 deregulates the expression of a number of genes that have a role in cell growth and proliferation in breast cancer. PINX1 overexpression decreases estrogen mediated proliferation of breast cancer cell lines, while its depletion shows the opposite effect. Taken together, these data show a novel molecular mechanism for PINX1 as an attenuator of estrogen receptor activity in breast cancer cell lines, furthering its role as a tumor suppressor gene in breast cancer.

Small heterodimer partner-interacting leucine zipper protein inhibits adipogenesis by regulating peroxisome proliferator-activated receptor γ activity

AIMS

Adipocytes play a critical role in energy balance. Growth of fat tissue is achieved via an increase in adipocyte mass and the formation of newly differentiated adipocytes from precursor cells. Understanding the cellular and molecular mechanisms of adipocyte differentiation is crucial for the study of obesity- and fat-related diseases. The present study was designed to study whether small heterodimer partner-interacting leucine zipper protein (SMILE), a novel co-repressor, could regulate differentiation of adipocyte in 3T3-L1 cells.

MATERIALS AND METHODS

Treatment of endoplasmic stress inducers, thapsigargin and tunicamycin, inhibited adipocyte differentiation, stimulated Smile mRNA expression, and repressed the expression of adiponectin (Adipoq) in 3T3-L1 pre-adipocyte. Overexpression of SMILE in 3T3-L1 cells decreased the expression of the mRNA encoding Adipoq, a major marker of adipocytes, significantly. Furthermore, knockdown of SMILE recovered the thapsigargin-mediated repression of Adipoq transcription. Co-immunoprecipitation experiments revealed that SMILE interacted physically with PPARγ in 3T3-L1 cells. In addition, chromatin immunoprecipitation experiments revealed that SMILE suppressed the binding affinity of PPARγ for the Adipoq promoter.

KEY FINDINGS

We demonstrate that SMILE controls adipocyte differentiation by regulating the transactivity of peroxisome proliferator-activated receptor γ (PPARγ).

SIGNIFICANCE

These findings demonstrate that SMILE represses adipocyte differentiation by regulating PPARγ transactivity; hence, SMILE is a potential regulator of PPARγ-related diseases.

The effect of Zhangfei/CREBZF on cell growth, differentiation, apoptosis, migration, and the unfolded protein response in several canine osteosarcoma cell lines

Background

We had previously shown that the bLZip domain-containing transcription factor, Zhangfei/CREBZF inhibits the growth and the unfolded protein response (UPR) in cells of the D–17 canine osteosarcoma (OS) line and that the effects of Zhangfei are mediated by it stabilizing the tumour suppressor protein p53. To determine if our observations with D-17 cells applied more universally to canine OS, we examined three other independently isolated canine OS cell lines—Abrams, McKinley and Gracie.

Results

Like D–17, the three cell lines expressed p53 proteins that were capable of activating promoters with p53 response elements on their own, and synergistically with Zhangfei. Furthermore, as with D–17 cells, Zhangfei suppressed the growth and UPR-related transcripts in the OS cell lines. Zhangfei also induced the activation of osteocalcin expression, a marker of osteoblast differentiation and triggered programmed cell death.

Conclusions

Osteosarcomas are common malignancies in large breeds of dogs. Although there has been dramatic progress in their treatment, these therapies often fail, leading to recurrence of the tumour and metastatic spread. Our results indicate that induction of the expression of Zhangfei in OS, where p53 is functional, may be an effective modality for the treatment of OS.

Construction and expression of lentiviral vectors encoding recombinant mouse CREBZF in NIH 3T3 cells

CREBZF, also known as Zhangfei or SMILE, is a member of the CREB/ATF protein family. CREBZF has mainly been considered as a basic region-leucine zipper transcription factor that functions in coordination with other transcription factors and plays a role in latent HSV-1 infection, apoptosis and the mammalian endoplasmic reticulum stress and unfolded protein response. In this study, we constructed recombinant lentiviral vectors for CREBZF short hairpin RNA (shRNA) expression and over-expression to improve understanding of the mechanisms regulating CREBZF. The CREBZF ORF sequence was cloned into the lentiviral shuttle plasmid pCD513B-1, and various shRNA oligonucleotides and one negative control (shN) were cloned into the pCD513B-U6 expression vector. The recombinant lentivirus was packaged and transduced into NIH 3T3 cells. CREBZF mRNA and protein expression were examined using real-time reverse transcription-polymerase chain reaction (RT-qPCR) and western blotting, respectively. The over-expression vector and the most effective shRNA vector significantly affected the expression of CREBZF mRNA and protein. Both of the CREBZF recombinant lentiviral vectors were successfully constructed. The over-expression vector significantly increased the expression of exogenous CREBZF and inhibited the growth of NIH 3T3 cells compared to controls. The most effective shRNA lentiviral vector, pCD513B-U6-CREBZF-shRNA-3, was transformed, leading to significant knockdown of the CREBZF gene. We conclude that CREBZF the recombinant lentiviral vectors are promising tools for regulating the expression of CREBZF in NIH 3T3 cells.

SMILE inhibits BMP-2-induced expression of osteocalcin by suppressing the activity of the RUNX2 transcription factor in MC3T3E1 cells

Small heterodimer partner interacting leucine zipper protein (SMILE) is an orphan nuclear receptor and a member of the bZIP family of proteins. Several recent studies have suggested that SMILE is a novel co-repressor that is involved in nuclear receptor signaling; however, the role of SMILE in osteoblast differentiation has not yet been elucidated. This study demonstrates that SMILE inhibits osteoblast differentiation by regulating the activity of Runt-related transcription factor-2 (RUNX2). Tunicamycin, an inducer of endoplasmic reticulum stress, stimulated SMILE expression. Bone morphogenetic protein-2-induced expression of alkaline phosphatase and osteocalcin, both of which are osteogenic genes, was suppressed by SMILE. The molecular mechanism by which SMILE affects osteocalcin expression was also determined. An immunoprecipitation assay revealed a physical interaction between SMILE and RUNX2 that significantly impaired the RUNX2-dependent activation of the osteocalcin gene. A ChIP assay revealed that SMILE repressed the ability of RUNX2 to bind to the osteocalcin gene promoter. Taken together, these findings demonstrate that SMILE negatively regulates osteocalcin via a direct interaction with RUNX2.

CREBZF expression and hormonal regulation in the mouse uterus

BACKGROUND

CREBZF is a member of the mammalian ATF/CREB family of the basic region-leucine zipper (bZIP) transcription factors. Two isoforms of CREBZF have been identified from the alternative usage of initiation codons, SMILE (long isoform of CREBZF) and Zhangfei (short isoform of CREBZF). Until recently, the physiological function of CREBZF in mammalian reproductions has not been reported.

METHODS

Multiple techniques were performed to investigate the spatiotemporal expression and hormonal regulation of the CREBZF gene in the mouse uterus and its role in embryo implantation.

RESULTS

Zhangfei was not detected in the mouse uterus. SMILE immunostaining was mainly expressed in the uterine luminal and glandular epithelium, and the expression levels of both SMILE mRNA and protein gradually decreased from days 1-3 of pregnancy, peaked on day 4, and then declined again on day 6. On day 5 of pregnancy, SMILE protein expression was detected only in the luminal epithelium at implantation sites compared with the expression at inter-implantation sites. SMILE protein was not detected in decidual cells from days 6-8 of pregnancy or artificial decidualisation. Furthermore, SMILE protein was not detected in the mouse uterus on days 3-6 of pseudopregnancy, and SMILE expression was also induced in the delayed-implantation uterus, indicating that the presence of an active blastocyst was required for SMILE expression at the implantation site. Oestrogen significantly stimulated SMILE expression in the ovariectomised mouse uterus. In addition, in cycling mice, high levels of SMILE protein and mRNA expression were also observed in proestrus and oestrus uteri.

CONCLUSIONS

Taken together, these results suggested that SMILE expression was closely related to mouse implantation and up-regulated by oestrogen.

Ursodeoxycholic acid inhibits liver X receptor α-mediated hepatic lipogenesis via induction of the nuclear corepressor SMILE

Small heterodimer partner interacting leucine zipper protein (SMILE) has been identified as a nuclear corepressor of the nuclear receptor (NRs) family. Here, we examined the role of SMILE in the regulation of nuclear receptor liver X receptor (LXR)-mediated sterol regulatory element binding protein-1c (SREBP-1c) gene expression. We found that SMILE inhibited T0901317 (T7)-induced transcriptional activity of LXR, which functions as a major regulator of lipid metabolism by inducing SREBP-1c, fatty acid synthase (FAS), and acetyl-CoA carboxylase (ACC) gene expression. Moreover, we demonstrated that SMILE physically interacts with LXR and represses T7-induced LXR transcriptional activity by competing with coactivator SRC-1. Adenoviral overexpression of SMILE (Ad-SMILE) attenuated fat accumulation and lipogenic gene induction in the liver of T7 administered or of high fat diet (HFD)-fed mice. Furthermore, we investigated the mechanism by which ursodeoxycholic acid (UDCA) inhibits LXR-induced lipogenic gene expression. Interestingly, UDCA treatment significantly increased SMILE promoter activity and gene expression in an adenosine monophosphate-activated kinase-dependent manner. Furthermore, UDCA treatment repressed T7-induced SREBP-1c, FAS, and ACC protein levels, whereas knockdown of endogenous SMILE gene expression by adenovirus SMILE shRNA (Ad-shSMILE) significantly reversed UDCA-mediated repression of SREBP-1c, FAS, and ACC protein levels. Collectively, these results demonstrate that UDCA activates SMILE gene expression through adenosine monophosphate-activated kinase phosphorylation, which leads to repression of LXR-mediated hepatic lipogenic enzyme gene expression.

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

Zhangfei/CREBZF, a basic region-leucine zipper (bLZip) transcription factor, is a potent suppressor of growth and the unfolded protein response (UPR) in some cancer cell lines, including the canine osteosarcoma cell line, D-17. However, the effects of Zhangfei are not universal, and it has no obvious effects on untransformed cells and some cancer cell lines, suggesting that Zhangfei may act through an intermediary that is either not induced or is defective in cells that it does not affect. Here we identify the tumor suppressor protein p53 as this intermediary. We show the following: in cells ectopically expressing Zhangfei, the protein stabilizes p53 and co-localizes with it in cellular nuclei; the bLZip domain of Zhangfei is required for its profound effects on cell growth and interaction with p53. Suppression of p53 by siRNA at least partially inhibits the effects of Zhangfei on the UPR and cell growth. The effects of Zhangfei on D-17 cells is mirrored by its effects on the p53-expressing human osteosarcoma cell line U2OS, while Zhangfei has no effect on the p53-null osteosarcoma cell line MG63. In U2OS cells, Zhangfei displaces the E3 ubiquitin ligase mouse double minute homolog 2 (Mdm2) from its association with p53, suggesting a mechanism for the effects of Zhangfei on p53.

Zhangfei/CREB-ZF - a potential regulator of the unfolded protein response

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.

Expression of a phosphorylated form of ATF4 in lung and non-small cell lung cancer tissues

ATF4 is a member of the cAMP-responsive element-binding protein family of basic zipper-containing proteins, a family of transcription factors phosphorylated at serines residues by protein kinase A. The family has been proved to be able to stimulate the transcription of the genes containing CRE elements. Elevated ATF4 expression was detected in some tumors including breast carcinoma compared to their corresponding nontumor tissues. p-ATF4 (ser 245), a phosphorylated form of ATF4 protein at serine 245 site, was believed to be an active type of this protein. However, its expression and clinical significance in malignant tumors including non-small cell lung cancer were not reported up to date. In the current study, we investigate the expression of p-ATF4 (ser 245) in non-small cell lung cancer using tissue microarray and immunohistochemistry. p-ATF4 (ser 245) immunostaining was detected in nucleus and cytoplasm in cancer cells and normal lung epithelial cells. Compared to bronchial epithelium and submucosal glands (total positive rate, 14.6% (12/82)), there was increased expression of p-ATF4 (ser 245) in non-small cell lung cancer cells (total positive rate, 42.7% (35/82)) (p < 0.05). In addition, increased expression of p-ATF4 (ser 245) was associated with lymph node metastasis and advanced TNM stages (III and IV) in non-small cell lung cancer (p < 0.05). Immunofluorescent staining confirmed nuclear and cytoplasmic expression of p-ATF4 (ser 245) in lung and cancer tissues, and also in non-small cell lung cancer cell lines including NCI-H157 and LTE cells. These results indicate that increased expression of p-ATF4 (ser 245) may contribute to cancer development of non-small cell lung cancer and may be a potential cancer marker.

JAB1/CSN5 inhibits the activity of Luman/CREB3 by promoting its degradation

Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER)-bound transcription factor that has been implicated in the ER stress response. In this study, we used the region of Luman containing the basic DNA-binding domain as bait in a yeast two-hybrid screen and identified the Jun activation domain-binding protein 1 (JAB1) or the COP9 signalosome complex unit 5 (CSN5) as an interacting protein. We confirmed their direct binding by glutathione S-transferase pull-down assays, and verified the existence of such interaction in the cellular environment by mammalian two-hybrid and co-immunoprecipitation assays. Deletion mapping studies revealed that the MPN domain in JAB1 was essential and sufficient for the binding. JAB1 also colocalized with Luman in transfected cells. More interestingly, the nuclear form of Luman was shown to promote the translocation of JAB1 into the nucleus. We found that overexpression of JAB1 shortened the half-life of Luman by 67%, and repressed its transactivation function on GAL4 and unfolded protein response element (UPRE)-containing promoters. We therefore propose that JAB1 is a novel binding partner of Luman, which negatively regulates the activity of Luman by promoting its degradation.

The transcription factor CREBZF is a novel positive regulator of p53

CREBZF is a member of the mammalian ATF/CREB family of transcription factors. Here, we describe a novel functional interaction between CREBZF and the tumor suppressor p53. CREBZF was identified in a yeast two-hybrid screen using HEY1, recently characterized as an indirect p53 activator, as bait. CREBZF interacts in vitro with both HEY1 and p53, and CREBZF expression stabilizes and activates p53. Moreover, CREBZF cooperates synergistically with HEY1 to enhance p53 transcriptional activity. On the other hand, partial depletion of endogenous CREBZF diminishes p53 protein levels and inhibits HEY1-mediated activation of p53. CREBZF-positive effects on p53 signaling may reflect, at least in part, an observed induction of posttranslational modifications in p53 known to prevent its degradation. CREBZF expression protects HCT116 cells from UV radiation-induced cell death. In addition, CREBZF expression confers sensitivity to 5-fluorouracil, a p53-activating chemotherapeutic drug. Our study suggests that CREBZF may participate in the modulation of p53 tumor suppressor function.

Mechanism for the induction of cell death in ONS-76 medulloblastoma cells by Zhangfei/CREB-ZF

Cells from medulloblastoma lines do not contain detectable amounts of the basic leucine-zipper protein Zhangfei. However, we have previously shown that expression of this protein in cells of the ONS-76 and UW228 medulloblastoma lines causes the cells to stop growing and develop processes that resemble neurites. Our objective was to determine the molecular mechanisms by which Zhangfei influences ONS-76 cells. We infected ONS-76 cells with adenovirus vectors expressing either Zhangfei or the control protein LacZ and then compared the following parameters in Zhangfei and LacZ-expressing cells: (a) markers of apoptosis, autophagy and macropinocytosis, (b) transcripts for genes involved in neurogenesis and apoptosis, (c) phosphorylation of peptide targets of selected cellular protein kinases, and (d) activation of transcription factors. Zhangfei-expressing cells appeared to succumb to apoptosis. Increased staining for autophagic vesicles and upregulated expression of autophagy response genes in these cells indicated that they were undergoing autophagy, possibly associated with apoptosis. Within our analysis, patterns of gene expression and phosphorylation-mediated signal transduction activity in Zhangfei-expressing cells indicated that the mitogen-activated protein kinase (MAPK) pathway was active. In addition, we found that the transcription factor Brn3a as well as factors implicated in differentiation were also active in Zhangfei-expressing cells. We tested the hypothesis that Zhangfei enhances the expression of Brn3a, a known inducer of TrkA, the high-affinity receptor for nerve growth factor (NGF). TrkA then engages NGF in an autocrine manner triggering the MAPK pathway and leading to differentiation of ONS-76 cells into neuron and glia-like cells-a process that eventually brings about cell death. We showed that: (a) Zhangfei could enhance transcription from the isolated Brn3a promoter, (b) ONS-76 cells produced NGF and (c) antibodies against NGF and inhibitors of TrkA and selected components of the MAPK pathway could partially restore the growth of Zhangfei-expressing ONS-76 cells.

CREBZF, a novel Smad8-binding protein

Smads are the secondary messengers of the transforming growth factor-β (TGF-β) signaling pathway. TGF-β receptors phosphorylate the Receptor Smads (R-Smads) upon ligand binding; activated R-Smads translocate to the nucleus and function as transcription factors. Among the R-Smads, Smads 1, 5, and 8 mainly mediate signals in the bone morphogenetic proteins (BMPs) pathways, while Smads 2/3 mediate TGF-β signaling. The regulation of Smads in the TGF-β signal pathway has been well defined, but the relationship of Smads 1, 5, and 8 to the BMP pathways has been relatively understudied. To understand the specific regulation of BMP mediating Smads, we performed yeast two-hybrid screening using the Mad homology 2(MH2) domain of Smad8 as bait. In this screening, novel Smad-binding protein, CREBZF-a basic region-leucine zipper (bZIP) transcription factor-was identified. The interaction of CREBZF and Smads 1, 5, and 8 was confirmed by immunoprecipitation in a human prostate cancer cell line. Overexpression of CREBZF inhibited the promoter activity of BMP response element and abolished the cell growth inhibition induced by BMP-6. Thus, CREBZF inhibits the function of BMP-6 by interacting with Smads. The identification of this novel Smads-binding protein, among others will help us understand the modulation of BMP-signaling pathways.

Mediation of glucolipotoxicity in INS-1 rat insulinoma cells by small heterodimer partner interacting leucine zipper protein (SMILE)

Sustained elevations of glucose and free fatty acid concentration have deleterious effects on pancreatic beta cell function. One of the hallmarks of such glucolipotoxicity is a reduction in insulin gene expression, resulting from decreased insulin promoter activity. Sterol regulatory element binding protein-1c (SREBP-1c), a lipogenic transcription factor, is related to the development of beta cell dysfunction caused by elevated concentrations of glucose and free fatty acid. Small heterodimer partner (SHP) interacting leucine zipper protein (SMILE), also known as Zhangfei, is a novel protein which interacts with SHP that mediates glucotoxicity in INS-1 rat insulinoma cells. Treatment of INS-1 cells with high concentrations of glucose and palmitate increased SREBP-1c and SMILE expression, and decreased insulin gene expression. Adenovirus-mediated overexpression of SREBP-1c in INS-1 cells induced SMILE expression. Moreover, adenovirus-mediated overexpression of SMILE (Ad-SMILE) in INS-1 cells impaired glucose-stimulated insulin secretion as well as insulin gene expression. Ad-SMILE overexpression also inhibited the expression of beta-cell enriched transcription factors including pancreatic duodenal homeobox factor-1, beta cell E box transactivator 2 and RIPE3b1/MafA, in INS-1 cells. Finally, in COS-1 cells, expression of SMILE inhibited the insulin promoter activity induced by these same beta-cell enriched transcription factors. These results collectively suggest that SMILE plays an important role in the development of beta cell dysfunction induced by glucolipotoxicity.

Effects of activating transcription factor 4 deficiency on carbohydrate and lipid metabolism in mammals

It has been shown that the mammalian activating transcription factor 4 (ATF4) is involved in many different physiological events, such as eye development, stress response, learning, and memory. However, several recent studies have demonstrated that ATF4 also plays an important role in the regulation of lipid and glucose metabolism, energy homeostasis, insulin secretion, and sensitivity, suggesting that ATF4 is a master regulator of metabolism. This review summarizes the most recent progress in the understanding of the novel roles of ATF4 in the regulation of metabolism.

Curcumin differentially regulates endoplasmic reticulum stress through transcriptional corepressor SMILE (small heterodimer partner-interacting leucine zipper protein)-mediated inhibition of CREBH (cAMP responsive element-binding protein H)

Curcumin (diferuloylmethane), a major active component of turmeric (Curcuma longa), is a natural polyphenolic compound. Herein the effect of curcumin on endoplasmic reticulum (ER) stress responsive gene expression was investigated. We report that curcumin induces transcriptional corepressor small heterodimer partner-interacting leucine zipper protein (SMILE) gene expression through liver kinase B1 (LKB1)/adenosine monophosphate-activated kinase (AMPK) signaling pathway and represses ER stress-responsive gene transcription in an ER-bound transcription factor specific manner. cAMP responsive element-binding protein H (CREBH) and activating transcription factor 6 (ATF6) are both ER-bound bZIP family transcription factors that are activated upon ER stress. Of interest, we observed that both curcumin treatment and SMILE overexpression only represses CREBH-mediated transactivation of the target gene but not ATF6-mediated transactivation. Knockdown of endogenous SMILE significantly releases the inhibitory effect of curcumin on CREBH transactivation. Intrinsic repressive activity of SMILE is observed in the Gal4 fusion system, and the intrinsic repressive domain is mapped to the C terminus of SMILE spanning amino acid residues 203-269, corresponding to the basic region leucine zipper (bZIP) domain. In vivo interaction assay revealed that through its bZIP domain, SMILE interacts with CREBH and inhibits its transcriptional activity. Interestingly, we observed that SMILE does not interact with ATF6. Furthermore, competition between SMILE and the coactivator peroxisome proliferator-activated receptor α (PGC-1α) on CREBH transactivation has been demonstrated in vitro and in vivo. Finally, chromatin immunoprecipitation assays revealed that curcumin decreases the binding of PGC-1α and CREBH on target gene promoter in a SMILE-dependent manner. Overall, for the first time we suggest a novel phenomenon that the curcumin/LKB1/AMPK/SMILE/PGC1α pathway differentially regulates ER stress-mediated gene transcription.

Negative regulation of IRF7 activation by activating transcription factor 4 suggests a cross-regulation between the IFN responses and the cellular integrated stress responses

Cells react to viral infection by exhibiting IFN-based innate immune responses and integrated stress responses, but little is known about the interrelationships between the two. In this study, we report a linkage between these two host-protective cellular mechanisms. We found that IFN regulatory factor (IRF)7, the master regulator of type I IFN gene expression, interacts with activating transcription factor (ATF)4, a key component of the integrated stress responses whose translation is induced by viral infection and various stresses. We have demonstrated that IRF7 upregulates ATF4 activity and expression, whereas ATF4 in return inhibits IRF7 activation, suggesting a cross-regulation between the IFN response and the cellular integrated stress response that controls host innate immune defense against viral infection.

Neurogenic transdifferentiation of human adipose-derived stem cells? A critical protocol reevaluation with special emphasis on cell proliferation and cell cycle alterations

Adipose-derived stem cells (ASCs) are reported to display multilineage differentiation potential, including neuroectodermal pathways. The aim of the present study was to critically re-evaluate the potential neurogenic (trans-)differentiation capacity of ASCs using a neurogenic induction protocol based on the combination of isobutylmethylxanthine (IBMX), indomethacin and insulin. ASCs isolated from lipo-aspirate samples of five healthy female donors were characterized and potential neurogenic (trans-)differentiation was assessed by means of immunohistochemistry and gene expression analyses. Cell proliferation and cell cycle alterations were studied, and the expression of CREB/ATF transcription factors was analyzed. ASCs expressed CD59, CD90 and CD105, and were tested negative for CD34 and CD45. Under neurogenic induction, ASCs adopted a characteristic morphology comparable to neur(on)al progenitors and expressed musashi1, β-III-tubulin and nestin. Gene expression analyses revealed an increased expression of β-III-tubulin, GFAP, vimentin and BDNF, as well as SOX4 in induced ASCs. Cell proliferation was significantly reduced under neurogenic induction; cell cycle analyses showed a G2-cell cycle arrest accompanied by differential expression of key regulators of cell cycle progression. Differential expression of CREB/ATF transcription factors could be observed on neurogenic induction, pointing to a decisive role of the cAMP-CREB/ATF system. Our findings may point to a potential neurogenic (trans-)differentiation of ASCs into early neur(on)al progenitors, but do not present definite evidence for it. Especially, the adoption of a neural progenitor cell-like morphology must not automatically be misinterpreted as a specific characteristic of a respective (trans-)differentiation process, as this may as well be caused by alterations of cell cycle progression.

Identification of bZIP interaction partners of viral proteins HBZ, MEQ, BZLF1, and K-bZIP using coiled-coil arrays

Basic-region leucine-zipper transcription factors (bZIPs) contain a segment rich in basic amino acids that can bind DNA, followed by a leucine zipper that can interact with other leucine zippers to form coiled-coil homo- or heterodimers. Several viruses encode proteins containing bZIP domains, including four that encode bZIPs lacking significant homology to any human protein. We investigated the interaction specificity of these four viral bZIPs by using coiled-coil arrays to assess self-associations as well as heterointeractions with 33 representative human bZIPs. The arrays recapitulated reported viral-human interactions and also uncovered new associations. MEQ and HBZ interacted with multiple human partners and had unique interaction profiles compared to any human bZIPs, whereas K-bZIP and BZLF1 displayed homospecificity. New interactions detected included HBZ with MAFB, MAFG, ATF2, CEBPG, and CREBZF and MEQ with NFIL3. These were confirmed in solution using circular dichroism. HBZ can heteroassociate with MAFB and MAFG in the presence of MARE-site DNA, and this interaction is dependent on the basic region of HBZ. NFIL3 and MEQ have different yet overlapping DNA-binding specificities and can form a heterocomplex with DNA. Computational design considering both affinity for MEQ and specificity with respect to other undesired bZIP-type interactions was used to generate a MEQ dimerization inhibitor. This peptide, anti-MEQ, bound MEQ both stably and specifically, as assayed using coiled-coil arrays and circular dichroism in solution. Anti-MEQ also inhibited MEQ binding to DNA. These studies can guide further investigation of the function of viral and human bZIP complexes.

Transcriptional corepressor SMILE recruits SIRT1 to inhibit nuclear receptor estrogen receptor-related receptor gamma transactivation

SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a corepressor of the glucocorticoid receptor, constitutive androstane receptor, and hepatocyte nuclear factor 4alpha. Here we show that SMILE also represses estrogen receptor-related receptor gamma (ERRgamma) transactivation. Knockdown of SMILE gene expression increases ERRgamma activity. SMILE directly interacts with ERRgamma in vitro and in vivo. Domain mapping analysis showed that SMILE binds to the AF2 domain of ERRgamma. SMILE represses ERRgamma transactivation partially through competition with coactivators PGC-1alpha, PGC-1beta, and GRIP1. Interestingly, the repression of SMILE on ERRgamma is released by SIRT1 inhibitors, a catalytically inactive SIRT1 mutant, and SIRT1 small interfering RNA but not by histone protein deacetylase inhibitor. In vivo glutathione S-transferase pulldown and coimmunoprecipitation assays validated that SMILE physically interacts with SIRT1. Furthermore, the ERRgamma inverse agonist GSK5182 enhances the interaction of SMILE with ERRgamma and SMILE-mediated repression. Knockdown of SMILE or SIRT1 blocks the repressive effect of GSK5182. Moreover, chromatin immunoprecipitation assays revealed that GSK5182 augments the association of SMILE and SIRT1 on the promoter of the ERRgamma target PDK4. GSK5182 and adenoviral overexpression of SMILE cooperate to repress ERRgamma-induced PDK4 gene expression, and this repression is released by overexpression of a catalytically defective SIRT1 mutant. Finally, we demonstrated that ERRgamma regulates SMILE gene expression, which in turn inhibits ERRgamma. Overall, these findings implicate SMILE as a novel corepressor of ERRgamma and recruitment of SIRT1 as a novel repressive mechanism for SMILE and ERRgamma inverse agonist.

Molecular characterization of SMILE as a novel corepressor of nuclear receptors

SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a coregulator in ER signaling. In this study, we have examined the effects of SMILE on other NRs (nuclear receptors). SMILE inhibits GR, CAR and HNF4α-mediated transactivation. Knockdown of SMILE gene expression increases the transactivation of the NRs. SMILE interacts with GR, CAR and HNF4α in vitro and in vivo. SMILE and these NRs colocalize in the nucleus. SMILE binds to the ligand-binding domain or AF2 domain of the NRs. Competitions between SMILE and the coactivators GRIP1 or PGC-1α have been demonstrated in vitro and in vivo. Furthermore, an intrinsic repressive activity of SMILE is observed in Gal4-fusion system, and the intrinsic repressive domain is mapped to the C-terminus of SMILE, spanning residues 203–354. Moreover, SMILE interacts with specific HDACs (histone deacetylases) and SMILE-mediated repression is released by HDAC inhibitor trichostatin A, in a NR-specific manner. Finally, ChIP (chromatin immunoprecipitation) assays reveal that SMILE associates with the NRs on the target gene promoters. Adenoviral overexpression of SMILE represses GR-, CAR- and HNF4α-mediated target gene expression. Overall, these results suggest that SMILE functions as a novel corepressor of NRs via competition with coactivators and the recruitment of HDACs.

SMILE, a new orphan nuclear receptor SHP-interacting protein, regulates SHP-repressed estrogen receptor transactivation

SHP (small heterodimer partner) is a well-known NR (nuclear receptor) co-regulator. In the present study, we have identified a new SHP-interacting protein, termed SMILE (SHP-interacting leucine zipper protein), which was previously designated as ZF (Zhangfei) via a yeast two-hybrid system. We have determined that the SMILE gene generates two isoforms [SMILE-L (long isoform of SMILE) and SMILE-S (short isoform of SMILE)]. Mutational analysis has demonstrated that the SMILE isoforms arise from the alternative usage of initiation codons. We have confirmed the in vivo interaction and co-localization of the SMILE isoforms and SHP. Domain-mapping analysis indicates that the entire N-terminus of SHP and the middle region of SMILE-L are involved in this interaction. Interestingly, the SMILE isoforms counteract the SHP repressive effect on the transactivation of ERs (estrogen receptors) in HEK-293T cells (human embryonic kidney cells expressing the large T-antigen of simian virus 40), but enhance the SHP-repressive effect in MCF-7, T47D and MDA-MB-435 cells. Knockdown of SMILE gene expression using siRNA (small interfering RNA) in MCF-7 cells increases ER-mediated transcriptional activity. Moreover, adenovirus-mediated overexpression of SMILE and SHP down-regulates estrogen-induced mRNA expression of the critical cell-cycle regulator E2F1. Collectively, these results indicate that SMILE isoforms regulate the inhibition of ER transactivation by SHP in a cell-type-specific manner and act as a novel transcriptional co-regulator in ER signalling.

Activating transcription factor 4 is critical for proliferation and survival in primary bone marrow stromal cells and calvarial osteoblasts

Activating transcription factor 4 (ATF4) is essential for bone formation. However, the mechanism of its actions in bone is poorly understood. The present study examined the role for ATF4 in the regulation of proliferation and survival of primary mouse bone marrow stromal cells (BMSCs) and osteoblasts. Results showed that Atf4(-/-) cells display a severe proliferative defect as measured by multiple cell proliferation assays. Cell cycle progression of Atf4(-/-) BMSCs was largely delayed with significant G1 arrest. Expression of cyclin D1 was decreased both at the mRNA and protein level. A similar proliferation defect was observed in Atf4(-/-) calvarial periosteal osteoblasts when compared with wt control. Knocking down Atf4 mRNA by small interfering RNA in MC3T3-E1 subclone 4 preosteoblasts markedly reduced expression of cyclin D1 and cell proliferation. In contrast, overexpression of ATF4 increased cyclin D1 expression as well as cell proliferation in Atf4(-/-) BMSCs. In addition, apoptosis was significantly increased in Atf4(-/-) BMSCs and calvarial periosteal osteoblasts relative to wt controls. Taken together, these results for the first time demonstrate that ATF4 is a critical regulator of proliferation and survival in BMSCs and osteoblasts in vitro and in vivo.

Zhangfei, a novel regulator of the human nerve growth factor receptor, trkA

The replication of herpes simplex virus (HSV) in epithelial cells, and during reactivation from latency in sensory neurons, depends on a ubiquitous cellular protein called host cell factor (HCF). The HSV transactivator, VP16, which initiates the viral replicative cycle, binds HCF as do some other cellular proteins. Of these, the neuronal transcription factor Zhangfei suppresses the ability of VP16 to initiate the replicative cycle. It also suppresses Luman, another cellular transcription factor that binds HCF. Interactions of nerve growth factor (NGF) and its receptor tropomyosin-related kinase (trkA) appear to be critical for maintaining HSV latency. Because the neuronal transcription factor Brn3a, which regulates trkA expression, has a motif for binding HCF, we investigated if Zhangfei had an effect on its activity. We found that Brn3a required HCF for activating the trkA promoter and Zhangfei suppressed its activity in non-neuronal cells. However, in neuron-like NGF-differentiated PC12 cells, both Brn3a and Zhangfei activated the trkA promoter and induced the expression of endogenous trkA. In addition, capsaicin, a stressor, which activates HSV in in vitro models of latency, decreased levels of Zhangfei and trkA transcripts in NGF-differentiated PC12 cells.

A novel protein, Luman/CREB3 recruitment factor, inhibits Luman activation of the unfolded protein response

Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER)-bound cellular transcription factor. It has been implicated in the mammalian unfolded protein response (UPR), as well as herpes simplex virus reactivation from latency in sensory neurons. Here, we report the identification of a novel Luman recruitment factor (LRF). Like Luman, LRF is a UPR-responsive basic-region leucine zipper protein that is prone to proteasomal degradation. Being a highly unstable protein, LRF interacts with Luman through the leucine zipper region and promotes Luman degradation. LRF was found to recruit the nuclear form of Luman to discrete nuclear foci, which overlap with the nuclear receptor coactivator GRIP1 bodies, and repress the transactivation activity of Luman. Compared to LRF+/+ mouse embryonic fibroblast (MEF) cells, the levels of CHOP, EDEM, and Herp were elevated in LRF-/- MEF cells. We propose that LRF is a negative regulator of the UPR. For Luman, it may represent another level of regulation following Luman proteolytic cleavage on the ER and nuclear translocation. In addition to inducing rapid Luman turnover, LRF may repress the transactivation potential of Luman by sequestering it in the LRF nuclear bodies away from key cofactors (such as HCF-1) that are required for transcriptional activation.

Identification and characterization of the DNA-binding properties of a Zhangfei homologue in Japanese pufferfish, Takifugu rubripes

Zhangfei is a basic region-leucine zipper (bZIP) transcription factor identified through its interaction with a herpesvirus-related host cell factor HCF1 (C1). Unlike most bZIP proteins, the mammalian Zhangfei protein does not bind DNA as homodimers. It is believed due to the absence of an asparagine residue in the basic region, which forms the DNA-recognition motif, NxxAAxxCR, in all bZIP proteins. Here, we report the identification and characterization of a novel Zhangfei homologue in Takifugu rubripes, which has an intact DNA-recognition motif by sequence analysis. We found that the pufferfish Zhangfei (pZF) appeared to have all the functional domains known in human Zhangfei, including the conserved HCF1-binding motif; however, pZF did not appear to bind DNA either. These findings suggest that the distinct property of the Zhangfei basic region is conserved during the evolution of vertebrates and that Zhangfei requires interaction with other proteins to regulate transcription from target promoters.