Role for activating transcription factor 3 in stress-induced beta-cell apoptosis

Host response to the attenuated poxvirus vector NYVAC: upregulation of apoptotic genes and NF-kappaB-responsive genes in infected HeLa cells

NYVAC has been engineered as a safe, attenuated vaccinia virus (VV) vector for use in vaccination against a broad spectrum of pathogens and tumors. Due to the interest in NYVAC-based vectors as vaccines and current phase I/II clinical trials with this vector, there is a need to analyze the human host response to NYVAC infection. Using high-density cDNA microarrays, we found 368 differentially regulated genes after NYVAC infection of HeLa cells. Clustering of the regulated genes identified six discrete gene clusters with altered expression patterns. Clusters 1 to 3 represented 47.5% of the regulated genes, with three patterns of gene activation kinetics, whereas clusters 4 to 6 showed distinct repression kinetics. Quantitative real-time reverse transcription-PCR analysis of selected genes validated the array data. Upregulated transcripts correlated with genes implicated in immune responses, including those encoding interleukin-1 receptor 2 (IL-1R2), IL-6, ISG-15, CD-80, and TNFSF7. NYVAC upregulated several intermediates of apoptotic cascades, including caspase-9, correlating with its ability to induce apoptosis. NYVAC infection also stimulated the expression of NF-kappaB1 and NF-kappaB2 as well as that of NF-kappaB target genes. Expression of the VV host range K1L gene during NYVAC infection prevented NF-kappaB activation, but not the induction of apoptosis. This study is the first overall analysis of the transcriptional response of human cells to NYVAC infection and provides a framework for future functional studies to evaluate this vector and its derivatives as human vaccines.

Characterization of ATF3 induction after ionizing radiation in human skin cells

The epidermis is a physiological barrier that protects an organism against pathogens and chemical or physical damage. The degree of skin injuries caused by radiation influences the subsequent survival from and prognosis of such injuries. Recently, we have identified, using microarray technology, genes regulated by high-dose radiation exposure in normal human epidermal keratinocytes. Our results suggest the possible role of ATF3 in the apoptosis pathway in radiation injuries caused by high-dose radiation exposure in human skin. In the present study, we characterized ATF3 induction after X-ray irradiation in normal human skin cells. Our results showed that the induction of ATF3 mRNA is rapid, transient, and at least in part mediated through a transcriptional mechanism after X-irradiation of normal human keratinocytes. In addition, ATF3 was induced and accumulated in the nuclei of keratinocytes upon treatment with an apoptosis inducer, anisomycin. Our results also suggested that the induction mechanism of ATF3 mRNA by X-irradiation may be different not only between epidermal keratinocytes and dermal fibroblasts, but also between dermal fibroblasts and lung fibroblasts.

ATF5 increases cisplatin-induced apoptosis through up-regulation of cyclin D3 transcription in HeLa cells

ATF5 transcription factor plays an essential role in hematopoietic and glioma cell survival and neuronal cell differentiation. Here, we report for the first time the pro-apoptosis role of ATF5 and identify Cyclin D3 as an ATF5-targeted apoptosis-related gene. The ectopic expression of ATF5 in HeLa cells could markedly increase cisplatin-induced apoptosis and the cleavage of Caspase-3, and induce Cyclin D3 mRNA expression via cooperation with E2F1 transcription factor. Moreover, the interference of Cyclin D3 expression by transfection with Cyclin D3 RNAi could protect cells from ATF5-mediated apoptosis induced by cisplatin, indicating the contribution of Cyclin D3 in ATF5-mediated apoptosis. Taken together, these results suggest that ATF5 increases cisplatin-induced apoptosis through up-regulation of Cyclin D3 transcription, which elicits survival signals in HeLa cells.

Reduction of spermatogenesis but not fertility in Creb3l4-deficient mice

Creb3l4 belongs to the CREB/ATF family of transcription factors that are involved in mediating transcription in response to intracellular signaling. This study shows that Creb3l4 is expressed at low levels in all organs and in different stages of embryogenesis but is present at very high levels in the testis, particularly in postmeiotic male germ cells. In contrast to CREB3L4 in the human prostate, of which specific expression was detected, Creb3l4 transcripts in the mouse prostate could be detected only by RT-PCR. To identify the physiological function of Creb3l4, the murine gene was inactivated by replacement with the gene encoding green fluorescent protein. Surprisingly, Creb3l4-deficient mice were born at expected ratios, were healthy, and displayed normal long-term survival rates. Despite a significant reduction in the number of spermatozoa in the epididymis of Creb3l4(-)(/)(-) mice, the breeding of mutant males with wild-type females was productive and the average litter size was not significantly altered in comparison to wild-type littermates. Further analyses revealed that the seminiferous tubules of Creb3l4(-)(/)(-) mice contained all of the developmental stages, though there was evidence for increased apoptosis of meiotic/postmeiotic germ cells. These results suggest that Creb3l4 plays a role in male germ cell development, but its loss is insufficient to completely compromise the production of spermatozoa.

Interaction of RNA-binding proteins HuR and AUF1 with the human ATF3 mRNA 3'-untranslated region regulates its amino acid limitation-induced stabilization

ATF3 expression is induced in cells exposed to a variety of stress conditions, including nutrient limitation. Here we demonstrated that the mechanism by which the ATF3 mRNA content is increased following amino acid limitation of human HepG2 hepatoma cells is mRNA stabilization. Analysis of ATF3 mRNA turnover revealed that the half-life was increased from about 1 h in control cells to greater than 8 h in the histidine-deprived state, demonstrating mRNA stabilization in response to nutrient deprivation. Treatment of HepG2 cells with thapsigargin, which causes endoplasmic reticulum stress, also increased the half-life of ATF3 mRNA. HuR is an RNA-binding protein that regulates both the stability and cytoplasmic/nuclear localization of mRNA species containing AU-rich elements. Another RNA-binding protein, AUF1, regulates target mRNA molecules by enhancing their decay. Amino acid limitation caused a slightly elevated mRNA level for HuR and AUF1 mRNA. The nuclear HuR protein content was unchanged, and AUF1 protein increased slightly after amino acid limitation, whereas the cytoplasmic levels of both HuR and AUF1 protein increased. Immunoprecipitation of HuR-RNA complexes followed by reverse transcriptase-PCR analysis showed that HuR interacted with ATF3 mRNA in vivo and that this interaction increased following amino acid limitation. In contrast, the interaction of AUF1 with the ATF3 mRNA is decreased in histidine-deprived cells relative to control cells. Suppression of HuR expression by RNA interference partially blocked the accumulation of ATF3 mRNA following amino acid deprivation. The results demonstrated that coordinated regulation of mRNA stability by HuR and AUF1 proteins contributes to the observed increase in ATF3 expression following amino acid limitation.

Activating transcription factor 3, a stress sensor, activates p53 by blocking its ubiquitination

Activating transcription factor 3 (ATF3) is rapidly induced by diverse environmental insults including genotoxic stress. We report herein that its interaction with p53, enhanced by genotoxic stress, stabilizes the tumor suppressor thereby augmenting functions of the latter. Overexpression of ATF3 (but not a mutated ATF3 protein (Delta102-139) devoid of its p53-binding region) prevents p53 from MDM2-mediated degradation and leads to increased transcription from p53-regulated promoters. ATF3, but not the Delta102-139 protein, binds the p53 carboxy-terminus and diminishes its ubiquitination and nuclear export. Genotoxic-stressed ATF3-null mouse embryonic fibroblasts, or cells in which ATF3 was reduced by small interference RNA, show inefficient p53 induction and impaired apoptosis compared with wild-type cells. ATF3-null cells (but not wild-type cells), which poorly accumulate p53, are transformed by oncogenic Ras. Thus, ATF3 is a novel stress-activated regulator of p53 protein stability/function providing the cell with a means of responding to a wide range of environmental insult, thus maintaining DNA integrity and protecting against cell transformation.

Identification of Skin injury-related genes induced by ionizing radiation in human keratinocytes using cDNA microarray

The skin is an external organ that is most frequently exposed to radiation. High-dose radiation initiates and promotes skin cancer and acute radiation injury. It is important to investigate the influence of high-dose radiation exposure on the skin at the molecular level to understand acute radiation injury. To identify genes that are associated with injury caused by high-dose radiation exposure of the skin, we used microarray technology to examine the effect of irradiation on approximately 1000 genes in normal human epidermal keratinocytes at 3 h postirradiation with a cytotoxic dose of X-ray (5 Gy). We found that 16 and 59 genes were up- and down-regulated respectively in the keratinocytes. Several apoptosis-related genes, for example, BAK and TSC-22, and anti-proliferative genes, for example, BTG-1 and BTG-3, were up-regulated. We focused on ATF3 because ATF3 is induced most strongly by X-irradiation, and its function in keratinocytes is unknown. The induction of the ATF3 mRNA and protein in keratinocytes following X-ray was confirmed by RT-PCR and western blot analysis. ATF3 was also induced and accumulated within the nuclei of keratinocytes after X-ray irradiation in vivo and in vitro. Exogenous EYFP-ATF3 also accumulated within the nuclei of keratinocytes. In the transient expression assay, EYFP-ATF3, but not EYFP, induced apoptosis in keratinocytes. Taken together, these results suggest that ATF3 plays a role in apoptosis in keratinocytes and is associated with skin injury caused by ionizing radiation.

Differential effects of Gq alpha, G14 alpha, and G15 alpha on vascular smooth muscle cell survival and gene expression profiles

Gqalpha family members (Gqalpha, G11alpha, G14alpha, and G15/16alpha) stimulate phospholipase Cbeta (PLCbeta) and inositol lipid signaling but differ markedly in amino acid sequence and tissue distribution predicting unappreciated functional diversity. To examine functional differences, we compared the signaling properties of Gqalpha, G14alpha, and G15alpha and their cellular responses in vascular smooth muscle cells (VSMC). Constitutively active forms of Gqalpha, G14alpha, or G15alpha elicit markedly different responses when introduced to VSMC. Whereas each Galpha stimulated PLCbeta to similar extents when expressed at equal protein levels, Gqalpha and G14alpha but not G15alpha initiated profound cell death within 48 h. This response was the result of activation of apoptotic pathways, because Gqalpha and G14alpha, but not G15alpha, stimulated caspase-3 activation and did not alter phospho-Akt, a regulator of cell survival pathways. Gqalpha and G14alpha stimulate nuclear factor of activated T cell (NFAT) activation in VSMC, but Galpha-induced cell death seems independent of PKC, InsP(3)/Ca(2+), and NFAT, in that pharmacological inhibitors of these pathways did not block cell death. Gene expression analysis indicates that Gqalpha, G14alpha, and G15alpha each elicit markedly different profiles of altered gene sets in VSMC after 24 h. Whereas all three Galpha stimulated changes (> or =2-fold) in 50 shared mRNA, Gqalpha and G14alpha (but not G15alpha) stimulated changes in 221 shared mRNA, many of which are reported to be pro-apoptotic and/or involved with TNF-alpha signaling. We were surprised to find that each Galpha also stimulated changes in nonoverlapping Galpha-specific gene sets. These findings demonstrate that Gqalpha family members activate both overlapping and distinct signaling pathways and are more functionally diverse than previously thought.

Gene expression profiling in conjunction with physiological rescues of IKKalpha-null cells with wild type or mutant IKKalpha reveals distinct classes of IKKalpha/NF-kappaB-dependent genes

Cellular responses to stress-like stimuli require the IkappaB kinase (IKK) signalsome (IKKalpha, IKKbeta, and NEMO/IKKgamma) to activate NF-kappaB-dependent genes. IKKbeta and NEMO/IKKgamma are required to release NF-kappaB p65/p50 heterodimers from IkappaBalpha, resulting in their nuclear migration and sequence-specific DNA binding; but IKKalpha was found to be dispensable for this initial phase of canonical NF-kappaB activation. Nevertheless, IKKalpha-/- mouse embryonic fibroblasts (MEFs) fail to express NF-kappaB targets in response to proinflammatory stimuli, uncovering a nuclear role for IKKalpha in NF-kappaB activation. However, it remains unknown whether the global defect in NF-kappaB-dependent gene expression of IKKalpha-/- cells is caused by the absence of IKKalpha kinase activity. We show by gene expression profiling that rescue of near physiological levels of wild type IKKalpha in IKKalpha-/- MEFs globally restores expression of their canonical NF-kappaB target genes. To prove that the kinase activity of IKKalpha was required on a genomic scale, the same physiological rescue was performed with a kinase-dead, ATP binding domain IKKalpha mutant (IKKalpha(K44M)). Remarkably, the IKKalpha(K44M) protein rescued approximately 28% of these genes, albeit in a largely stimulus-independent manner with the notable exception of several genes that also acquired tumor necrosis factor-alpha responsiveness. Thus the IKKalpha-containing signalsome unexpectedly functions in the presence and absence of extracellular signals in both kinase-dependent and -independent modes to differentially modulate the expression of five distinct classes of IKKalpha/NF-kappaB-dependent genes.

Transcriptional profile of a myotube starvation model of atrophy

Skeletal muscle wasting is a pervasive phenomenon that can result from a wide range of pathological conditions as well as from habitual muscular inactivity. The present work describes a cell-culture condition that induces significant atrophy in skeletal muscle C2C12 myotubes. The failure to replenish differentiation media in mature myotubes leads to rapid atrophy (53% in diameter), which is referred to here as starvation. Affymetrix microarrays were used to develop a transcriptional profile of control (fed) vs. atrophied (nonfed) myotubes. Myotube starvation was characterized by an upregulation of genes involved in translational inhibition, amino acid biosynthesis and transport, and cell cycle arrest/apoptosis, among others. Downregulated genes included several structural and regulatory elements of the extracellular matrix as well as several elements of Wnt/frizzled and TGF-β signaling pathways. Interestingly, the characteristic transcriptional upregulation of the ubiquitin-proteasome system, calpains, and cathepsins known to occur in multiple in vivo models of atrophy were not seen during myotube starvation. With the exception of the downregulation of extracellular matrix genes, serine protease inhibitor genes, and the upregulation of the translation initiation factor PHAS-I, this model of atrophy in cell culture has a transcriptional profile quite distinct from any study published to date with atrophy in whole muscle. These data show that, although the gross morphology of atrophied muscle fibers may be similar in whole muscle vs. myotube culture, the processes by which this phenotype is achieved differ markedly.

GeneChip analyses of global transcriptional responses of murine macrophages to the lethal toxin of Bacillus anthracis

We performed GeneChip analyses on RNA from Bacillus anthracis lethal toxin (LeTx)-treated RAW 264.7 murine macrophages to investigate global effects of anthrax toxin on host cell gene expression. Stringent analysis of data revealed that the expression of several mitogen-activated protein kinase kinase-regulatory genes was affected within 1.5 h post-exposure to LeTx. By 3.0 h, the expression of 103 genes was altered, including those involved in intracellular signaling, energy production, and protein metabolism.

Characterization and expression pattern of two zebrafishatf7 genes

Members of the ATF/CREB (activating transcription factor/cAMP-responsive element binding protein) transcription factor family play diverse roles in controlling cell proliferation, apoptosis, and oncogenesis, as well as in embryonic development of vertebrates. We identified two zebrafish orthologs of human ATF7 gene: atf7a and atf7b. Whole-mount in situ hybridization shows that zebrafish atf7a is first expressed in the notochord precursors at 80% epiboly stage and then in the developing notochord during segmentation. The expression of atf7a is positively regulated by ntl, flh, and spr2, which are involved in development of the notochord. In contrast, atf7b is maternally expressed and during embryogenesis its mRNA is ubiquitously distributed, showing an expression pattern similar to that of mammalian Atf7.

Adenylyl cyclase type VI gene transfer reduces phospholamban expression in cardiac myocytes via activating transcription factor 3

Cardiac-directed expression of adenylyl cyclase type VI (AC(VI)) increases stimulated cAMP production, improves heart function, and increases survival in cardiomyopathy. In contrast, pharmacological agents that increase intracellular levels of cAMP have detrimental effects on cardiac function and survival. We wondered whether effects that are independent of cAMP might be responsible for these salutary outcomes associated with AC(VI) expression. We therefore conducted a series of experiments focused on how gene transcription is influenced by AC(VI) in cultured neonatal rat cardiac myocytes, with a particular focus on genes that might influence cardiac function. We found that overexpression of AC(VI) down-regulated mRNA and protein expression of phospholamban, an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase. We determined that the cAMP-responsive-like element in the phospholamban (PLB) promoter was critical for down-regulation by AC(VI). Overexpression of AC(VI) did not alter the expression of CREB, CREM, ATF1, ATF2, or ATF4 proteins. In contrast, overexpression of AC(VI) increased expression of ATF3 protein, a suppressor of transcription. Following AC(VI) gene transfer, when cardiac myocytes were stimulated with isoproterenol or NKH477, a water-soluble forskolin analog that directly stimulates AC, expression of ATF3 protein was increased even more, which correlated with reduced expression of PLB. We then showed that AC(VI)-induced ATF3 protein binds to the cAMP-responsive-like element on the PLB promoter and that overexpression of ATF3 in cardiac myocytes inhibits PLB promoter activity. These findings indicate that AC(VI) has effects on gene transcription that are not directly dependent on cAMP generation.