Rapid and preferential induction of ATF3 transcription in response to low doses of UVA light

Atf3 defines a population of pulmonary endothelial cells essential for lung regeneration

Following acute injury, the capillary vascular bed in the lung must be repaired to reestablish gas exchange with the external environment. Little is known about the transcriptional and signaling factors that drive pulmonary endothelial cell (EC) proliferation and subsequent regeneration of pulmonary capillaries, as well as their response to stress. Here, we show that the transcription factor Atf3 is essential for the regenerative response of the mouse pulmonary endothelium after influenza infection. Atf3 expression defines a subpopulation of capillary ECs enriched in genes involved in endothelial development, differentiation, and migration. During lung alveolar regeneration, this EC population expands and increases the expression of genes involved in angiogenesis, blood vessel development, and cellular response to stress. Importantly, endothelial cell-specific loss of Atf3 results in defective alveolar regeneration, in part through increased apoptosis and decreased proliferation in the endothelium. This leads to the general loss of alveolar endothelium and persistent morphological changes to the alveolar niche, including an emphysema-like phenotype with enlarged alveolar airspaces lined with regions that lack vascular investment. Taken together, these data implicate Atf3 as an essential component of the vascular response to acute lung injury that is required for successful lung alveolar regeneration.

Multifunctional Non-Coding RNAs Mediate Latent Infection and Recurrence of Herpes Simplex Viruses

Herpes simplex viruses (HSVs) often cause latent infection for a lifetime, leading to repeated recurrence. HSVs have been engineered as oncolytic HSVs. The mechanism of the latent infection and recurrence remains largely unknown, which brings great challenges and limitations to eliminate HSVs in clinic and engineer safe oHSVs. Here, we systematically reviewed the latest development of the multi-step complex process of HSV latency and reactivation. Significantly, we first summarized the three HSV latent infection pathways, analyzed the structure and expression of the LAT1 and LAT2 of HSV-1 and HSV-2, proposed the regulation of LAT expression by four pathways, and dissected the function of LAT mediated by five LAT products of miRNAs, sRNAs, lncRNAs, sncRNAs and ORFs. We further analyzed that application of HSV LAT deletion mutants in HSV vaccines and oHSVs. Our review showed that deleting LAT significantly reduced the latency and reactivation of HSV, providing new ideas for the future development of safe and effective HSV therapeutics, vaccines and oHSVs. In addition, we proposed that RNA silencing or RNA interference may play an important role in HSV latency and reactivation, which is worth validating in future.

Sufentanil Alleviates Intrathecal Lidocaine Induced Prolonged Sensory and Motor Impairments but not the Spinal Histological Injury in Rats

Spinal anesthesia has evolved into a safe and widely accepted method of anesthesia. Synergy between opioids and local anesthetics further increases the quality of analgesia and decreases the dose requirement of both local anesthetics and opioids. However, over the last decades compelling evidence suggested that lidocaine could be more neurotoxic than other commonly used local anesthetics. Whether opioids can modify the local anesthetics-induced neurotoxicity is largely unexplored. Here, we investigated the effect of sufentanil on the neurotoxicity induced by intrathecal lidocaine in a rat model. Our data showed that 5 µg/ml sufentanil didn't deteriorate nor reduce the histopathological injuries induced by intrathecal application of 10% lidocaine in a rat model. However, it did alleviate sensory and motor function impairments induced by 10% lidocaine. Repeated intrathecal injection of 5 µg/ml sufentanil also decreased the paw withdraw threshold compared to the baseline. An increase in expression of activating transcription factor 3, a stress response gene, as a marker for injured neurons, was also detected in lidocaine-induced neurotoxicity, while 5 µg/ml sufentanil inhibited lidocaine-induced the upregulation of activating transcription factor 3. These results suggest that sufentanil alleviates lidocaine induced sensory and motor impairments, and did not worsen histopathological injury induced by intrathecal lidocaine.

The Stress-responsive Gene ATF3 Mediates Dichotomous UV Responses by Regulating the Tip60 and p53 Proteins

The response to UV irradiation is important for a cell to maintain its genetic integrity when challenged by environmental genotoxins. An immediate early response to UV irradiation is the rapid induction of activating transcription factor 3 (ATF3) expression. Although emerging evidence has linked ATF3 to stress pathways regulated by the tumor suppressor p53 and the histone acetyltransferase Tip60, the role of ATF3 in the UV response remains largely unclear. Here, we report that ATF3 mediated dichotomous UV responses. Although UV irradiation enhanced the binding of ATF3 to Tip60, knockdown of ATF3 expression decreased Tip60 stability, thereby impairing Tip60 induction by UV irradiation. In line with the role of Tip60 in mediating UV-induced apoptosis, ATF3 promoted the death of p53-defective cells in response to UV irradiation. However, ATF3 could also activate p53 and promote p53-mediated DNA repair, mainly through altering histone modifications that could facilitate recruitment of DNA repair proteins (such as DDB2) to damaged DNA sites. As a result, ATF3 rather protected the p53 wild-type cells from UV-induced apoptosis. Our results thus indicate that ATF3 regulates cell fates upon UV irradiation in a p53-dependent manner.

Role of activating transcription factor 3 in the synthesis of latency-associated transcript and maintenance of herpes simplex virus 1 in latent state in ganglia

A key property of herpes simplex viruses (HSVs) is their ability to establish latent infection in sensory or autonomic ganglia and to reactivate on physical, hormonal, or emotional stress. In latently infected ganglia, HSVs express a long noncoding RNA, a latency-associated transcript (LAT), which plays a key role in maintaining latently infected neurons, but not viral proteins. To investigate the events leading to reactivation, we examined the use of ganglionic organ cultures that enable rapid reactivation in medium containing antibody to nerve growth factor (NGF) or delayed reactivation in medium containing NGF and epidermal growth factor (EGF). Here we report the discovery that activating transcription factor 3 (ATF3), a stress response protein, profoundly affects the interaction of HSV with its host. Specifically, (i) ATF3 is induced by stress, such as inhibition of protein synthesis or infection; (ii) in infected cells, ATF3 enhances the accumulation of LAT by acting on the response elements in the promoter of the LAT precursor RNA; (iii) ATF3 is induced nearly 100-fold in ganglionic organ cultures; and (iv) ATF3 plays a key role in the maintenance of the latent state, inasmuch as expression of ATF3 bereft of the C-terminal activation domain acts as a dominant negative factor, inducing HSV gene expression in ganglionic organ cultures harboring latent virus and incubated in medium containing NGF and EGF. Thus, ATF3 is a component of a cluster of cellular proteins that together with LAT maintain the integrity of the neurons harboring latent virus.

Induction of ATF3 gene network by triglyceride-rich lipoprotein lipolysis products increases vascular apoptosis and inflammation

OBJECTIVE

Elevation of triglyceride-rich lipoproteins (TGRLs) contributes to the risk of atherosclerotic cardiovascular disease. Our work has shown that TGRL lipolysis products in high physiological to pathophysiological concentrations cause endothelial cell injury; however, the mechanisms remain to be delineated.

APPROACH AND RESULTS

We analyzed the transcriptional signaling networks in arterial endothelial cells exposed to TGRL lipolysis products. When human aortic endothelial cells in culture were exposed to TGRL lipolysis products, activating transcription factor 3 (ATF3) was identified as a principal response gene. Induction of ATF3 mRNA and protein was confirmed by quantitative reverse-transcription polymerase chain reaction and Western blot respectively. Immunofluorescence analysis showed that ATF3 accumulated in the nuclei of cells treated with lipolysis products. Nuclear expression of phosphorylated c-Jun N-terminal kinase (JNK), previously shown to be an initiator of the ATF3 signaling cascade, also was demonstrated. Small interfering RNA (siRNA)-mediated inhibition of ATF3 blocked lipolysis products-induced transcription of E-selectin and interleukin-8, but not interleukin-6 or nuclear factor-κB. c-Jun, a downstream protein in the JNK pathway, was phosphorylated, whereas expression of nuclear factor-κB-dependent JunB was downregulated. Additionally, JNK siRNA suppressed ATF3 and p-c-Jun protein expression, suggesting that JNK is upstream of the ATF3 signaling pathway. In vivo studies demonstrated that infusion of TGRL lipolysis products into wild-type mice induced nuclear ATF3 accumulation in carotid artery endothelium. ATF3(-/-) mice were resistant to vascular apoptosis precipitated by treatment with TGRL lipolysis products. Also peripheral blood monocytes isolated from postprandial humans had increased ATF3 expression as compared with fasting monocytes.

CONCLUSIONS

This study demonstrates that TGRL lipolysis products activate ATF3-JNK transcription factor networks and induce endothelial cells inflammatory response.

Activating transcription factor 3 and the nervous system

Activating transcription factor 3 (ATF3) belongs to the ATF/cyclic AMP responsive element binding family of transcription factors and is often described as an adaptive response gene whose activity is usually regulated by stressful stimuli. Although expressed in a number of splice variants and generally recognized as a transcriptional repressor, ATF3 has the ability to interact with a number of other transcription factors including c-Jun to form complexes which not only repress, but can also activate various genes. ATF3 expression is modulated mainly at the transcriptional level and has markedly different effects in different types of cell. The levels of ATF3 mRNA and protein are normally very low in neurons and glia but their expression is rapidly upregulated in response to injury. ATF3 expression in neurons is closely linked to their survival and the regeneration of their axons following axotomy, and that in peripheral nerves correlates with the generation of a Schwann cell phenotype that is conducive to axonal regeneration. ATF3 is also induced by Toll-like receptor (TLR) ligands but acts as a negative regulator of TLR signaling, suppressing the innate immune response which is involved in immuno-surveillance and can enhance or reduce the survival of injured neurons and promote the regeneration of their axons.

Comparative gene responses to collected ambient particles in vitro: endothelial responses

Epidemiologic studies associate exposure to ambient particulate matter (APM) with increased cardiovascular mortality. Since both pulmonary inflammation and systemic circulation of ultrafine particles are hypothesized as initiating cardiovascular effects, we examined responses of potential target cells in vitro. Human aortic endothelial cells (HAEC) were exposed to 10 μg/ml fine and ultrafine APM collected in an urban setting in summer 2006 or winter 2007 in the San Joaquin Valley, California. RNA isolated after 3 h was analyzed with high-density oligonucleotide arrays. Summer APM treatment affected genes involved in xenobiotic and oxidoreductase activity, transcription factors, and inflammatory responses in HAEC, while winter APM had a robust xenobiotic but lesser inflammatory response. Real-time polymerase chain reaction analysis confirmed that particulate matter (PM)-treated HAEC increased mRNA levels of xenobiotic response enzymes CYP1A1, ALDH1A3, and TIPARP and cellular stress response transcription factor ATF3. Inflammatory response genes included E-selectin, PTGS2, CXCL-2 (MIP-2α), and CCL-2 (MCP-1). Multiplex protein assays showed secretion of IL-6 and MCP-1 by HAEC. Since induction of CYP1A1 is mediated through the ligand-activated aryl hydrocarbon receptor (AhR), we demonstrated APM induced AhR nuclear translocation by immunofluorescence and Western blotting and activation of the AhR response element using a luciferase reporter construct. Inhibitor studies suggest differential influences of polycyclic aromatic hydrocarbon signaling, ROS-mediated responses and endotoxin alter stress and proinflammatory endothelial cell responses. Our findings demonstrate gene responses correlated with current concepts that systemic inflammation drives cardiovascular effects of particulate air pollution. We also demonstrate a unique pattern of gene responses related to xenobiotic metabolism in PM-exposed HAEC.

The potential role of ATF3 on immune response is regulated by BMP10 through Smad dependent pathway

It is hypothesis that ATF3 is a downstream component of BMP10. The possible function of ATF3 on immune response is partially regulated by BMP10 via Smad dependent pathway. BMP10 is highly expressed in blood cells during embryonic development based on our in situ hybridization. The expression of ATF3 is enhanced by BMP10 in overexpression transgenic mice. Both BMP10 and ATF3 can response to stress stimulate, and ATF3 is well understood as a stress inducible gene which possible contributes to immune response. The Smad dependent pathway is well established for BMP10 in regulation expression of downstream targets. It would be interesting for us to determine the relationship between BMP10 and ATF3, especially to understand the mechanism of BMP10 and ATF3 effecting on heart development, as well as immune response exposed to stress stimulates.

Activating transcription factor 3 confers protection against ventilator-induced lung injury

RATIONALE

Ventilator-induced lung injury (VILI) significantly contributes to mortality in patients with acute respiratory distress syndrome, the most severe form of acute lung injury. Understanding the molecular basis for response to cyclic stretch (CS) and its derangement during high-volume ventilation is of high priority.

OBJECTIVES

To identify specific molecular regulators involved in the development of VILI.

METHODS

We undertook a comparative examination of cis-regulatory sequences involved in the coordinated expression of CS-responsive genes using microarray analysis. Analysis of stretched versus nonstretched cells identified significant enrichment for genes containing putative binding sites for the transcription factor activating transcription factor 3 (ATF3). To determine the role of ATF3 in vivo, we compared the response of ATF3 gene-deficient mice to wild-type mice in an in vivo model of VILI.

MEASUREMENTS AND MAIN RESULTS

ATF3 protein expression and nuclear translocation is increased in the lung after mechanical ventilation in wild-type mice. ATF3-deficient mice have greater sensitivity to mechanical ventilation alone or in conjunction with inhaled endotoxin, as demonstrated by increased cell infiltration and proinflammatory cytokines in the lung and bronchoalveolar lavage, and increased pulmonary edema and indices of tissue injury. The expression of stretch-responsive genes containing putative ATF3 cis-regulatory regions was significantly altered in ATF3-deficient mice.

CONCLUSIONS

ATF3 deficiency confers increased sensitivity to mechanical ventilation alone or in combination with inhaled endotoxin. We propose ATF3 acts to counterbalance CS and high volume-induced inflammation, dampening its ability to cause injury and consequently protecting animals from injurious CS.

Cellular and sub-cellular responses to UVA in relation to carcinogenesis

PURPOSE

UVA radiation (315-400 nm) contributes to skin aging and carcinogenesis. The aim of this review is to consider the mechanisms that underlie UVA-induced cellular damage, how this damage may be prevented or repaired and the signal transduction processes that are elicited in response to it.

RESULTS

Exposure to ultraviolet (UV) light is well-established as the causative factor in skin cancer. Until recently, most work on the mechanisms that underlie skin carcinogenesis focused on shorter wavelength UVB radiation (280-315 nm), however in recent years there has been increased interest in the contribution made by UVA. UVA is able to cause a range of damage to cellular biomolecules including lipid peroxidation, oxidized protein and DNA damage, such as 8-oxoguanine and cyclobutane pyrimidine dimers. Such damage is strongly implicated in both cell death and malignant transformation and cells have a number of mechanisms in place to mitigate the effects of UVA exposure, including antioxidants, DNA repair, and stress signalling pathways.

CONCLUSIONS

The past decade has seen a surge of interest in the biological effects of UVA exposure as its significance to the process of photo-carcinogenesis has become increasingly evident. However, unpicking the unique complexity of the cellular response to UVA, which is only now becoming apparent, will be a major challenge for the field of photobiology in the 21st century.

Purification, cDNA cloning, and expression profiles of the cyclobutane pyrimidine dimer photolyase of Xenopus laevis

Photolyase is a light-dependent enzyme that repairs pyrimidine dimers in DNA. Two types of photolyases have been found in frog Xenopus laevis, one for repairing cyclobutane pyrimidine dimers (CPD photolyase) and the other for pyrimidine-pyrimidone (6-4)photoproduct [(6-4)photolyase]. However, little is known about the former type of the Xenopus photolyases. To characterize this enzyme and its expression profiles, we isolated the entire coding region of a putative CPD photolyase cDNA by extending an EST (expressed sequence tag) sequence obtained from the Xenopus database. Nucleotide sequence analysis of the cDNA revealed a protein of 557 amino acids with close similarity to CPD photolyase of rat kangaroo. The identity of this cDNA was further established by the molecular mass (65 kDa) and the partial amino acid sequences of the major CPD photolyase that we purified from Xenopus ovaries. The gene of this enzyme is expressed in various tissues of Xenopus. Even internal organs like heart express relatively high levels of mRNA. A much smaller amount was found in skin, although UV damage is thought to occur most frequently in this tissue. Such expression profiles suggest that CPD photolyase may have roles in addition to the photorepair function.

Construction and activity assay of the activating transcription factor 3 reporter vector pATF/CRE-luc

Activating transcription factor 3 (ATF3), a member of the activating transcription factor/cAMP responsive element binding protein (ATF/CREB) family of transcription factors, is induced by many physiological stresses. To investigate the activity of ATF/CREB in cells with physiological stresses, we developed a practical reporter vector, the plasmid pATF/CRE-luc, bearing activating transcription factor/cAMP responsive element (ATF/CRE) binding sites. This plasmid was constructed by inserting three repeats of the ATF/CRE binding element into the plasmid pG5luc, replacing the GAL-4 binding sites. The plasmids pACT/ATF3 and pATF/CRE-luc were transfected into HeLa and NIH3T3 cells, respectively, and the results showed that the expression of luciferase was increased in a dose-dependent manner on plasmid pACT/ATF3. The data suggested that the plasmid pATF/CRE-luc could be used as a sensitive and convenient reporter system of ATF3 activity.

Effects of chlorpromazine with and without UV irradiation on gene expression of HepG2 cells

Damage to DNA can trigger a variety of stress-related signals that alter the expression of genes associated with numerous biological pathways. In this study, we have used a cDNA microarray representing 1089 genes related to DNA damage and repair, cell cycle, transcription, metabolism and other toxicologically important cell functions to identify genes regulated in response to DNA damage in HepG2 cells induced by UV-activated chlorpromazine (CPZ). CPZ itself is not genotoxic but, upon UV irradiation with a non-genotoxic dose in the UVA range, it produces reactive free radical intermediates with DNA damaging properties. Genotoxicity in HepG2 cells was assessed concomitantly to gene expression profiling using the Comet assay. Kinetic studies were performed at a non-cytotoxic but clearly photogenotoxic concentration of CPZ (1.25 microg/ml) to characterize gene expression profiles at four different time points (3, 7, 15, 23 h) post short-term treatment. The results obtained from repeated experiments display a time-dependent expression pattern of up-regulated and repressed genes with distinct peaks in the number of differentially expressed genes at the 7 and 23 h time points. Most of the genes with altered expression belonged to the functional categories of cell cycle regulation and cell proliferation. A comparison with published expression profiles established in response to other genotoxic compounds showed low levels of concordance, which is most likely caused by the fact that extremely different testing conditions were used.

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.

Photodecomposition and phototoxicity of natural retinoids

Sunlight is a known human carcinogen. Many cosmetics contain retinoid-based compounds, such as retinyl palmitate (RP), either to protect the skin or to stimulate skin responses that will correct skin damaged by sunlight. However, little is known about the photodecomposition of some retinoids and the toxicity of these retinoids and their sunlight-induced photodecomposition products on skin. Thus, studies are required to test whether topical application of retinoids enhances the phototoxicity and photocarcinogenicity of sunlight and UV light. Mechanistic studies are needed to provide insight into the disposition of retinoids in vitro and on the skin, and to test thoroughly whether genotoxic damage by UV-induced radicals may participate in any toxicity of topically applied retinoids in the presence of UV light. This paper reports the update information and our experimental results on photostability, photoreactions, and phototoxicity of the natural retinoids including retinol (ROH), retinal, retinoid acid (RA), retinyl acetate, and RP (Figure 1).

Identification of genes responsive to UV-A radiation in human lens epithelial cells using complementary DNA microarrays

UV-A radiation produces cataract in animals, enhances photoaging of the lens and skin and increases the phototoxicity of drugs. However, the nature of genes that are activated or repressed after cellular exposure to UV-A radiation remains enigmatic. Because lens epithelial cells exposed to UV-A radiation undergo apoptosis 4 h after exposure to the stress, we sought to establish the change in gene expression in cells by UV-A radiation using gene expression profiling using complementary DNA microarrays containing about 12 000 genes. We identified 78 genes abnormally expressed in UV-A-irradiated cells (showing >2.5-fold change at P < 0.05). These genes are implicated in various biological processes, including signal transduction and nucleic acid binding, and genes encoding enzymes. A majority of the genes were downregulated. Our analysis revealed that the expression of genes for the transcription factors ATF-3 and Pilot increased four-fold, whereas the gene for the apoptosis regulator NAPOR-1 decreased five-fold. These changes were confirmed by real-time quantitative reverse transcriptase-polymerase chain reaction. The calpain large polypeptide 3 (CANP3) gene also increased nine-fold after UV-A radiation. In addition, peroxisomal biogenesis factor 7, glucocorticoid receptor-alpha and tumor-associated calcium signal transducer genes decreased three- to eight-fold. Western blot analysis further confirmed the increase in protein expression of ATF-3 and CANP3 and decreased expression of glucocorticoid receptor-alpha in the irradiated cells. Surprisingly, most of these genes had not been previously shown to be modulated by UV-A radiation. Our results show that human lens epithelial cells respond to a single dose of UV-A radiation by enhancing or suppressing functionally similar sets of genes, some of which have opposing functions, around the time at which apoptosis occurs. These studies support the intriguing concept that activation of competing pathways favoring either cell survival or death is a means to coordinate the response of cells to UV-A stress.

A Protein Sequestering System Reveals Control of Cellular Programs by the Transcriptional Coactivator HCF-1

The mammalian transcriptional coactivator HCF-1 is a critical component of the multiprotein herpes simplex virus immediate early gene enhancer core complex. The protein has also been implicated in basic cellular processes such as cell-cycle progression, transcriptional coactivation, and mRNA processing. Functions have been attributed to HCF-1 primarily from analyses of protein-protein interactions and from the cell-cycle-arrested phenotype of an HCF-1 temperature-sensitive mutant. However, neither the mechanisms involved nor specific cellular transcriptional targets have been identified. As the protein is essential for cell viability and proliferation, a genetic system was developed to specifically sequester the nuclear factor in the cell cytoplasm in a regulated manner. This approach exhibits no significant cell toxicity yet clearly demonstrates the requirement of available nuclear HCF-1 for herpes simplex virus immediate early gene expression during productive infection. Additionally, cellular transcriptional events were identified that contribute to understanding the functions ascribed to the protein and implicate the protein in events that impact the regulation of critical cellular processes.

Functional genomics of UV radiation responses in human cells

The gene expression responses of MCF-7, a p53 wild-type (wt) human cell line, were monitored by cDNA microarray hybridization after exposure to different wavelengths of UV irradiation. Equitoxic doses of UVA, UVB, and UVC radiation were used to reduce survival to 37%. The effects of suramin, a signal pathway inhibitor, on the gene expression responses to the three UV wavelengths were also compared in this model system. UVB radiation triggered the broadest gene expression responses, and 172 genes were found to be consistently responsive in at least two-thirds of independent UVB experiments. These UVB radiation-responsive genes encode proteins with diverse cellular roles including cell cycle control, DNA repair, signaling, transcription, protein synthesis, protein degradation, and RNA metabolism. The set of UVB-responsive genes included most of the genes responding to an equitoxic dose of UVC radiation, plus additional genes that were not strongly triggered by UVC radiation. There was also some overlap with genes responding to an equitoxic dose of UVA radiation, although responses to this lower energy UV radiation were overall weaker. Signaling through growth factor receptors and other cytokine receptors was shown to have a major role in mediating UV radiation stress responses, as suramin, which inhibits such receptors, attenuated responses to UV radiation in nearly all the cases. Inhibition by suramin was greater for UVC than for UVB irradiation. This probably reflects the more prominent role in UVB damage response of signaling by reactive oxygen species, which would not be affected by suramin. Our results with suramin demonstrate the power of cDNA microarray hybridization to illuminate the global effects of a pharmacologic inhibitor on cell signaling.