TLR4-A Pertinent Player in Radiation-Induced Heart Disease?

The heart is one of the organs that is sensitive to developing delayed adverse effects of ionizing radiation (IR) exposure. Radiation-induced heart disease (RIHD) occurs in cancer patients and cancer survivors, as a side effect of radiation therapy of the chest, with manifestation several years post-radiotherapy. Moreover, the continued threat of nuclear bombs or terrorist attacks puts deployed military service members at risk of exposure to total or partial body irradiation. Individuals who survive acute injury from IR will experience delayed adverse effects that include fibrosis and chronic dysfunction of organ systems such as the heart within months to years after radiation exposure. Toll-like receptor 4 (TLR4) is an innate immune receptor that is implicated in several cardiovascular diseases. Studies in preclinical models have established the role of TLR4 as a driver of inflammation and associated cardiac fibrosis and dysfunction using transgenic models. This review explores the relevance of the TLR4 signaling pathway in radiation-induced inflammation and oxidative stress in acute as well as late effects on the heart tissue and the potential for the development of TLR4 inhibitors as a therapeutic target to treat or alleviate RIHD.

Effects of Whole and Partial Heart Irradiation on Collagen, Mast Cells, and Toll-like Receptor 4 in the Mouse Heart.

In radiation therapy of tumors in the chest, such as in lung or esophageal cancer, part of the heart may be situated in the radiation field. This can lead to the development of radiation-induced heart disease. The mechanisms by which radiation causes long-term injury to the heart are not fully understood, but investigations in pre-clinical research models can contribute to mechanistic insights. Recent developments in X-ray technology have enabled partial heart irradiation in mouse models. In this study, adult male and female C57BL/6J mice were exposed to whole heart (a single dose of 8 or 16 Gy) and partial heart irradiation (16 Gy to 40% of the heart). Plasma samples were collected at 5 days and 2 weeks after the irradiation for metabolomics analysis, and the cardiac collagen deposition, mast cell numbers, and left ventricular expression of Toll-like receptor 4 (TLR4) were examined in the irradiated and unirradiated parts of the heart at 6 months after the irradiation. Small differences were found in the plasma metabolite profiles between the groups. However, the collagen deposition did not differ between the irradiated and unirradiated parts of the heart, and radiation did not upregulate the mast cell numbers in either part of the heart. Lastly, an increase in the expression of TLR4 was seen only after a single dose of 8 Gy to the whole heart. These results suggest that adverse tissue remodeling was not different between the irradiated and unirradiated portions of the mouse heart. While there were no clear differences between male and female animals, additional work in larger cohorts may be required to confirm this result, and to test the inhibition of TLR4 as an intervention strategy in radiation-induced heart disease.

Small animal models of localized heart irradiation.

A subset of cancer patients treated with radiation therapy may experience radiation-induced heart disease (RIHD) that develops within weeks to several years after cancer treatment. Rodent models are most commonly used to examine the biological effects of local X-rays in the heart and test potential strategies to reduce RIHD. While developments in technology over the last decades have changed the procedures for local heart irradiation in animal models, the X-ray settings and radiation doses have remained quite consistent in time and between different research laboratories. This chapter provides a protocol for whole heart irradiation in rodent models, using an X-ray machine with cone beam computed tomography (CBCT) capabilities. Some methods for the quantification of common histological changes after whole heart irradiation in the rodent are also described.

Analysis of Radiation-Induced Changes in Cell Cycle and DNA Damage of Murine Hematopoietic Stem Cells by Multi-Color Flow Cytometry.

Exposure of bone marrow to genotoxic stress such as ionizing radiation (IR) results in a rapid decline of peripheral blood cells and stimulates entry of the normally quiescent hematopoietic stem cells (HSCs) into the cell cycle to reconstitute the hematopoietic system. While several protocols have employed flow cytometry analysis of bone marrow cells to study changes in specific cell populations with respect to cell cycle proliferation and/or expression of γ-H2AX, a marker of DNA damage, these parameters were examined in separate panels. Here, we describe a flow cytometry-based method specifically designed to examine cell cycle distribution using Ki-67 and FXCycle violet in combination with γ-H2AX in HSCs and hematopoietic progenitor cells (HPCs) within the same sample. This method is very useful, particularly in studies involving genotoxic stresses such as IR, which substantially reduce the absolute numbers of HSCs and HPCs available for staining. Additionally, we describe several important considerations for the analysis of markers of HSCs in irradiated versus unirradiated samples. Examples include the use of fluorescence minus one (FMO) controls, the gating strategy for markers whose expression is typically impacted by IR such as Sca1, tips for staining of intracellular antigens like Ki67, and ensuring the detection of signal from at least 500 events in each gate to ensure robustness of the results. © 2021 Wiley Periodicals LLC. Basic Protocol: Immunostaining protocol for bone marrow mononuclear cells using a multi-fluorophore panel.

Simultaneous exposure to chronic irradiation and simulated microgravity differentially alters immune cell phenotype in mouse thymus and spleen.

Deep-space missions may alter immune cell phenotype in the primary (e.g., thymus) and secondary (e.g., spleen) lymphoid organs contributing to the progression of a variety of diseases. In deep space missions, astronauts will be exposed to chronic low doses of HZE radiation while being in microgravity. Ground-based models of long-term uninterrupted exposures to HZE radiation are not yet available. To obtain insight in the effects of concurrent exposure to microgravity and chronic irradiation (CIR), mice received a cumulative dose of chronic 0.5 Gy gamma rays over one month ± simulated microgravity (SMG). To obtain insight in a dose rate effect, additional mice were exposed to single acute irradiation (AIR) at 0.5 Gy gamma rays. We measured proportions of immune cells relative to total number of live cells in the thymus and spleen, stress level markers in plasma, and change in body weight, food consumption, and water intake. CIR affected thymic CD3+/CD335+ natural killer T (NK-T) cells, CD25+ regulatory T (Treg) cells, CD27+/CD335- natural killer (NK1) cells and CD11c+/CD11b- dendritic cells (DCs) differently in mice subjected to SMG than in mice with normal loading. No such effects of CIR on SMG as compared to normal loading were observed in cell types from the spleen. Differences between CIR and AIR groups (both under normal loading) were found in thymic Treg and DCs. Food consumption, water intake, and body weight were less after coexposure than singular or no exposure. Compared to sham, all treatment groups exhibited elevated plasma levels of the stress marker catecholamines. These data suggest that microgravity and chronic irradiation may interact with each other to alter immune cell phenotypes in an organ-specific manner and appropriate strategies are required to reduce the health risk of crewmembers.

C/EBPδ protects from radiation-induced intestinal injury and sepsis by suppression of inflammatory and nitrosative stress.

Ionizing radiation (IR)-induced intestinal damage is characterized by a loss of intestinal crypt cells, intestinal barrier disruption and translocation of intestinal microflora resulting in sepsis-mediated lethality. We have shown that mice lacking C/EBPδ display IR-induced intestinal and hematopoietic injury and lethality. The purpose of this study was to investigate whether increased IR-induced inflammatory, oxidative and nitrosative stress promote intestinal injury and sepsis-mediated lethality in Cebpd-/- mice. We found that irradiated Cebpd-/- mice show decreased villous height, crypt depth, crypt to villi ratio and expression of the proliferation marker, proliferating cell nuclear antigen, indicative of intestinal injury. Cebpd-/- mice show increased expression of the pro-inflammatory cytokines (Il-6, Tnf-α) and chemokines (Cxcl1, Mcp-1, Mif-1α) and Nos2 in the intestinal tissues compared to Cebpd+/+ mice after exposure to TBI. Cebpd-/- mice show decreased GSH/GSSG ratio, increased S-nitrosoglutathione and 3-nitrotyrosine in the intestine indicative of basal oxidative and nitrosative stress, which was exacerbated by IR. Irradiated Cebpd-deficient mice showed upregulation of Claudin-2 that correlated with increased intestinal permeability, presence of plasma endotoxin and bacterial translocation to the liver. Overall these results uncover a novel role for C/EBPδ in protection against IR-induced intestinal injury by suppressing inflammation and nitrosative stress and underlying sepsis-induced lethality.

Gamma-Tocotrienol Protects the Intestine from Radiation Potentially by Accelerating Mesenchymal Immune Cell Recovery.

Natural antioxidant gamma-tocotrienol (GT3), a vitamin E family member, provides intestinal radiation protection. We seek to understand whether this protection is mediated via mucosal epithelial stem cells or sub-mucosal mesenchymal immune cells. Vehicle- or GT3-treated male CD2F1 mice were exposed to total body irradiation (TBI). Cell death was determined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Villus height and crypt depth were measured with computer-assisted software in tissue sections. Functional activity was determined with an intestinal permeability assay. Immune cell recovery was measured with immunohistochemistry and Western blot, and the regeneration of intestinal crypts was assessed with ex vivo organoid culture. A single dose of GT3 (200 mg/kg body weight (bwt)) administered 24 h before TBI suppressed cell death, prevented a decrease in villus height, increased crypt depth, attenuated intestinal permeability, and upregulated occludin level in the intestine compared to the vehicle treated group. GT3 accelerated mesenchymal immune cell recovery after irradiation, but it did not promote ex vivo organoid formation and failed to enhance the expression of stem cell markers. Finally, GT3 significantly upregulated protein kinase B or AKT phosphorylation after TBI. Pretreatment with GT3 attenuates TBI-induced structural and functional damage to the intestine, potentially by facilitating intestinal immune cell recovery. Thus, GT3 could be used as an intestinal radioprotector.

Loss of C/EBPδ Exacerbates Radiation-Induced Cognitive Decline in Aged Mice due to Impaired Oxidative Stress Response.

Aging is characterized by increased inflammation and deterioration of the cellular stress responses such as the oxidant/antioxidant equilibrium, DNA damage repair fidelity, and telomeric attrition. All these factors contribute to the increased radiation sensitivity in the elderly as shown by epidemiological studies of the Japanese atomic bomb survivors. There is a global increase in the aging population, who may be at increased risk of exposure to ionizing radiation (IR) as part of cancer therapy or accidental exposure. Therefore, it is critical to delineate the factors that exacerbate age-related radiation sensitivity and neurocognitive decline. The transcription factor CCAAT enhancer binding protein delta (C/EBPδ) is implicated with regulatory roles in neuroinflammation, learning, and memory, however its role in IR-induced neurocognitive decline and aging is not known. The purpose of this study was to delineate the role of C/EBPδ in IR-induced neurocognitive decline in aged mice. We report that aged Cebpd-/- mice exposed to acute IR exposure display impairment in short-term memory and spatial memory that correlated with significant alterations in the morphology of neurons in the dentate gyrus (DG) and CA1 apical and basal regions. There were no significant changes in the expression of inflammatory markers. However, the expression of superoxide dismutase 2 (SOD2) and catalase (CAT) were altered post-IR in the hippocampus of aged Cebpd-/- mice. These results suggest that Cebpd may protect from IR-induced neurocognitive dysfunction by suppressing oxidative stress in aged mice.

Cebpd Is Essential for Gamma-Tocotrienol Mediated Protection against Radiation-Induced Hematopoietic and Intestinal Injury.

Gamma-tocotrienol (GT3) confers protection against ionizing radiation (IR)-induced injury. However, the molecular targets that underlie the protective functions of GT3 are not yet known. We have reported that mice lacking CCAAT enhancer binding protein delta (Cebpd-/-) display increased mortality to IR due to injury to the hematopoietic and intestinal tissues and that Cebpd protects from IR-induced oxidative stress and cell death. The purpose of this study was to investigate whether Cebpd mediates the radio protective functions of GT3. We found that GT3-treated Cebpd-/- mice showed partial recovery of white blood cells compared to GT3-treated Cebpd⁺/+ mice at 2 weeks post-IR. GT3-treated Cebpd-/- mice showed an increased loss of intestinal crypt colonies, which correlated with increased expression of inflammatory cytokines and chemokines, increased levels of oxidized glutathione (GSSG), S-nitrosoglutathione (GSNO) and 3-nitrotyrosine (3-NT) after exposure to IR compared to GT3-treated Cebpd+/+ mice. Cebpd is induced by IR as well as a combination of IR and GT3 in the intestine. Studies have shown that granulocyte-colony stimulating factor (G-CSF), mediates the radioprotective functions of GT3. Interestingly, we found that IR alone as well as the combination of IR and GT3 caused robust augmentation of plasma G-CSF in both Cebpd⁺/+ and Cebpd-/- mice. These results identify a novel role for Cebpd in GT3-mediated protection against IR-induced injury, in part via modulation of IR-induced inflammation and oxidative/nitrosative stress, which is independent of G-CSF.

Loss of C/EBPδ enhances IR-induced cell death by promoting oxidative stress and mitochondrial dysfunction.

Exposure of cells to ionizing radiation (IR) generates reactive oxygen species (ROS). This results in increased oxidative stress and DNA double strand breaks (DSBs) which are the two underlying mechanisms by which IR causes cell/tissue injury. Cells that are deficient or impaired in the cellular antioxidant response are susceptible to IR-induced apoptosis. The transcription factor CCAAT enhancer binding protein delta (Cebpd, C/EBPδ) has been implicated in the regulation of oxidative stress, DNA damage response, genomic stability and inflammation. We previously reported that Cebpd-deficient mice are sensitive to IR and display intestinal and hematopoietic injury, however the underlying mechanism is not known. In this study, we investigated whether an impaired ability to detoxify IR-induced ROS was the underlying cause of the increased radiosensitivity of Cebpd-deficient cells. We found that Cebpd-knockout (KO) mouse embryonic fibroblasts (MEFs) expressed elevated levels of ROS, both at basal levels and after exposure to gamma radiation which correlated with increased apoptosis, and decreased clonogenic survival. Pre-treatment of wild type (WT) and KO MEFs with polyethylene glycol-conjugated Cu-Zn superoxide dismutase (PEG-SOD) and catalase (PEG-CAT) combination prior to irradiation showed a partial rescue of clonogenic survival, thus demonstrating a role for increased intracellular oxidants in promoting IR-induced cell death. Analysis of mitochondrial bioenergetics revealed that irradiated KO MEFs showed significant reductions in basal, adenosine triphosphate (ATP)-linked, maximal respiration and reserved respiratory capacity and decrease in intracellular ATP levels compared to WT MEFs indicating they display mitochondrial dysfunction. KO MEFs expressed significantly lower levels of the cellular antioxidant glutathione (GSH) and its precursor- cysteine as well as methionine. In addition to its antioxidant function, GSH plays an important role in detoxification of lipid peroxidation products such as 4-hydroxynonenal (4-HNE). The reduced GSH levels observed in KO MEFs correlated with elevated levels of 4-HNE protein adducts in irradiated KO MEFs compared to respective WT MEFs. We further showed that pre-treatment with the GSH precursor, N-acetyl L-cysteine (NAC) prior to irradiation showed a significant reduction of IR-induced cell death and increases in GSH levels, which contributed to the overall increase in clonogenic survival of KO MEFs. In contrast, pre-treatment with the GSH synthesis inhibitor- buthionine sulfoximine (BSO) further reduced the clonogenic survival of irradiated KO MEFs. This study demonstrates a novel role for C/EBPδ in protection from basal as well as IR-induced oxidative stress and mitochondrial dysfunction thus promoting post-radiation survival.

C/EBPδ deficiency sensitizes mice to ionizing radiation-induced hematopoietic and intestinal injury.

Knowledge of the mechanisms involved in the radiation response is critical for developing interventions to mitigate radiation-induced injury to normal tissues. Exposure to radiation leads to increased oxidative stress, DNA-damage, genomic instability and inflammation. The transcription factor CCAAT/enhancer binding protein delta (Cebpd; C/EBPδ is implicated in regulation of these same processes, but its role in radiation response is not known. We investigated the role of C/EBPδ in radiation-induced hematopoietic and intestinal injury using a Cebpd knockout mouse model. Cebpd-/- mice showed increased lethality at 7.4 and 8.5 Gy total-body irradiation (TBI), compared to Cebpd+/+ mice. Two weeks after a 6 Gy dose of TBI, Cebpd-/- mice showed decreased recovery of white blood cells, neutrophils, platelets, myeloid cells and bone marrow mononuclear cells, decreased colony-forming ability of bone marrow progenitor cells, and increased apoptosis of hematopoietic progenitor and stem cells compared to Cebpd+/+ controls. Cebpd-/- mice exhibited a significant dose-dependent decrease in intestinal crypt survival and in plasma citrulline levels compared to Cebpd+/+ mice after exposure to radiation. This was accompanied by significantly decreased expression of γ-H2AX in Cebpd-/- intestinal crypts and villi at 1 h post-TBI, increased mitotic index at 24 h post-TBI, and increase in apoptosis in intestinal crypts and stromal cells of Cebpd-/- compared to Cebpd+/+ mice at 4 h post-irradiation. This study uncovers a novel biological function for C/EBPδ in promoting the response to radiation-induced DNA-damage and in protecting hematopoietic and intestinal tissues from radiation-induced injury.

Characterization of transgenic Gfrp knock-in mice: implications for tetrahydrobiopterin in modulation of normal tissue radiation responses.

AIMS: The free radical scavenger and nitric oxide synthase cofactor, 5,6,7,8-tetrahydrobiopterin (BH4), plays a well-documented role in many disorders associated with oxidative stress, including normal tissue radiation responses. Radiation exposure is associated with decreased BH4 levels, while BH4 supplementation attenuates aspects of radiation toxicity. The endogenous synthesis of BH4 is catalyzed by the enzyme guanosine triphosphate cyclohydrolase I (GTPCH1), which is regulated by the inhibitory GTP cyclohydrolase I feedback regulatory protein (GFRP). We here report and characterize a novel, Cre-Lox-driven, transgenic mouse model that overexpresses Gfrp. RESULTS: Compared to control littermates, transgenic mice exhibited high transgene copy numbers, increased Gfrp mRNA and GFRP expression, enhanced GFRP-GTPCH1 interaction, reduced BH4 levels, and low glutathione (GSH) levels and differential mitochondrial bioenergetic profiles. After exposure to total body irradiation, transgenic mice showed decreased BH4/7,8-dihydrobiopterin ratios, increased vascular oxidative stress, and reduced white blood cell counts compared with controls. INNOVATION AND CONCLUSION: This novel Gfrp knock-in transgenic mouse model allows elucidation of the role of GFRP in the regulation of BH4 biosynthesis. This model is a valuable tool to study the involvement of BH4 in whole body and tissue-specific radiation responses and other conditions associated with oxidative stress.

Pharmacological targeting of the thrombomodulin-activated protein C pathway mitigates radiation toxicity.

Tissue damage induced by ionizing radiation in the hematopoietic and gastrointestinal systems is the major cause of lethality in radiological emergency scenarios and underlies some deleterious side effects in patients undergoing radiation therapy. The identification of target-specific interventions that confer radiomitigating activity is an unmet challenge. Here we identify the thrombomodulin (Thbd)-activated protein C (aPC) pathway as a new mechanism for the mitigation of total body irradiation (TBI)-induced mortality. Although the effects of the endogenous Thbd-aPC pathway were largely confined to the local microenvironment of Thbd-expressing cells, systemic administration of soluble Thbd or aPC could reproduce and augment the radioprotective effect of the endogenous Thbd-aPC pathway. Therapeutic administration of recombinant, soluble Thbd or aPC to lethally irradiated wild-type mice resulted in an accelerated recovery of hematopoietic progenitor activity in bone marrow and a mitigation of lethal TBI. Starting infusion of aPC as late as 24 h after exposure to radiation was sufficient to mitigate radiation-induced mortality in these mice. These findings suggest that pharmacologic augmentation of the activity of the Thbd-aPC pathway by recombinant Thbd or aPC might offer a rational approach to the mitigation of tissue injury and lethality caused by ionizing radiation.

Identification of a Src tyrosine kinase/SIAH2 E3 ubiquitin ligase pathway that regulates C/EBPδ expression and contributes to transformation of breast tumor cells.

The transcription factor CCAAT/enhancer-binding protein delta (C/EBPδ, CEBPD) is a tumor suppressor that is downregulated during breast cancer progression but may also promote metastasis. Here, we have investigated the mechanism(s) regulating C/EBPδ expression and its role in human breast cancer cells. We describe a novel pathway by which the tyrosine kinase Src downregulates C/EBPδ through the SIAH2 E3 ubiquitin ligase. Src phosphorylates SIAH2 in vitro and leads to tyrosine phosphorylation and activation of SIAH2 in breast tumor cell lines. SIAH2 interacts with C/EBPδ, but not C/EBPß, and promotes its polyubiquitination and proteasomal degradation. Src/SIAH2-mediated inhibition of C/EBPδ expression supports elevated cyclin D1 levels, phosphorylation of retinoblastoma protein (Rb), motility, invasive properties, and survival of transformed cells. Pharmacological inhibition of Src family kinases by SKI-606 (bosutinib) induces C/EBPδ expression in an SIAH2-dependent manner, which is necessary for "therapeutic" responses to SKI-606 in vitro. Ectopic expression of degradation-resistant mutants of C/EBPδ, which do not interact with SIAH2 and/or cannot be polyubiquitinated, prevents full transformation of MCF-10A cells by activated Src (Src truncated at amino acid 531 [Src-531]) in vitro. These data reveal that C/EBPδ expression can be regulated at the protein level by oncogenic Src kinase signals through SIAH2, thus contributing to breast epithelial cell transformation.

C/EBP{delta} targets cyclin D1 for proteasome-mediated degradation via induction of CDC27/APC3 expression.

The transcription factor CCAAT/enhancer binding protein delta (C/EBPdelta, CEBPD, NFIL-6beta) has tumor suppressor function; however, the molecular mechanism(s) by which C/EBPdelta exerts its effect are largely unknown. Here, we report that C/EBPdelta induces expression of the Cdc27 (APC3) subunit of the anaphase promoting complex/cyclosome (APC/C), which results in the polyubiquitination and degradation of the prooncogenic cell cycle regulator cyclin D1, and also down-regulates cyclin B1, Skp2, and Plk-1. In C/EBPdelta knockout mouse embryo fibroblasts (MEF) Cdc27 levels were reduced, whereas cyclin D1 levels were increased even in the presence of activated GSK-3beta. Silencing of C/EBPdelta, Cdc27, or the APC/C coactivator Cdh1 (FZR1) in MCF-10A breast epithelial cells increased cyclin D1 protein expression. Like C/EBPdelta, and in contrast to cyclin D1, Cdc27 was down-regulated in several breast cancer cell lines, suggesting that Cdc27 itself may be a tumor suppressor. Cyclin D1 is a known substrate of polyubiquitination complex SKP1/CUL1/F-box (SCF), and our studies show that Cdc27 directs cyclin D1 to alternative degradation by APC/C. These findings shed light on the role and regulation of APC/C, which is critical for most cellular processes.

Evidence for a cancer-specific switch at the CDK4 promoter with loss of control by both USF and c-Myc.

USF and c-Myc are basic helix-loop-helix transcription factors with similar DNA-binding specificities, but antagonistic effects on cellular transformation. In order to determine how these opposite functions correlate with the transcriptional activities of the two factors on particular downstream targets, we investigated the roles of USF and c-Myc in expression of CDK4, a known direct target of c-Myc. Overexpression of either c-Myc or USF2, but not USF1, stimulated the expression of CDK4 promoter-driven reporter genes in the non-tumorigenic mammary epithelial MCF-10A cells. Dominant-negative mutants specific to either Myc or USF family proteins inhibited reporter gene activity as well as endogenous CDK4 expression, demonstrating involvement of both USF and Myc in CDK4 transcriptional control. In contrast, in two different breast cancer cell lines where USF is transcriptionally inactive and c-Myc is overexpressed, CDK4 promoter activity was no longer responsive to either transcription factor. Accordingly, chromatin immunoprecipitation revealed significantly lower levels of both USF and c-Myc bound to the endogenous CDK4 promoter in breast cancer cells than in MCF-10A cells, with a concomitant decrease in associated histone H3 acetylation. These results suggest that a major switch in the transcriptional control of CDK4 occurs during breast carcinogenesis, with likely alteration of cell cycle regulation.

The IGF2 receptor is a USF2-specific target in nontumorigenic mammary epithelial cells but not in breast cancer cells.

The antiproliferative activities of the USF proteins and the frequent loss of USF function in cancer cells suggest a role for these ubiquitous transcription factors in tumor suppression. However, the cellular targets that mediate the effects of USF on cellular proliferation and transformation remain uncharacterized. IGF2R, with multiple functions in both normal growth and cancer, was investigated here as a possible USF target in both nontumorigenic and tumorigenic breast cell lines. The 5'-flanking sequences of the human IGF2R gene contain multiple, highly conserved E boxes almost identical to the consensus USF DNA-binding sequence. These E boxes were found to be essential for IGF2R promoter activity in the nontumorigenic mammary epithelial cell line MCF-10A. USF1 and USF2 bound the IGF2R promoter in vitro, and both USF1 and USF2, but not c-Myc, were present within the IGF2R promoter-associated chromatin in vivo. Overexpressed USF2, but not USF1, transactivated the IGF2R promoter, and IGF2R mRNA was markedly decreased by expression of a USF-specific dominant negative mutant, identifying IGF2R as a USF2 target. IGF2R promoter-driven expression was USF-independent in both MCF-7 and MDA-MB-231 breast cancer cell lines, suggesting that a defect in USF function may contribute to down-regulation of IGF2R expression in cancer cells.