Evaluation of different pharmacokinetically guided IV busulfan exposure ranges on adult patient outcomes after hematopoietic stem cell transplantation.

Conditioning intensity contributes significantly to outcomes in allogeneic hematopoietic stem cell transplantation (allo-HSCT). We evaluated two myeloablative conditioning dosing ranges of intravenous (IV) busulfan (Bu) in combination with fludarabine in 70 patients. In 2015, our practice changed to target busulfan area under the curve (AUC) of ≥ 19.7 mg*h/L. We assessed responses in patients receiving busulfan AUCs of < 19.7 mg*h/L (Low-Bu) and ≥ 19.7 mg*h/L (High-Bu). At 18-month median follow-up, no differences in overall survival (OS) and relapse-free survival (RFS) were found between Low-Bu and High-Bu groups (p = 0.35 and p = 0.29, respectively). Relapses occurred in 25.7% of patients. No differences in median time to relapse were noted. Minimal residual disease (MRD)-positive patients had a shorter median OS and RFS than MRD-negative patients. No differences were found in OS and RFS between Low-Bu and High-Bu groups in MRD-positive patients (p = 0.86 and p = 0.83, respectively), or MRD-negative patients (p = 0.56 and p = 0.38, respectively). Non-relapsed mortality (NRM) at 100 days was 3.4% vs. 4.1% in the Low-Bu vs. High-Bu groups. There were no significant differences in the incidence of acute-graft-versus-host disease (aGVHD) (71.4% vs. 63.4%) or chronic GVHD (cGVHD) (48.3% vs. 43.9%) between the groups. The cumulative incidence of grades III-IV aGVHD was 24.1% in Low-Bu group and 22.4% in High-Bu group. In conclusion, targeting a busulfan AUC of > 19.7 mg*h/L with fludarabine does not appear to add an advantage in OS and RFS.

Cryopreserved versus fresh peripheral blood allogeneic stem cell transplantation outcomes in patients receiving post-transplant cyclophosphamide for graft-versus-host prophylaxis during the COVID-19 pandemic: a single center experience.

BACKGROUND/OBJECTIVE: Cryopreservation of grafts is not common practice in allogeneic hematopoietic stem cell transplant (HSCT) recipients. However, our center had to use cryopreserved cells for allogeneic HSCT during the COVID-19 pandemic to avoid delays in transplantation due to uncertainty regarding patient and donor exposures. STUDY DESIGN: We retrospectively evaluated post-transplant engraftment and survival outcomes of adult patients who received cryopreserved versus fresh allografts during the COVID-19 pandemic. RESULTS: Fifty-five patients with hematologic malignancies received either cryopreserved (n = 34) or fresh (n = 21) allogeneic HSCT using peripheral blood stem cells between January 2020 and December 2020. At a median follow-up time of 15 months, cryopreserved allograft recipients had significantly lower overall survival (OS) (p = 0.02). They also experienced significantly delayed neutrophil (p = 0.01) and platelet engraftments (p < 0.0001), as well as higher red blood cell transfusion-dependence after day + 60 (67.6% vs. 28.6%; p = 0.01). Significantly more cryopreserved allograft recipients received donor lymphocyte infusion than fresh allograft recipients (35.3% vs. 4.8%, p = 0.01). Neither relapse-free survival nor non-relapse mortality differed significantly between the two groups. CONCLUSION: Cryopreservation of allografts in combination with post-transplant cyclophosphamide may negatively affect engraftment and OS outcomes in HSCT recipients.

Mutational profiling of myeloid neoplasms associated genes may aid the diagnosis of acute myeloid leukemia with myelodysplasia-related changes.

AML with myelodysplasia-related changes (AML-MRC) is a subtype of AML known to have adverse prognosis. The karyotype abnormalities in AML-MRC have been well established; however, relatively little has been known about the role of gene mutation profiles by next generation sequencing. 177 AML patients (72 AML-MRC and 105 non-MRC AML) were analyzed by NGS panel covering 53 AML related genes. AML-MRC showed statistically significantly higher frequency of TP53 mutation, but lower frequencies of mutations in NPM1, FLT3-ITDLow, FLT3-ITDHigh, FLT3-TKD, NRAS, and PTPN11 than non-MRC AML. Supervised tree-based classification models including Decision tree, Random forest, and XGboost, and logistic regression were used to evaluate if the mutation profiles could be used to aid the diagnosis of AML-MRC. All methods showed good accuracy in differentiating AML-MRC from non-MRC AML with AUC (area under curve) of ROC ranging from 0.69 to 0.78. Additionally, logistic regression indicated 3 independent factors (age and mutations of TP53 and FLT3) could aid the diagnosis AML-MRC. Using weighted factors, a AML-MRC risk scoring equation was established for potential application in clinical setting: +1x(Age ≥ 65) + 3 x (TP53 mutation) - 2 x (FLT3 mutation). Using a cutoff score of 0, the accuracy of the risk score was 0.76 with sensitivity of 0.77 and specificity of 0.75 for predicting the diagnosis of AML-MRC. Further studies with larger sample sizes are warranted to further evaluate the potential of using gene mutation profiles to aid the diagnosis of AML-MRC.

Adverse Impact of DNA Methylation Regulatory Gene Mutations on the Prognosis of AML Patients in the 2017 ELN Favorable Risk Group, Particularly Those Defined by NPM1 Mutation.

The 2017 ELN risk stratification has been widely adopted, but some studies have suggested the outcomes are heterogenous within the ELN risk groups and may be affected by other co-existing genetic mutations. This study evaluated the impact of DNA methylation regulatory gene (TET2, IDH1/2, DNMT3A, SETBP1) mutations (DMRGM) evaluated by NGS in the outcome of AML patients in each ELN risk group. A total of 114 patients were analyzed with a median follow-up of 12 months. Overall, 30.7% (35/114) of patients had DMRGM. DMRGM status had no impact on CR rate in each ELN risk group. The OS, however, was significantly shorter in patients with DMRGM compared to those without DMRGM (median OS: 12 vs. 33 months, p = 0.0053). Multivariate analysis showed DMRGM status was an independent unfavorable factor for OS (HR: 2.704, 95% CI: 1.451-5.041, p = 0.0017). The adverse OS impact of DMRGM was only observed in the ELN favorable group (7 months vs. not reached, p = 0.0001), but not in the intermediate or adverse group. Among the favorable group with DMRGM (n = 16), DMRGM occurred predominantly in cases with mutated NPM1 (15/16, or 93.8%). Our results suggest that DMRGM adversely impact the outcomes of ELN favorable group patients, particularly those with mutated NPM1. Further studies are warranted to confirm our observations.

Comparison of Myeloablative versus Reduced-Intensity Conditioning Regimens in Allogeneic Stem Cell Transplantation Recipients with Acute Myelogenous Leukemia with Measurable Residual Disease-Negative Disease at the Time of Transplantation: A Retrospective Cohort Study.

The ideal conditioning intensity in allogeneic hematopoietic stem cell transplantation (HSCT) is evolving. Previous prospective studies comparing myeloablative conditioning (MAC) versus reduced-intensity conditioning (RIC) regimens in adults with acute myelogenous leukemia (AML) have shown mixed results. In many of these studies, patients were not stratified based on measurable residual disease (MRD). We evaluated the effect of conditioning intensity on the outcomes of AML patients in complete remission (CR) with flow cytometry evidence of MRD negativity. A total of 135 patients age 20 to 75 years with AML in CR1 or CR2 and flow cytometry evidence of MRD negativity who underwent allogeneic HSCT at our center between 2011 and 2019 were evaluated. We compared overall survival (OS), relapse-free survival (RFS), nonrelapse mortality (NRM), relapse, and acute and chronic graft-versus-host disease (GVHD) in recipients of MAC (n = 89) and RIC (n = 46). Although the patients receiving RIC were older (62 versus 51 years; P < .0001), there were no statistically significant differences between the groups in terms of Eastern Cooperative Oncology Group and European Leukemia Network risk criteria and disease status (CR1 or CR2) at the time of transplantation. At a median follow-up of 24.6 months, no statistically significant differences in OS (hazard ratio [HR], 0.78; 95% confidence interval [CI] 0.42 to 1.42, P = .411) or RFS (HR, 1.004; 95% CI, 0.48 to 2.09, P = .99) were identified. The cumulative incidence of NRM (HR, 0.595; 95% CI, 0.24 to 1.48; P = .2644) and relapse (HR, 1.007; 95% CI, 0.45 to 2.23; P = .9872) was not different between the 2 groups. Grade II-IV and grade III-IV acute GVHD were more frequent in the MAC group (39.3% verses 19.9% [P = .018] and 19.3% versus 2.3% [P < .001], respectively), as was moderate/severe chronic GVHD (23.6% versus 15.8%; P = .038). Our data indicate that conditioning intensity did not appear to affect OS, RFS, NRM, and relapse risk in patients with MRD-negative AML as measured by flow cytometry. RIC resulted in less severe acute and chronic GVHD.

Impact of Next-Generation Sequencing Cell-free Pathogen DNA Test on Antimicrobial Management in Adults with Hematological Malignancies and Transplant Recipients with Suspected Infections.

Infections in adult patients with hematological malignancies (HM) and stem cell transplant (SCT) recipients are a significant cause of morbidity and mortality. A timely diagnosis of infections can have a major impact on outcomes. Tools that help rule out infectious causes of fever can decrease antibiotic use, toxicities, hospitalization costs, and potentially decrease antibiotic resistance in the long term. We retrospectively evaluated the ability of cell-free DNA next-generation sequencing (NGS) testing in the timely identification of pathogenic microorganisms and its impact on the antimicrobial management of immunocompromised patients with hematologic malignancies. In the period between 2018 to 2020, 95 samples were reviewed, of which 31 adult patients (32 tests) had hematologic malignancies or were recipients of SCT. The NGS tests were performed in the following patients: (a) patients with prolonged fever and negative conventional tests, (b) persistent fever despite positive conventional test and appropriate antimicrobials, and (c) fever-free patients with imaging suspicious for infection. The median time from fever to NGS sampling was 5 days (range, 1-28). The median time to NGS results was 2 days (range, 1-6). The NGS resulted in an escalation of antibiotics in 28% of cases (9/32) and de-escalation of antibiotics in 31% of cases (10/32). Overall, NGS testing changed management in nearly 59% (19/32) of patients. The sensitivity and specificity of NGS to detect clinically significant infection was 80% and 58%, respectively. The test identified uncommon and difficult to diagnose organisms such as Nocardia, Legionella, Toxoplasma and Pneumocystis jirovecii resulting in rapid antimicrobial interventions. In conclusion, in patients with HM or SCT recipients, microbial cell-free DNA sequencing allowed rapid and actionable treatment. This strategy can target appropriate antibiotic use, avoid overtreatment, and potentially decrease the hospital length-of-stay.

Conditioning regimen intensity and low-dose azacitidine maintenance after allogeneic hematopoietic cell transplantation for acute myeloid leukemia.

Azacitidine (AZA) maintenance following allogeneic hematopoietic cell transplantation (HCT) for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) may reduce relapse risk and improve survival. Given logistic and toxicity-related challenges, identifying subgroups appropriate for this approach is an unmet need. Using data from two centers, we retrospectively compared event-free survival (EFS) and overall survival (OS) of AML and MDS patients who received AZA maintenance (n = 59) with historic controls (n = 90). Controls were selected according to the following criteria: no death, relapse, or Grade III-IV acute GVHD for 100 days after transplant. In multivariable analysis, AZA maintenance yielded significantly improved EFS (p = 0.019) and OS (p = 0.011). Outcomes differed according to regimen intensity. For reduced-intensity transplant, EFS (p = 0.004) and OS (p = 0.004) were significantly improved and equivalent to myeloablative transplant. A significant benefit following myeloablative transplant was not observed. Within the limitation of its retrospective nature, this study suggests that AZA maintenance improves outcomes following reduced-intensity HCT, comparable to myeloablative HCT.

Eltrombopag for Treating Thrombocytopenia after Allogeneic Stem Cell Transplantation.

Thrombocytopenia after allogeneic hematopoietic stem cell transplantation (allo-SCT) can pose significant problems in management of patients. Eltrombopag is a small-molecule thrombopoietin receptor agonist that has been approved for use in immune thrombocytopenic purpura and aplastic anemia; but its use after allo-SCT is limited. Between 2014 and 2017, we treated 13 patients with eltrombopag for poor platelet engraftment without evidence of relapse at the time of initiation, including 6 patients with primary platelet engraftment failure and 7 with secondary platelet engraftment failure. Eltrombopag was started at an initial dose of 25 or 50 mg per day, and dose adjustments were made in accordance with the manufacturer's recommendation. The cumulative incidence of platelet recovery to ≥50,000/µL without the need for transfusion for at least 7 days was defined as response. The overall response rate was 62% (n = 8). Of the 6 patients with primary isolated platelet failure, 3 (50%) responded, and of the 7 patients with secondary platelet failure, 5 (71%) responded. The median time to response was 33 days (range, 11 to 68 days). In addition, no significant differences in platelet recovery were noted in patients with adequate and decreased bone marrow megakaryocytic reserve (60% and 67%, respectively). Although eltrombopag was well tolerated, and no patient discontinued treatment because of adverse events, only 3 patients were alive at the end of the observation period, with relapse and graft-versus-host disease accounting for majority of the deaths. This suggested that despite the relatively good overall response rate to eltrombopag, inadequate platelet engraftment is a harbinger of poor outcome in allo-SCT.

Progressive multifocal leukoencephalopathy after allogeneic stem cell transplantation: Case report and review of the literature.

Progressive multifocal leukoencephalopathy (PML) is a rare, yet typically fatal complication of allogeneic stem cell transplantation. It is caused by reactivation of the John Cunningham (JC) virus in an immunocompromised host. This report describes an unfortunate case of PML in a recipient of an allogeneic stem cell transplant for acute myelogenous leukemia. The JC virus was undetectable in the patient's cerebrospinal fluid by polymerase chain reaction (PCR); however, a positive diagnosis was made after a brain biopsy. This and other published cases demonstrate that recipients of allogeneic stem cells can develop PML. Moreover, early diagnosis of the disease is often difficult and, as demonstrated in this case, screening with PCR does not appear to have strong diagnostic significance. With no effective treatment presently available, restoration of immune function is the only intervention that can affect prognosis. Further prospective studies are needed to understand the pathophysiology and treatment of this disease.

Aryl hydrocarbon receptor regulates the cholesterol biosynthetic pathway in a dioxin response element-independent manner.

UNLABELLED: The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Activation of AhR mediates the expression of target genes (e.g., CYP1A1) by binding to dioxin response element (DRE) sequences in their promoter region. To understand the multiple mechanisms of AhR-mediated gene regulation, a microarray analysis on liver isolated from ligand-treated transgenic mice expressing a wild-type (WT) Ahr or a DRE-binding mutant Ahr (A78D) on an ahr-null background was performed. Results revealed that AhR DRE binding is not required for the suppression of genes involved in cholesterol synthesis. Quantitative reverse-transcription polymerase chain reaction performed on both mouse liver and primary human hepatocyte RNA demonstrated a coordinated repression of genes involved in cholesterol biosynthesis, namely, HMGCR, FDFT1, SQLE, and LSS after receptor activation. An additional transgenic mouse line was established expressing a liver-specific Ahr-A78D on a Cre(Alb)/Ahr(flox/flox) background. These mice displayed a similar repression of cholesterol biosynthetic genes, compared to Ahr(flox/flox) mice, further indicating that the observed modulation is AhR specific and occurs in a DRE-independent manner. Elevated hepatic transcriptional levels of the genes of interest were noted in congenic C57BL/6J-Ah(d) allele mice, when compared to the WT C57BL/6J mice, which carry the Ah(b) allele. Down-regulation of AhR nuclear translocator levels using short interfering RNA in a human cell line revealed no effect on the expression of cholesterol biosynthetic genes. Finally, cholesterol secretion was shown to be significantly decreased in human cells after AhR activation. CONCLUSION: These data firmly establish an endogenous role for AhR as a regulator of the cholesterol biosynthesis pathway independent of its DRE-binding ability, and suggest that AhR may be a previously unrecognized therapeutic target.

Ah receptor represses acute-phase response gene expression without binding to its cognate response element.

Repression of the nuclear factor-kappaB (NF-kappaB) pathway has been extensively researched because of its pivotal role in inflammation. We investigated the potential of the aryl hydrocarbon receptor (AHR) to suppress NF-kappaB regulated-gene expression, especially acute-phase genes, such as serum amyloid A (Saa). Using AHR mutants, it was determined that nuclear translocation and heterodimerization with AHR-nuclear translocator are essential, but DNA binding is not involved in AHR-mediated Saa repression. A number of AHR ligands were capable of repressing Saa3 expression. AHR activation leads to a decrease in RELA and C/EBP/beta recruitment to and histone acetylation at Saa3 gene promoter. A battery of acute-phase genes (eg C-reactive protein and haptoglobin) induced by cytokine exposure was repressed by AHR activation in mouse hepatocytes. Dietary exposure to an AHR ligand represses cytokine-induced acute-phase response in the liver. Use of a human liver-derived cell line revealed similar repression of Saa mRNA levels and secreted protein. Repression of AHR expression also enhanced Saa induction in response to cytokines, suggesting that AHR is capable of constitutively repressing Saa gene expression. These results establish a role for AHR in inflammatory signaling within the liver, presenting a new therapeutic opportunity, and signify AHR's ability to function in a DNA-independent manner.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid derivative, is an activator of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AHR) is a ligand-regulated transcription factor that can be activated by structurally diverse chemicals, ranging from environmental carcinogens to dietary metabolites. Evidence supporting a necessary role for the AHR in normal biology has been established; however, identification of key endogenous ligand/activator remains to be established. Here, we report the ability of 12(R)-hydroxy-5(Z),8(Z),10(E), 14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid metabolite produced by either a lipoxygenase or cytochrome P-450 pathway, to act as a potent indirect modulator of the AHR pathway. In contrast, structurally similar HETE isomers failed to demonstrate significant activation of the AHR. Electrophoretic mobility shift assays, together with ligand competition binding experiments, have demonstrated the inability of 12(R)-HETE to directly bind or directly activate the AHR to a DNA binding species in vitro. However, cell-based xenobiotic-responsive element-driven luciferase reporter assays indicate the ability of 12(R)-HETE to modulate AHR activity, and quantitation of induction of an AHR target gene confirmed 12(R)-HETE's ability to activate AHR-mediated transcription, even at high nanomolar concentrations in human hepatoma (HepG2)- and keratinocyte (HaCaT)-derived cell lines. One explanation for these results is that a metabolite of 12(R)-HETE is acting as a direct ligand for the AHR. However, several known metabolites failed to exhibit AHR activity. The ability of 12(R)-HETE to activate AHR target genes required receptor expression. These results indicate that 12(R)-HETE can serve as a potent activator of AHR activity and suggest that in normal and inflammatory disease conditions in skin, 12(R)-HETE is produced, perhaps leading to AHR activation.

Evidence for an aryl hydrocarbon receptor-mediated cytochrome p450 autoregulatory pathway.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor responsible for mediating the cellular response to the toxic compound 2,3,7,8,-tetrachlorodibenzo-p-dioxin. An essential role for the AhR in cellular biology has been established previously, but no high-affinity endogenous ligand has yet been identified. We have confirmed the presence of a putative endogenous ligand(s) in CV-1 cells through transient transfection with various cytochrome P450 isoforms. Expression of cytochromes P450 1A1, 1A2, or 1B1 reduced AhR-mediated luciferase reporter activity, whereas cytochrome P450 2E1 exhibited no significant effect. Studies with 2,4,3',5'-tetramethoxystilbene, a potent and specific inhibitor of cytochrome P450 1B1, was able to partially block cytochrome P450 1B1-mediated reduction in reporter gene activity. These results provide evidence of the existence of a possible feedback mechanism in which AhR-regulated cytochromes P450 from the CYP1A and CYP1B families are able to metabolically alter putative endogenous ligand(s). Several experiments were performed to provide initial characterization of these putative endogenous ligands, including electrophoretic mobility shift assay analyses, which demonstrated that these ligands directly activate the AhR. Soluble extracts from various C57BL/6J and Ahr-null mouse tissues were also analyzed for the presence of AhR activators. Studies revealed that Ahr-null mouse lung tissue had a 4-fold increase in AhR-mediated reporter activity in cells. Quantitative polymerase chain reaction analysis revealed that lung tissue exhibits relatively high constitutive CYP1A1 mRNA levels. These results suggest that there is an autoregulatory feedback loop between the AhR and cytochrome P450 1A1 in mouse lung.

Aryl-hydrocarbon receptor activation regulates constitutive androstane receptor levels in murine and human liver.

UNLABELLED: The aryl-hydrocarbon receptor (AhR) is a basic helix-loop-helix/Per-Arnt-Sim transcription factor that can be activated by exogenous as well as endogenous ligands. AhR is traditionally associated with xenobiotic metabolism. In an attempt to identify novel target genes, C57BL/6J mice were treated with beta-naphthoflavone (BNF), a known AhR ligand, and genome-wide expression analysis studies were performed using high-density microarrays. Constitutive androstane receptor (CAR) was found to be one of the differentially regulated genes. Real-time quantitative polymerase chain reaction (qPCR) verified the increase in CAR messenger RNA (mRNA) level. BNF treatment did not increase CAR mRNA in AhR-null mice. Time-course studies in mice revealed that the regulation of CAR mRNA mimicked that of Cyp1A1, a known AhR target gene. To demonstrate that the increase in CAR mRNA translates to an increase in functional CAR protein, mice were sequentially treated with BNF (6 hours) followed by the selective CAR agonist, TCPOBOP (3 hours). qPCR revealed an increase in the mRNA level of Cyp2b10, previously known to be regulated by CAR. This also suggests that CAR protein is present in limiting amounts with respect to its transactivation ability. Finally, CAR was also up-regulated in primary human hepatocytes in response to AhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene. CONCLUSION: This study identifies a mode of up-regulating CAR and potentially expands the role of AhR in drug metabolism. This study also demonstrates in vivo up-regulation of CAR through chemical exposure.

Endogenous hepatic expression of the hepatitis B virus X-associated protein 2 is adequate for maximal association with aryl hydrocarbon receptor-90-kDa heat shock protein complexes.

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that acts as an environmental sensor by binding to a variety of xenobiotics. AHR activation serves to combat xenotoxic stress by inducing metabolic enzyme expression in the liver. The hepatitis B virus X-associated protein (XAP2) is a component of the cytosolic AHR complex and modulates AHR transcriptional properties in vitro and in cell culture and yeast systems. Expression of XAP2 is low in liver compared with other nonhepatic tissues and the AHR exhibits high ligand-induced transcriptional activity. Because XAP2 has been demonstrated to repress AHR activity, we hypothesized that XAP2 may be limiting in liver and that increasing XAP2 levels would attenuate AHR transcriptional activity. To this end, transgenic mice were generated that exhibit hepatocyte-specific elevation in XAP2 expression. Transgenic XAP2 expression was restricted to liver, and its ability to complex with the AHR was verified. Gene expression experiments were performed by inducing AHR transcriptional activity with beta-naphthoflavone via intraperitoneal injection, and mRNA quantification was done by real-time polymerase chain reaction. Wild-type and transgenic animals showed little difference in constitutive or ligand-induced CYP1A1; CYP1A2; UDP glucuronosyltransferase 1A2; NAD(P)H dehydrogenase, quinone 1; constitutive androstane receptor; or nuclear factor erythroid 2-related factor 2 mRNA expression. Sucrose density fractionation and AHR immunoprecipitation experiments found little or no stoichiometric increase in bound XAP2 to the AHR between genotypes. Gene array studies were performed to identify novel XAP2-regulated targets. Taken together, this work shows that despite the relatively low level of XAP2 in liver, it is not a limiting component in AHR regulation.

The aryl hydrocarbon receptor directly regulates expression of the potent mitogen epiregulin.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to cause a large number of adverse effects, mediated largely by its binding to the aryl-hydrocarbon receptor (AhR) and subsequent modulation of gene expression. It is thought that AhR mediates these effects through the untimely and disproportionate expression of specific genes. However, the exact mechanism, or the genes involved, through which TCDD leads to these effects is still unknown. This study reports the discovery of a novel target gene, epiregulin, which is regulated by TCDD-activated AhR. Epiregulin is a growth regulator which belongs to the epidermal growth factor (EGF) family. Using real time quantitative PCR (qPCR), it was established that TCDD upregulates epiregulin gene expression. The promoter region of epiregulin has a dioxin responsive element (DRE) 56 nucleotides upstream of the transcription start site, along with three potential Sp1 binding sites. Chromatin immunoprecipitation (ChIP) assays with an anti-AhR antibody showed promoter occupancy upon TCDD treatment. Luciferase reporter assays using a vector harboring the first 125 base pairs of the epiregulin rat promoter revealed an increase in signal on TCDD treatment, which was lost upon mutation of the DRE. Epiregulin and TCDD treatment mediated a dose-dependent increase in primary mouse keratinocyte growth. These results demonstrate that AhR directly increases epiregulin expression, which could play an important role in TCDD mediated tumor promotion observed in rodent models.