Hepatic transcriptional networks induced by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin.

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) serves as a prototype for a range of environmental toxicants and as a pharmacologic probe to study signal transduction by the aryl hydrocarbon receptor (AHR). Despite a detailed understanding of how TCDD exposure leads to the transcriptional up-regulation of cytochrome P450-dependent monooxygenases, we know little about how compounds like TCDD lead to a variety of AHR-dependent toxic end points such as liver pathology, terata, thymic involution, and cancer. Using an acute exposure protocol and the toxic response of the mouse liver as a model system, we have begun a detailed microarray analysis to describe the transcriptional changes that occur after various TCDD doses and treatment times. Through correlation analysis of time- and dose-dependent toxicological end points, we are able to identify coordinately responsive transcriptional events that can be defined as primary transcriptional events and downstream events that may represent mechanistically linked sequelae or that have potential as biomarkers of toxicity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces premature activation of the KLF2 regulon during thymocyte development.

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) causes numerous and diverse toxic events via activation of the aryl hydrocarbon receptor, including atrophy of the thymus. Exposure to TCDD induces acute thymocyte cell loss, which occurs concomitantly with proliferation arrest and premature emigration of triple negative (TN; CD4(-), CD8(-), CD3(-)) T cell progenitors. In this report, we demonstrate that TCDD exposure results in dysregulation of KLF2 (Kruppel-like factor 2) expression in developing thymocytes. The Klf2 gene encodes an Sp1-like zinc finger transcription factor that functions as a central regulator of T lymphocyte proliferation and trafficking. During normal thymocyte development, KLF2 is expressed exclusively in CD4 and CD8 single positive T cells and promotes a nonproliferative, promigratory phenotype. In mice exposed to TCDD, however, the Klf2 gene is prematurely expressed in TN thymocytes. Administration of a 100 microg/kg dose of TCDD results in a approximately 15-fold induction of KLF2 as early as the TN2 (CD44(+), CD25(+)) stage of development and immediately precedes acute cell loss in the TN3, TN4, and double positive (CD4(+), CD8(+)) cell stages. Induction of KLF2 occurs within 12 h of TCDD exposure and is fully dependent on expression of the aryl hydrocarbon receptor. In addition, TCDD exposure alters the expression of several factors comprising the KLF2 regulon, including Edg1/S1P(1), beta(7) integrin, CD52, Cdkn2d (cyclin-dependent kinase inhibitor 2D), s100a4, and IL10R alpha. These findings indicate that the pollutant TCDD interferes with early thymopoeisis via ectopic expression of the KLF2 regulon.