Background: We have recently discovered a new toxic effect of the aryl hydrocarbon receptor (AhR) ligand 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD or dioxin). Specifically, exposure during pregnancy impairs mammary gland development and suppresses the coordinated induction of milk proteins, resulting in impaired lactation and neonatal mortality. Objectives/Hypothesis: The objectives of the proposed studies are (1) to further characterize this novel finding and (2) to identify the lactogenic regulatory pathways adversely affected by exposure to dioxin. The hypothesis for these studies is that AhR activation during pregnancy disrupts the normal signaling that directs pregnancy-associated mammary development and milk protein gene expression, resulting in impaired epithelial cell differentiation and lactation. Specific Aims: 1).To determine whether defects in lactogenesis result from direct effects on mammary tissue, we will cross-transplant mammary tissue from wild-type and AhR-null mice. 2) To identify the mechanism underlying impaired milk production, we will determine whether exposure to TCDD deregulates the activation of NF-kappaB-, STATSa- and C/EBbeta-mediated signalling pathways in mammary epithelial cells. 3) To determine whether stunted glandular development during pregnancy results from deregulation of proliferation, differentiation, apoptosis or defects in multiple pathways, we will further characterize the effects of exposure to TCDD on these processes in mammary cells during pregnancy. 4) To identify additional molecular pathways that are deregulated following exposure to TCDD, we will compare the expression of factors known to regulate to glandular differentiation and lactogenesis in glands derived from vehicle- and TCDD-treated pregnant mice using a combination of gene expression profiling and immunocytochemical methods. Mammary tissue from AhR-null mice will be used to distinguish defects that are directly AhR-mediated from defects that arise due to an upstream lesion. Significance: The proposed studies address an area that is clinically-relevant but has received very little attention. An estimated 3-6 million mothers of live infants annually are either unable to or have significant difficulty initiating breastfeeding. The causes of this problem are not clear, and very little is known about the effects of exposure to environmental contaminants on lactogenesis. Furthermore, since the mechanisms that control lactogenesis also regulate proliferation and differentiation in other organs, and exposure to AhR ligands disrupts the proliferation and differentiation of epithelial cells in other tissues, findings from these studies will have broad biological significance, and will help us better understand the mechanisms by which dioxin-like chemicals adversely affect epithelial cells throughout the body.