Effect of Dioxin on Normal Human Hematopoietic and Leukemic Cells.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the prototypical agonist of the aryl hydrocarbon receptor (AhR), a member of the erb-A family that also includes receptors for steroids, thyroid hormones, peroxisome proliferators and retinoids. When bound to dioxin, the AhR can bind to DNA and alter the expression of various genes, including cytokines and growth factors. TCDD has a large number of biological effects, such as long-lasting skin disease, cardiovascular disease, diabete and cancer. An increase risk of Hodgkin’s disease, non-Hodgkin’s lymphoma and myeloid leukemia was observed after 15 years in the population exposed to dioxin following the 1976 accident in Seveso, Italy. In this study, we analysed the effect of escalating doses of TCDD on the human myeloid leukemic cell lines HL60 (promyelocytic leukemia) and K562 (chronic myelogenous leukemia), as well as on human CD34+ progenitor cells from the leukapheresis of normal donors stimulated with G-CSF. The possible specific modulation of gene expression induced by TCDD was then tested by means of macroarray analyses (Clontech), the results of which were successfully validated by real-time quantitative PCR on five candidate genes. The dioxin dose capable of inhibiting the growth of 50% of the colonies in semisolid medium (IC50 calculated using the CalcuSyn computer program) was 33 nM for HL60, 28 nM for K562, and 20 nM for the CD34+ cells. The HL60, K562 and CD34+ cells tested before and after exposure to 20 nM of TCDD allowed us to identify a series of modulated genes. The up-regulated genes included LUCA15 putative tumour suppressor, cyclin-D binding Myb-like protein (hDMP1), MAPK/ERK kinase 6, ras-related protein RAB2, RalB GTP-binding protein, transcription factor ZFM1, and BRCA1-associated ring domain protein (BARD1). The down-regulated genes included found c-myc, TNF alpha precursor, ICAM1, MIP1-beta, interferon gamma-induced protein precursor (gamma-IP10), granulocyte-macrophage colony stimulating factor (GM-CSF), monocyte chemotactic protein 1 precursor (MCP1). All of these genes are variously involved in the processes of proliferation, differentiation and transformation in hematological and tumoral cell models. On the basis of these data, we can hypothesise that the inhibition of clonogenic potential and the gene expression induced by TCDD exposure on normal CD34+ progenitor cells plays a role in the neoplastic transformation of hemopoietic stem cells. This supports the epidemiological data indicating an increased hematological cancer risk in the population accidentally exposed to TCDD.