2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits growth factor withdrawal-induced apoptosis in the human mammary epithelial cell line, MCF-10A

Previous studies have demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases cell recovery in the human mammary epithelial cell line M CF-10A grown under growth factor-restricted conditions. TCDD was also found to mimic growth factor signaling pathways by stimulating the tyrosine phos phorylation of numerous effector molecules, and increased phosphatidylinosi tol 3-kinase (PI3K) activity in the absence of exogenously added growth fac tors. In the present studies, we have expanded on these initial results to show that TCDD (3-30 nM) increases cell recovery on days 2-6 by as much as 80% when insulin or epidermal growth factor (EGF) was removed from the medi a. The mechanism for this effect appears to be complex as TCDD inhibited ap optosis stimulated by EGF, or EGF and insulin, withdrawal by almost 80% as determined by Annexin V binding, However, withdrawal of insulin alone did n ot induce apoptosis even though TCDD did increase cell number in its absenc e. These results were corroborated by immunoblot analysis of poly(ADP-ribos e) polymerase cleavage. Since TCDD stimulates PI3K activity, the phosphoryl ation status of Akt, a serine/threonine kinase that mediates PI3K-dependent inhibition of apoptosis, was examined. Immunoblot analysis revealed that T CDD causes a transient increase in the phosphorylated form of Akt that peak s at 6 h and disappears by 12 h, It appears that EGF stimulates an anti-apo ptotic pathway, while insulin signals a pro-mitogenic pathway. By stimulati ng or mimicking one or both of these pathways TCDD may alter tightly regula ted growth pathways in the MCF-10A cell line.