Disruption of cell-cell contact maximally but transiently activates AhR-mediated transcription in 10T1/2 fibroblasts

The aryl hydrocarbon receptor (AhR) is activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but activation without an exogenous ligand also occurs when normal cell-cell contact is prevented. Suspension of several C3H10T1/2 fibroblast clonal sub-lines that contain an integrated AhR-responsive reporter produced a time course and level of reporter activation and CYP1B1 induction that paralleled TCDD stimulation in confluent monolayer culture. Suspension activation was, however, more transient. Loss of cell-cell contact at low density also activated these reporters independent of cell cycle changes to levels comparable to TCDD stimulation of confluent cells. Loss of cell-cell contact may, therefore, activate AhR. Suspension and TCDD activations exhibited comparable nuclear translocation of AhR and then AhR/ARNT complex formation. Each AhR activation process was equally attenuated by inhibition of, respectively, HSP90 ATPase, the 26S proteosome, and by depletion of intracellular Ca2+. By contrast, the AhR antagonist alpha-naphthoflavone (alphaNF) blocked ligand-stimulated AhR activity, but not activation through loss of cell-cell contact. Suspension-induced reporter activation was selectively enhanced by LiCl, which prevented GSK-3beta effects on the simultaneously released beta-catenin. The effects of suspension and LiCl on reporters were reversed by Ro-31-8220, which did not affect beta-catenin, TCDD-activation processes, or AhR turnover. Neither LiCl nor Ro-31-8220 altered suspension-induced AhR/ARNT complex formation. Loss of cell-cell contact permits nuclear translocation and AhR activation that is largely replicated after TCDD binding, but with activity differences due to contact-sensitive factors functioning after AhR/ARNT complex formation. (C) 2004 Elsevier Inc. All rights reserved.