IMMUNOHISTOCHEMICAL DOUBLE-STAINING FOR AH RECEPTOR AND ARNT IN HUMANEMBRYONIC PALATAL SHELVES

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator p rotein (ARNT) are basic-helix-loop-helix-PAS (HLH) proteins involved i n transcriptional regulation. Polycyclic aromatic halogenated chemical s, of which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most pot ent, bind to the AhR. In the cellular cytoplasm, the AhR exists as a c omplex with the heat shock protein HSP90 and other small peptides. Thi s complex dissociates following ligand binding and then the ligand-bou nd AhR binds ARNT. The ligand-AhR-ARNT complex interacts with a specif ic, nuclear DNA sequence, the dioxin response element (DRE), altering transcription of a regulated gene. Studies in hepatoma cell lines indi cate that both proteins are required for regulation of transcription. in this study, AhR and ARNT were localized immunohistochemically in hu man embryonic palatal cells and specific patterns of expression were s een for each protein. A double-staining protocol revealed that epithel ial cells expressed both AhR and ARNT, but in mesenchyme and nasal spi ne cartilege individual cells were identified which expressed either A hR or ARNT. This heterogeneous pattern may be a means of suppressing t ranscriptional regulation and also suggests the existence of other, un identified basic-helix-loop-helix partner(s). The heterogeneous expres sion pattern may also reflect a complex role for these HLH proteins as transcriptional regulators of embryonic development. (C) 1994 Wiley-L iss, Inc.