| Research Abstract |
(1) AhR-mediated signal transduction
The AhR is a ligand-activated transcription factor that acts in concert with the AhR nuclear translocator (ARNT), and alters gene expression in response to environmental contaminants such as TCDD. We showed direct evidence for nucleo-cytoplasmic shuttling of AhR and also showed the biological significance of the shutting in terms of the transcriptional activation of its target gene, CYP1A1. [J. Biochem., 127, 503-509 (2000)]. We also investigated the role of the LXXLL-motif of the AhR relative to its subcellular localization and transactivation of CYP1A1. This motif at amino acids 50-54 is important for regulating the localization and transcriptional activation of AhR. [J. Biochem., 131, 79-85 (2002)].
(2) Physiological function of AhR
AhR contains signals for both nuclear localization (NLS) and export (NES). Here, we intended to reveal what or how physiological signals affect AhR intracellular distribution and function. The ligand-dependent nuclear im
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port of AhR was inhibited by phosphorylation at the two protein kinase C sites adjacent to the bipartite NLS, while nuclear export was suppressed by the phosphorylation of the NES. The cell density, but not the cell cycle, influenced the subcellular distribution of AhR in keratinocyte cell line HaCaT, with predominantly nuclear AhR in sparse, even both in nuclear and cytoplasmic in subconfluence but predominantly cytoplasmic in confluence. Leptomycin B promoted a nuclear accumulation of AhR irrespective of cell density. The novel cell density-dependent AhR relocalization was affected by exposure to SB203580, okadaic acid and low Ca^<2+> concentration. The stable transfectants of HaCaT carrying the xenobiotic responsive element (XRE) showed an association between fee XRE-mediated transcription and AhR relocalization. Our data provide evidence that cell density regulates the intracellular localization and function of AhR caused by p38 MAPK-dependent phosphorylation and dephosphorylation of the NES. Less
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