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Activation of mitogen-activated protein kinase pathways induces antioxidant response element-mediated gene expression
1Department of Pharmaceutics and Pharmacodynamics and Center for Pharmaceutical Biotechnology, College of Pharmacy, University of Illinois, Chicago, Illinois 60612, USA.
Abstract:
Antioxidant response element (ARE) regulates the induction of a number of cellular antioxidant and detoxifying enzymes. However, the signaling pathways that lead to ARE activation remain unknown. Here, we report that the expression of mitogen-activated protein (MAP) kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1), transforming growth factor-beta-activated kinase (TAK1), and apoptosis signal-regulating kinase (ASK1) in HepG2 cells activated the ARE reporter gene, whereas the expression of their dominant-negative mutants impaired ARE activation by the chemicals sodium arsenite and mercury chloride. Coexpression of downstream kinases, MAP kinase kinase 4, MAP kinase kinase 6, and c-Jun NH(2)-terminal kinase-1, but not MAP kinase kinase 3 and p38, augmented ARE activation by MEKK1, TAK1, and ASK1. The coexpression of a basic leucine zipper transcription factor Nrf2 but not c-Jun also greatly enhanced the activation of reporter gene by MEKK1, TAK1, and ASK1; however, a dominant-negative mutant of Nrf2 (NF-E2-related factor 2) blocked this event. Furthermore, when overexpressed, MEKK1, TAK1, and ASK1 induced the expression of heme oxygenase-1, a gene regulated by ARE, and the cotransfection with the dominant-negative mutant of Nrf2 abolished the induction. Taken together, these results suggest that MAP kinase pathways that are activated by MEKK1, TAK1, and ASK1 may link chemical signals to Nrf2, leading to the activation of ARE-dependent genes.
Insights
Mitogen-activated protein kinase (MAPK) pathways activated by MEKK1, TAK1, and ASK1 link chemical signals to Nrf2, activating antioxidant response element (ARE)-dependent genes.
Area of Science:
- Cellular signaling pathways
- Gene regulation
- Toxicology
Background:
- The antioxidant response element (ARE) controls the expression of crucial antioxidant and detoxifying enzymes.
- The specific signaling cascades that trigger ARE activation are not fully understood.
Purpose of the Study:
- To elucidate the signaling pathways involved in the activation of the antioxidant response element (ARE).
- To investigate the role of specific mitogen-activated protein (MAP) kinases in ARE activation by chemical stressors.
Main Methods:
- Expression of various MAP kinase kinases (MEKK1, TAK1, ASK1) and their mutants in HepG2 cells.
- Assays for ARE reporter gene activation using chemicals like sodium arsenite and mercury chloride.
- Analysis of downstream kinase involvement and the role of transcription factor Nrf2.
Main Results:
- MEKK1, TAK1, and ASK1 expression activated the ARE reporter gene.
- Dominant-negative mutants of these kinases inhibited ARE activation by chemicals.
- Downstream kinases (MAPK4, MAPK6, JNK1) and Nrf2 augmented ARE activation, while a dominant-negative Nrf2 mutant blocked it.
- Overexpression of MEKK1, TAK1, and ASK1 induced heme oxygenase-1 expression, which was abolished by Nrf2 inhibition.
Conclusions:
- MAP kinase pathways initiated by MEKK1, TAK1, and ASK1 are critical for linking chemical stimuli to Nrf2.
- This signaling cascade ultimately leads to the activation of ARE-dependent genes, playing a key role in cellular defense mechanisms.