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The ubiquitous PM component Zn2+ induces HO-1 expression through multiple targets in the Nrf2/Keap1 signaling pathway
Citation:
Silbajoris, R., L. Dailey, P. Wages, A. Speen, Steve Simmons, P. Bromberg, AND J. Samet. The ubiquitous PM component Zn2+ induces HO-1 expression through multiple targets in the Nrf2/Keap1 signaling pathway. Presented at American Thoracic Society, San Diego, CA, May 16 - 21, 2014.
Impact/Purpose:
This is an abstract of scientfic work to be presented at the American Thoracic Society meeting May 16-21, 2014.
Description:
Oxidant stress can play an important role in particulate matter (PM)–mediated toxicity in the respiratory tract. Zinc (Zn2+) is a ubiquitous component of ambient PM that induces adverse responses such as inflammatory and adaptive gene expression in human airway epithelial cells. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a transcription factor that controls the expression of genes whose transcriptional expression is regulated by the antioxidant response element (ARE), including the adaptive protein hemoxygenase 1 (HO-1). Under normal conditions, Nrf2 is sequestered and degraded in the cytoplasm through binding to Kelch-like ECH-associated protein 1 (Keap1). Oxidative stress modifies Keap1, resulting in the accumulation of Nrf2 and its translocation to the nucleus where it binds to the ARE. BACH1 is a member of the cap'n'collar type of basic region leucine zipper factor protein family that competes with Nrf2 by binding to the ARE, thereby acting as an additional transcriptional repressor of genes such as HO-1. We reported previously that treatment of human bronchial epithelial cells BEAS 2B with Zn2+ induces a dramatic increase in HO-1 expression in these cells. In order to elucidate the molecular basis for Zn2+-induced adaptive gene expression, in the current study we investigated Nrf2/ARE-dependent mechanisms of Zn2+ induced expression of HO-1 in BEAS 2B cells. BEAS 2B cells transduced with a synthetic ARE-luciferase reporter construct showed a 4-fold increase in transcriptional activity when stimulated with Zn2+, but not in cells expressing a vector encoding a scrambled ARE sequence. Overexpression of wild type Keap1 or a version in which the 3 amino acids responsible for Zn2+ binding and activation of Keap1 were mutated led to significant reductions in HO-1 mRNA expression in BEAS 2B cells exposed to 10-50 uM Zn2+, indicating the involvement of Nrf2 in this effect. In addition, Zn2+ exposure modulated levels of BACH1 protein measured by Western blotting, suggesting that Zn2+ can lift the repressive tone that BACH1 exerts on HO-1 expression. Taken together these findings show that Zn2+ can interact with multiple targets in the Nrf2/ARE signaling pathway to cause simultaneous signaling events that synergize to induce robust adaptive gene expression in BEAS 2B cells. These studies identify molecular signaling mechanisms through which critical PM components perturb intracellular quiescence and lead to adverse tissue responses to pollutant inhalation.