Signaling mechanisms for gene regulation by metals and metal mixtures

摘要

Numerous epidemiological studies associate chronic inhalation of metal mixtures with increased risk of pulmonary diseases. Although exposure to metal mixtures is of great public health relevance, the integration of cellular responses to metals within these mixtures that promote disease is poorly understood. This dissertation investigated the hypothesis that chromium (VI) (Cr(VI)) stimulates signaling that alters transcriptional complexes to silence protective gene induction by nickel (Ni). In airway epithelial (BEAS-2B) cells, Cr(VI) activated signal transducer and activator of transcription 1 (STAT1)-dependent signaling within 1 h of exposure. This activation was dependent on Src family kinases (SFKs) since inhibiting SFKs prevented Cr(VI)-stimulated STAT1 signaling. Moreover, Cr(VI) activated STAT1 in wild-type mouse embryonic fibroblast (MEF) cells, but no response was observed in MEF cells null for the SFKs, Src, Yes, and Fyn. However, reconstituting human Fyn in the deficient MEF cells restored the Cr(VI) response. These data indicate that Cr(VI)-activated STAT1 is mediated by Fyn. This signaling may be detrimental as STAT1 has been implicated as an inflammatory mediator in asthma patients that is specifically activated in bronchial epithelial cells (1). Metallothionein (MT) and vascular endothelial growth factor A (VEGFA) are involved in protecting the lung from injury by sequestering metals and promoting wound repair, respectively. Ni-induced MT2A, the most abundant human isoform, required zinc (Zn) redistribution which directly activated metal transcription factor-1 (MTF-1). A prolonged induction was mediated by secondary signaling pathways. Cr(VI) negatively regulated the secondary pathway and had no effect on Zn mobilization. For VEGFA induction, Ni activated a complex signaling cascade involving ERK. Ni-stimulated ERK was upstream of hypoxia-inducible factor-1á (HIF-1á) and Src-mediated Sp1 transactivation. Cr(VI) inhibited Ni-activated ERK, HIF-1á stabilization, Src phosphorylation, and VEGFA induction. The current study demonstrated that Cr(VI)-activated STAT1 is responsible for the silencing of inducible genes. In BEAS-2B cells stably expressing STAT1 shRNA, Cr(VI) no longer had an inhibitory effect on Ni-induced MT or VEGFA mRNA expression and positively interacted with Ni to induce both genes. These data indicate that Cr(VI)-activated STAT1 may play a role in the pathogenesis of Cr(VI)-induced pulmonary diseases by silencing the protective gene transcription in the airway epithelium.