Metal-Binding Properties of a Dicysteine-Containing Motif in Protein Tyrosine Kinases

Tight junction disruption by cadmium in an in vitro human airway tissue model

Background

The cadmium (Cd) present in air pollutants and cigarette smoke has the potential of causing multiple adverse health outcomes involving damage to pulmonary and cardiovascular tissue. Injury to pulmonary epithelium may include alterations in tight junction (TJ) integrity, resulting in impaired epithelial barrier function and enhanced penetration of chemicals and biomolecules. Herein, we investigated mechanisms involved in the disruption of TJ integrity by Cd exposure using an in vitro human air-liquid-interface (ALI) airway tissue model derived from normal primary human bronchial epithelial cells.

Methods

ALI cultures were exposed to noncytotoxic doses of CdCl₂ basolaterally and TJ integrity was measured by Trans-Epithelial Electrical Resistance (TEER) and immunofluorescence staining with TJ markers. PCR array analysis was used to identify genes involved with TJ collapse. To explore the involvement of kinase signaling pathways, cultures were treated with CdCl₂ in the presence of kinase inhibitors specific for cellular Src or Protein Kinase C (PKC).

Results

Noncytotoxic doses of CdCl₂ resulted in the collapse of barrier function, as demonstrated by TEER measurements and Zonula occludens-1 (ZO-1) and occludin staining. CdCl₂ exposure altered the expression of several groups of genes encoding proteins involved in TJ homeostasis. In particular, down-regulation of select junction-interacting proteins suggested that a possible mechanism for Cd toxicity involves disruption of the peripheral junctional complexes implicated in connecting membrane-bound TJ components to the actin cytoskeleton. Inhibition of kinase signaling using inhibitors specific for cellular Src or PKC preserved the integrity of TJs, possibly by preventing occludin tyrosine hyperphosphorylation, rather than reversing the down-regulation of the junction-interacting proteins.

Conclusions

Our findings indicate that acute doses of Cd likely disrupt TJ integrity in human ALI airway cultures both through occludin hyperphosphorylation via kinase activation and by direct disruption of the junction-interacting complex.

Chromium(VI) stimulates Fyn to initiate innate immune gene induction in human airway epithelial cells

Mechanisms for pathogenic metal signaling in airway injury or disease promotion are poorly understood. It is widely believed that one mechanism for pathogenic and possible carcinogenic effects of inhaled chromium (Cr(VI)) is inhibition of inducible gene transactivation. However, we recently reported that Cr(VI) inhibition of Sp1-dependent transactivation required signal transducer and activator of transcription 1 (STAT1)-dependent expression of an inhibitory protein in airway epithelium. Thus, Cr(VI) exposures can induce genes, and we hypothesized that this induction resulted from Cr(VI) signaling through an innate immune-like STAT1-dependent pathway initiated by Fyn. Exposure of human airway epithelial (BEAS-2B) cells to Cr(VI) selectively transactivated the STAT-responsive interferon-stimulated response element (ISRE) and induced ISRE-driven transactivation of interferon regulatory factor 7 (IRF7), without affecting the gamma interferon-activated site (GAS)-driven IRF1 expression. Cr(VI)-induced IRF7 was absent or greatly reduced in cells that lacked STAT1, were treated with the Src family kinase inhibitor, PP2, or lacked Fyn. Expressing Fyn, but not Src, in mouse embryonic fibroblasts cells null for Src, Yes, and Fyn restored Cr(VI)-stimulated STAT1 tyrosine phosphorylation and IRF7 expression. Finally, shRNA knockdown of Fyn in BEAS-2B cells prevented Cr(VI)-activated STAT1 transactivation of IRF7. These data support a novel mechanism through which Cr(VI) stimulates Fyn to initiate interferon-like signaling for STAT1-dependent gene transactivation.