The adaptor molecule Disabled-2 (Dab2) has been shown to link cell surface receptors to downstream signaling pathways. Using a small-pool cDNA screening strategy, we identify that the N-terminal domain of Dab2 interacts with Dishevelled-3 (Dvl-3), a signaling mediator of the Wnt pathway. Ectopic expression of Dab2 in NIH-3T3 mouse ®broblasts attenuates canonical Wnt/b-catenin-mediated signaling, including accumulation of b-catenin, activation of b-catenin/ T-cell-speci®c factor/lymphoid enhancer-binding factor 1-dependent reporter constructs, and endogenous cyclin D1 induction. Wnt stimulation leads to a timedependent dissociation of endogenous Dab2±Dvl-3 and Dvl-3±axin interactions in NIH-3T3 cells, while Dab2 overexpression leads to maintenance of Dab2±Dvl-3 association and subsequent loss of Dvl-3±axin interactions. In addition, we ®nd that Dab2 can associate with axin in vitro and stabilize axin expression in vivo. Mouse embryo ®broblasts which lack Dab2 exhibit constitutive Wnt signaling as evidenced by increased levels of nuclear b-catenin and cyclin D1 protein levels. Based on these results, we propose that Dab2 functions as a negative regulator of canonical Wnt signaling by stabilizing the b-catenin degradation complex, which may contribute to its proposed role as a tumor suppressor. IntroductionThe Wnt family of secreted glycoproteins play key roles in embryonic development, regulating cell proliferation, motility and cell fate (Cadigan and Nusse, 1997;Dale, 1998) and in adult tissues where aberrant Wnt signaling has been shown to contribute to a variety of human cancers (Polakis, 2000;Bienz and Clevers, 2000). Wnt ligand binding to its cognate receptors can stimulate several distinct signaling pathways including the canonical Wnt±b-catenin and non-canonical planar cell polarity± convergent extension (PCP±CE) pathways. Required for activation of both of these pathways is the common mediator dishevelled (Dvl) which interprets signals from various receptors and transmits them to different effector molecules. In the canonical pathway, activation of the frizzled and LRP5/6 co-receptors results in recruitment of Dvl which relays the signal to a complex composed of adenomatous polyposis coli (APC), axin, glycogen synthase kinase 3b (GSK3b) and b-catenin (Polakis, 2000;Wharton, 2003). In the absence of Wnt signaling, b-catenin is phosphorylated on N-terminal serine and threonine residues, targeting it for degradation by the ubiquitin±proteasome pathway (Kitagawa et al., 1999). In the presence of Wnt, association of Dvl with axin prevents GSK3b from phosphorylating b-catenin, leading to stabilization of b-catenin and its translocation to the nucleus. In the nucleus, b-catenin complexes with members of the T-cell-speci®c factor/lymphoid enhancerbinding factor 1 (TCF/LEF-1) transcription factor family to regulate target genes which include c-myc and cyclin D1 (He et al., 1998;Shtutman et al., 1999). PCP signaling, responsible for proper orientation of photoreceptor cells in Drosophila, and vertebrate CE, responsi...

Cadmium is a toxic heavy metal ranked seventh on the Priority List of Hazardous Substances. As a byproduct of smelters, cadmium is a prevalent environmental contaminant. It is also a major component of cigarette smoke, and its inhalation is associated with decreased pulmonary function, lung cancer, and chronic obstructive pulmonary disease. Ion channels, including the cystic fibrosis transmembrane conductance regulator (CFTR), play a central role in maintaining fluid homeostasis and lung functions. CFTR is mostly expressed in epithelial cells, and little is known about the effect of cadmium exposure on lung epithelial cell function. We show that exposure to cadmium decreases the expression of the CFTR protein and subsequent chloride transport in human airway epithelial cells in vitro. Impairment of CFTR protein expression was also observed in vivo in the lung of mice after intranasal instillation of cadmium. We established that the inhibitory effect of cadmium was not a nonspecific effect of heavy metals, as nickel had no effect on CFTR protein levels. Finally, we show that selected antioxidants, including alpha-tocopherol (vitamin E), but not N-acetylcysteine, can prevent the cadmium-induced suppression of CFTR. In summary, we have identified cadmium as a regulator of the CFTR chloride channel present in lung epithelial cells. Future strategies to prevent the deleterious effect of cadmium on epithelial cells and lung functions may benefit from the finding that alpha-tocopherol protects CFTR expression and function.

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Airway epithelial cells in the lung are the first line of defense against pathogens and environmental pollutants. Inhalation of the environmental pollutant cadmium has been linked to the development of lung cancer and chronic obstructive pulmonary disease, which are diseases characterized by chronic inflammation. To address the role of airway epithelial cells in cadmium-induced lung inflammation, we investigated how cadmium regulates secretion of interleukin 8 (IL-8) by airway epithelial cells. We show that exposure of human airway epithelial cells to subtoxic doses of cadmium in vitro promotes a characteristic inflammatory cytokine response consisting of IL-8, but not IL-1β or tumor necrosis factor-alpha. We also found that intranasal delivery of cadmium increases lung levels of the murine IL-8 homologs macrophage inflammatory protein-2 and keracinocyte-derived chemokine and results in an influx of Gr1+ cells into the lung. We determined that inhibition of the nuclear factor-κB (NF-κB) pathway had no effect on cadmium-induced IL-8 secretion by human airway epithelial cells, suggesting that IL-8 production was mediated through an NF-κB-independent pathway. Mitogen-activated protein kinases (MAPKs) are often involved in proinflammatory signaling. Cadmium could activate the main MAPKs (i.e., p38, JNK, and Erk1/2) in human airway epithelial cells. However, only pharmacological inhibition of Erk1/2 pathway or knockdown of the expression of Erk1 and Erk2 using small interfering RNAs suppressed secretion of IL-8 induced by cadmium. Our findings identify cadmium as a potent activator of the proinflammatory cytokine IL-8 in lung epithelial cells and reveal for the first time the role of an NF-κB-independent but Erk1/2-dependent pathway in cadmium-induced lung inflammation.

Cadmium is a toxic metal present in the environment and its inhalation can lead to pulmonary disease such as lung cancer and chronic obstructive pulmonary disease. These lung diseases are characterized by chronic inflammation. Here we show that exposure of human airway epithelial cells to cadmium promotes a polarized apical secretion of IL-6 and IL-8, two pivotal pro-inflammatory cytokines known to play an important role in pulmonary inflammation. We also determined that two distinct pathways controlled secretion of these proinflammatory cytokines by human airway epithelial cells as cadmium-induced IL-6 secretion occurs via an NF-κB dependent pathway, whereas IL-8 secretion involves the Erk1/2 signaling pathway. Interestingly, the natural antioxidant curcumin could prevent both cadmium-induced IL-6 and IL-8 secretion by human airway epithelial cells. In conclusion, curcumin could be used to prevent airway inflammation due to cadmium inhalation.