The soluble form of a disintegrin and metalloprotease 33 promotes angiogenesis: implications for airway remodeling in asthma.

BACKGROUND: A disintegrin and metalloprotease (ADAM)-33 is a susceptibility gene for asthma and chronic obstructive pulmonary disease whose function remains unknown. OBJECTIVE: Because asthmatic bronchoalveolar lavage fluid contains high levels of soluble ADAM33 (sADAM33), which includes the catalytic domain, we postulated that its release from cell membranes might play functional roles in airway remodeling by promoting angiogenesis. METHODS: The proangiogenic activity of the highly purified catalytic domain of ADAM33 or a catalytically inactive mutant was studied in vitro (Matrigel assay), ex vivo (human embryonic/fetal lung explants) and in vivo (chorioallantoic membrane assay). The regulation of sADAM33 release from cells overexpressing full-length ADAM33 and its biological activity were characterized. RESULTS: We show that the purified catalytic domain of ADAM33, but not its inactive mutant, causes rapid induction of endothelial cell differentiation in vitro, and neovascularization ex vivo and in vivo. We also show that TGF-beta(2) enhances sADAM33 release from cells overexpressing full-length ADAM33 and that this truncated form is biologically active. CONCLUSION: The discovery that sADAM33 promotes angiogenesis defines it as a tissue remodeling gene with potential to affect airflow obstruction and lung function independently of inflammation. As TGF-beta(2) enhances sADAM33 release, environmental factors that cause epithelial damage may synergize with ADAM33 in asthma pathogenesis, resulting in a disease-related gain of function. This highlights the potential for interplay between genetic and environmental factors in this complex disease.

Beta-catenin/T-cell factor-mediated transcription is modulated by cell density in human bronchial epithelial cells.

The embryonic Wnt/beta-catenin ('canonical') pathway has been implicated in epithelial regeneration. To investigate the role of Wnt signal transduction in the airways, we characterised the expression of key pathway components in human bronchial epithelial cells (HBEC) and studied the influence of cell density on pathway activity, using sub-confluent cells in log-phase growth as a simple model of repairing epithelium. Primary HBEC and H292 bronchial epithelial cells were found to express TCF-4, TCF-3 and isoforms of LEF-1, transcription factors that are regulated by Wnt signalling. The cells also had the potential to respond to Wnt signalling through expression of several members of the Frizzled receptor family, including FZD-5 and -6. In confluent H292 cells, 20 mM lithium and 25% v/v Wnt-3a conditioned medium induced 4.5-fold (p = 0.008) and 1.4-fold (p = 0.006) increases in TOPflash activity, respectively. Under conditions of reduced cell density, TOPflash activity increased 1.8-fold (p = 0.002) in association with increased nuclear localisation of hypophosphorylated (active) beta-catenin and increased cell proliferation. This up-regulation in reporter activity occurred independently of EGF receptor activation and could not be recapitulated by use of low-calcium medium to disrupt cadherin-mediated cell-cell adhesion, but was associated with changes in FZD-6 expression. We conclude that reactivation of this embryonic pathway may play an important role in bronchial epithelial regeneration, and that modulation of Fzd-6 receptors may regulate Wnt signalling at confluence. Recognising that many chronic inflammatory disorders of the airways involve epithelial damage and repair, altered Wnt signalling might contribute to disease pathogenesis or progression.

ADAM33: a newly identified gene in the pathogenesis of asthma.

There is much to find out about this fascinating and complex molecule in relation to the development and progression of asthma. Added to it are three further new asthma/allergy genes identified by positional cloning: PDH Finger Protein II (PHF11) on chromosome 13q14, which encodes NY-REN-34 a protein first described in patients with renal cell carcinoma [67]; Dipeptidyl diptidase 10 (DDP10) on chromosome 2q14 [68]; and G protein-coupled receptor for asthma susceptibility (GPRA) on chromosome 7p [69]. For each of these genes, as is the case for ADAM33, determining their normal function(s) and how these become disordered in asthma is the future challenge.

ADAM33: a newly identified protease involved in airway remodelling.

Asthma is a complex disorder in which major genetic and environmental factors interact to both initiate the disease and modify its progression. While asthma is recognised as a disorder of the conducting airways characterised by Th2-directed inflammation, it is being increasingly apparent that alteration of the structural cells of the airways (airway remodelling) is also fundamental to disease chronicity and severity. The gene ADAM33, encoding a novel member of a identified as an asthma susceptibility gene as the result of a positional cloning effort in a cohort of families recruited form the UK and USA. Subsequent genetic studies have now provided evidence that ADAM33 may be involved in determining lung function throughout life, associated with early life lung function as well as increased decline therapeutic intervention in asthma and future work will focus on the mechanisms by which it alters lung function and bronchial hyperresponsiveness.

The genetics of asthma: ADAM33 as an example of a susceptibility gene.

The ability to identify novel disease genes by positional cloning led to the identification of a disintegrin and metalloprotease (ADAM)33 gene on chromosome 20p13 as a susceptibility gene for asthma. Case-control and family-based association studies have mostly confirmed a link between ADAM33 and asthma. Its restricted expression to mesenchymal cells as well as its association with bronchial hyperresponsiveness and accelerated decline in lung function over time point strongly to its involvement in the structural airway components of asthma, such as remodeling. Extensive alternative splicing, expression during branching morphogensis in the developing fetus, impaired lung function in childhood, the production of a soluble form linked to chronic asthma, and tight epigenetic regulation indicate a level of complexity in the way ADAM33 influences disease phenotype. Its recent association with chronic obstructive pulmonary disease as well as with asthma and lung development points to functions relating to airway wall modeling and remodeling as a general morphogenetic repair gene rather than being restricted to asthma.