Asthma is an inflammatory disease characterised by tissue remodelling. A prominent feature of this remodelling is an increase in the number and size of the blood vessels- formed from pre-existing capillaries – angiogenesis (Siddiqui et al., 2007; Wilson, 2003). This is triggered by many different endogenous angiogenic stimulators such as vascular endothelial growth factor (VEGF), and inhibited by endogenous angiogenic inhibitors such as tumstatin. Tumstatin is the non-collagenous domain (NC1) of the collagen IV α3 chain which, when cleaved, inhibits endothelial cell proliferation and induces apoptosis. Experiments described in this thesis have for the first time demonstrated the absence of tumstatin in the airways of individuals with asthma and lymphangioleiomyomatosis (LAM) as well as the functional responses to tumstatin as an angiogenic inhibitor, both in vitro and in vivo, in the airway.
Although tumstatin was absent from the airways of asthmatic and LAM individuals it was present in the airways of individuals with no airways disease, chronic obstructive pulmonary disease, bronchiectasis and cystic fibrosis. No significant difference was seen in the levels of the Goodpasture Binding Protein (GPBP), a phosphorylating protein responsible for the alternate folding of tumstatin, between asthmatic, LAM and individuals with no airways disease. The αvβ3 integrin, reported to be necessary for the activity of tumstatin, as well as the individual αv and β3 sub-units were shown to be equally expressed in the airways of all patient groups. Co-localisation of tumstatin, VEGF and the αvβ3 integrin was seen in the disease free airways, however, a different pattern of VEGF and the αvβ3 integrin expression was observed in asthmatic and LAM airways with minimal co-localisation.
Tumstatin was detected in serum and bronchoalveolar lavage fluid (BAL-f) samples from asthmatics and individuals with no airway disease, however there was no significant difference in the level of expression between the two groups. It was demonstrated that the tumstatin detected in the serum and BAL-f samples from asthmatics and individuals with no airway disease was part of the whole collagen IV α3 chain and not in its free and potentially active form.
The ability of recombinant tumstatin to inhibit tube formation and proliferation of primary pulmonary endothelial cells was demonstrated for the first time. Further, the functional response of tumstatin was demonstrated in vivo in a mouse model of allergic airway disease. Tumstatin inhibited angiogenesis in the airway and decreased airway hyperresponsiveness.
Whether there is potential for tumstatin, or a derivative thereof, to be of therapeutic value in airways diseases in which angiogenesis is a component should be the subject of future studies.