Altered protein tyrosine phosphorylation in asthmatic bronchial epithelium

A disease-related, corticosteroid-insensitive increase in the expression of epidermal growth factor (EGF) receptor (EGFR) tyrosine kinase in asthmatic bronchial epithelium has been shown previously by the current authors. To determine whether this is associated with enhanced intracellular signalling, the aim of this study was to evaluate epithelial tyrosine phosphorylation. Bronchial biopsies were analysed for the presence of phosphotyrosine by immunohistochemistry. Bronchial epithelial cells were exposed to EGF, hydrogen peroxide or tumour necrosis factor-alpha in vitro for measurement of tyrosine phosphorylated signalling intermediates and interleukin (IL)-8 release. Phosphotyrosine was increased significantly in the epithelium of severe asthmatics when compared with controls or mild asthmatics; however, in mild asthma, phosphotyrosine levels were significantly decreased when compared with controls. There was no significant difference between phosphotyrosine levels before or after 8 weeks of treatment with budesonide. Stimulation of bronchial epithelial cells resulted in tyrosine phosphorylation of several proteins, including EGFR, Shc and p42/p44 mitogen-activated protein kinase. In the presence of salbutamol, a transient partial suppression of EGFR phosphorylation occurred, whereas dexamethasone was without effect. Neither salbutamol nor dexamethasone inhibited EGF-stimulated IL-8 release. These data indicate that regulation of protein tyrosine kinase activity is abnormal in severe asthma. The epidermal growth factor receptor and/or other tyrosine kinase pathways may contribute to persistent, corticosteroid-unresponsive inflammation in severe asthma.