br Introduction The airway epithelium
Introduction The airway epithelium is the first barrier against inhaled insults and the final barrier against internal forces (hydrostatic, inflammatory, and ischemic) that disrupt water and solute movement across the epithelium. This barrier is formed by adhesion of airway and alveolar epithelial CDK4 inhibitor mg through the adhesive properties of junctional proteins. We have previously shown that the receptor tyrosine kinase (TK) HER2 is important in maintaining this epithelial cell barrier [, , , , ]. HER2 is a class I membrane bound receptor TK with four other family members; EGFR (HER1), HER3, and HER4. On ligand binding these receptors homo- or hetero-dimerize, and activate their intrinsic TK domains, resulting in auto-phosphorylation and downstream signaling [7,8]. The diversity of receptor combinatorial events is greatly simplified in the lung as HER1 (EGFR) and HER4 are expressed at lower levels relative to HER2 and HER3 [6,9], and HER3 is the preferred dimerization partner for HER2 . Therefore, the HER2/HER3 heterodimer is the predominant signaling complex in the lung. However, HER2 has no known ligand [11,12], and HER3 is catalytically inactive [10,11]. Therefore, in order to signal, the receptor components cooperate. Membrane bound NRG-1, the HER3 ligand, expressed at high levels in the pulmonary epithelium [9,13], is shed by IL-1β induced activation of ADAM17 . NRG-1 binds with HER3 in an autocrine fashion , and NRG-1/HER3 associates with HER2, activating HER2's TK domain. The NRG-1 and HER2/HER3 interaction is elegantly regulated in polarized epithelial cells by a spatially defined mechanism with segregation of the ligand apically, and the receptors laterally . Activation of the HER2/HER3 signal pathway results in an alteration of the epithelial cell barrier through HER2 mediated phosphorylation of β-catenin, leading to dissociation of β-catenin from E-cadherin which results in decreased E-cadherin mediated cell adhesion and increased paracellular leak. In bronchoalveolar lavage (BAL) samples from patients with acute lung injury (ALI) requiring intubation and mechanical ventilation, BAL NRG-1 levels inversely correlated with ventilator free days, confirming shedding of NRG-1 during lung injury and indicating activation of this pathway in inflammatory ALI . Blocking this pathway in animal models of lung injury decreased the extent of injury and improved survival. Inhibition of HER2/HER3 signaling in bleomycin lung injury models using either a transgenic mouse with a dominant negative HER3 incapable of HER2 activation or a monoclonal antibody that blocks HER2 activation, resulted in reduced pulmonary fibrosis and mortality [, , ]. A number of other inflammatory mediators are elevated in the alveolus and airways during the early phase of lung injury including Interleukin-6 (IL-6) [15,16]. IL-6 acts as a major pro-inflammatory mediator for the induction of the acute phase response , leading to a wide range of local and systemic changes including fever, leucocyte recruitment and activation. However, IL-6 is multi-functional, its effects context dependent and increased levels seen in lung injury can either contribute to or prevent organ injury . Some studies find a beneficial effect of IL-6 during lung injury [, , , , ]. This is seen in IL-6KO mice that have more severe pulmonary inflammation and injury in response to LPS aspiration and mechanical ventilation when compared to wild-type animals [23,24]. Treatment with exogenous IL-6 rescues the knock-out phenotype with decreased alveolar permeability, lung edema and reduced histological lung damage, and in wild type mice IL-6 decreased lung leukocyte infiltration following a LPS challenge or mechanical ventilation [23,24]. Others have reported a detrimental effect with circulating IL-6 mediating lung injury after acute kidney injury or pancreatitis, and promoting pulmonary fibroblast proliferation [, , ]. Clearly, IL-6 plasma levels have been associated with higher morbidity and mortality [15,28].