Cationic Proteins and Bronchial Hyperresponsiveness

Abstract

Chronic airway inflammation in asthma is characterized by the preponderance of eosinophils in the airway walls and lumens. Eosinophils are unique among granulocytes in that they contain intracytoplasmic granules, which stain with eosin. These granules contain a number of preformed enzymes and other proteins, including cationic proteins such as major basic protein (MBP), eosinophil cationic protein, eosinophil-derived neurotoxin, and eosinophil peroxidase. Other granule enzymes include lysophospholipase, collagenase, b -glucuronidase, and arylsulfatase B (1). The principal granule protein in eosinophils is MBP, a highly positively charged, 117–amino acid polypeptide with a molecular weight of 14 kD. It is named for its highly basic isoelectric point (calculated to be 10.9) that is due to its large number of arginine residues (2). It is encoded from a 3.3-kb, five-intron gene located on chromosome 11 (3). MBP is synthesized in the cell as a pre-proprotein and is then processed and transported as a relatively neutral pH, 33-kD proprotein, presumably to protect the eosinophil from the toxic effects of MBP (4). Pro-MBP is converted into mature MBP after its sequestration in the granule crystalloid cores. Upon release, physiologic concentrations of MBP upregulate expression of intercellular adhesion molecule-1 on cultured human nasal epithelial cells (5). Moreover, MBP can induce eosinophil degranulation and the production of interleukin 8 by these cells (6). Thus, MBP can act as an autocrine factor to potentiate further eosinophil recruitment and activation in the airways. In addition, the highly cationic MBP reacts readily with acidic lipids, disordering lipid membranes and resulting in fusion and lysis (4). Accordingly, MBP is a potent bacteriocidal and helminthotoxic agent that is also toxic to tumor cells and other mammalian cells, including the airway epithelium. Naturally occurring cationic polypeptides such as MBP have diverse cytotoxic and noncytotoxic effects on many different cell types due to interactions with cell-surface anion charges. The importance of the cationic charge is supported by the observation that synthetic polycations of similar size and charge, such as polyL -arginine and polyL lysine, mimic many of the effects of MBP. Specific mechanisms are not known, but interactions of polycations with cell-surface anionic charges could interfere with normal cell function in a variety of ways, including the blockade of ion channels, inhibition of transporters, compromise of membrane integrity, and binding to or aggregation of integral membrane proteins (7–11). However, cationic charge alone may not be the only mechanism of action for naturally occurring cationic polypeptides. For example, contractions of guinea pig tracheal smooth muscle by MBP are inhibited by atropine, whereas the contractions due to synthetic polycations of similar charge are not (12). Thus, all of the effects of MBP may not be attributed to cationic charge. Interest in eosinophil-derived cationic proteins, especially MBP, was stimulated by evidence implicating them in the pathogenesis of bronchial hyperresponsiveness (13, 14) (Table 1). In ovalbumin-sensitized guinea pigs and in patients with asthma, a significant correlation exists between the concentrations of MBP and methacholine hyperresponsiveness (15, 16). Additional evidence suggests that neutralization of endogenously secreted MBP, either with a polyanionic peptide or with antibodies to MBP, can prevent antigen-induced bronchial hyperreactivity in sensitized animals. Specifically, the polyanion polyL -glutamic acid inhibits the bronchial hyperresponsiveness induced by MBP in primates (17) as well as the increase in vagal responsiveness induced by antigen provocation in guinea pigs (18). Similarly, anti-MBP antibodies suppress the development of bronchial hyperreactivity after antigen challenge in sensitized guinea pigs without affecting airway eosinophilia (19). Such studies suggest that MBP plays a key role in the alterations of airway function that accompany eosinophilic airway inflammation. Initial in vivo (20) and in vitro studies (21, 22, 23) suggested that the effects of MBP on airway smooth-muscle contractility occurred indirectly, via effects on the airway epithelium. Exposure to MBP or synthetic polycations damages the airway epithelial barrier (24, 25) and increases epithelial permeability to hydrophilic tracers (26). Such actions would expose the underlying airway smooth muscle to higher concentrations of bronchoactive substances. Nevertheless, MBP also exerts important nontoxic effects on the airway epithelium. It has been shown that MBP affects epithelial eicosanoid metabolism, altering the balance between constricting and dilating prostaglandins (27, 28). Furthermore, recent evidence shows that polycations inhibit L -arginine uptake by airway epithelial cells (8) and that a deficiency in nitric oxide production contributes to polycation-induced airway hyperreactivity (9). ( Received in original form February 14, 2000 )