1. Field of the Invention
This invention relates generally to the field of bronchotherapeutics. More specifically, this invention relates to treatment of airway diseases such as asthma.
2. Description of the Related Art
Asthma is a global problem with serious morbidity, mortality and economic burden. Treatment of asthma often requires toxic doses of adrenergic bronchodilators and steroids. An increase in bronchial smooth muscle mass due to hypertrophy and hyperplasia is an important element of airway structural changes, the remodeling process occurring in asthma. The increase in bronchial smooth muscle mass may play a critical role in the development of airway hyper-reactivity (AHR), the hallmark of asthma. However, the pathogenesis of bronchial smooth muscle hyperplasia/hypertrophy is not well understood, despite the knowledge in this area disclosed over the last decade.
Airway inflammation is a key feature of asthma. Of the inflammatory mediators associated with asthma, histamine, lysosomal hydrolases, endothelin-1, thromboxane, and tryptase have been reported to directly stimulate airway smooth muscle cells proliferation. In addition, smooth muscle cells with asthma phenotype can produce a number of growth factors and cytokines that may contribute to the inflammatory process and airway hyper-reactivity. Finally, airway remodeling, including airway smooth muscle cells proliferation, has been shown to correlate with airway responsiveness.
Lysosomal Hydrolases
The mitogenic function of lysosomal hydrolases in airway smooth muscle cells has been studied. Lysosomal hydrolases are produced and secreted by inflammatory cells, i.e., mast cells, macrophages, neutrophils, and eosinophils, in response to various soluble and particulate stimuli such as antigen bound to IgE antibody, β-glucan and zymosan. These protease-resistant lysosomal hydrolases are important putative mediators responsible for airway smooth muscle cell proliferation occurring in asthma. Increased levels of these enzymes in bronchoalveolar lavage fluid are associated with experimentally induced asthma in humans and guinea pigs.
Lysosomal hydrolases are often used as inflammatory markers. Moreover, it has been shown that leukotrienes, LTB4 and LTC4, augment the synthesis and secretion of hexosaminidases in macrophages. It has been shown that lysosomal hydrolases (β-hexosaminidases (Hex A, Hex B) and β-glucuronidase) and mannosylated human or bovine serum albumin stimulate proliferation of bovine tracheal myocytes. At physiologically and pathologically relevant concentrations, Hex A and Hex B stimulate DNA synthesis and increase cell number.
The mitogenic action of purified human placental b hexosaminidases is mediated by 175 kD mannose recognizing receptors (ASM-MR) which share structural, topological, and functional properties with macrophage mannose receptor (Mø-MR). A monoclonal antibody (mAb15) directed to the extracellular domain of Mø-MR acts as an agonistic antibody for DNA synthesis. Unlike most glycoproteins and membrane glycoconjugates of the majority of higher organisms, lysosomal hydrolases possess high mannose chains as a remnant of the mannose-phosphate targeting marker. Lysosomal hydrolases are thus taken up by either mannose receptor or cation independent mannose-6-phosphate (MPRCI/IGF-II) receptor.
Mannose Recognizing Receptors
Macrophage mannose receptor (Mø-MR) is the first recognized member of a growing family of mutilectin receptor proteins involved in molecular scavenging of glycoconjugates with terminal mannose, N-acetylglucosamine or fucose. It has specialized high affinity binding sites involved in a pattern recognition and host defense against pathogens such as Pneumocystis carinii, Mycobacterium tuberculosis, Candida albicans, and Leishmania donovani. Other members of mannose receptor family are secretory phospholipase A2 receptor, DEC205, and Endo180. Endo180 is an endocytic Ca++-dependent lectin receptor expressed in fibroblasts, endothelial cells and macrophages.
Airway smooth muscle cells express mannose receptors (ASM-MR). While ASM-MR and Mø-MR bind to mannose affinity columns, Endo180 binds to N-acetylglucosamine affinity matrix but not mannose affinity column. However, Endo180 recognizes mannose residues by the second domain of carbohydrate recognition domain (CRD) that is a low affinity binding domain. Each of these receptors mediates endocytosis, but physiological ligands have only been identified for Mø-MR, ASM-MR and secretory phospholipase A2 (sPLA2) receptor. Indeed, sPLA2 receptor and dendritic cell receptor DEC205 do not contain the conserved residues necessary for Ca++-dependent sugar binding.
The features and functions of individual domains of macrophage mannose receptor have been studied extensively. The N-terminal cysteine-rich domain contains N-acetylgalactosamine 4-sulphate binding site involved in recognition of leutropin. No specific function of fibronectin type II (FN II) repeat domain of Mø-MR has been determined. In other proteins containing FN II such as gelatinases and possibly in fibronectin, these domains are involved in binding of gelatin. Of the carbohydrate recognition domains (CRD) of Mø-MR, CRD4-5 form a protease-resistant ligand-binding core essential for high affinity binding of mutivalent ligands. However, maximum affinity for yeast mannan is achieved when CRD4-8 are intact. The cytoplasmic domain of Mø-MR contains a di-aromatic motif, Tyr18-Phe19, which is important in endosomal sorting.
Among the mannose receptor family members, airway smooth muscle cell mannose receptor (ASMC-MR) has unique mitogenic function (Table 1). Unique ligand binding property of the ASM-MR may have evolved in order to bind mannosyl-rich lysosomal hydrolases released from lung mast cells and macrophages in direct contact with bronchial smooth muscle.
ASMC-MR mitogenic activation is accompanied by a transient elevation of cyclic adenosine monophosphate (cAMP), activation of protein kinase C (PKC) and p21Ras- and PKC-dependent activation of p44/42MAPK. The onset of p44/42MAPK activation in response to the ASM-MR activation is late (30 min) and the activation lasts for 4 hours. This sustained activation of p44/42MAPK is considered an important signal to cause cell proliferation.
While the main treatment of asthma therapy, i.e., inhaled corticosteroids, effectively reduces inflammation and remodeling of epithelium and basement membrane, no agents have been proven effective in reducing smooth muscle mass in asthmatic patients. Mannan is a carbohydrate cell wall component of yeast such as Saccharomyces cerevisiae and is a mannose receptor blocker. The mannan component of microbial cell wall renders pattern recognition by host cellular receptors and serves as a virulence factor for the organism by supporting differentiation and protecting from starvation and/or stress conditions. The inventors recognize that pathological airway smooth muscle cells proliferation may be controlled by blocking airway smooth muscle cells mannose receptor (ASMC-MR) with mannan or other anti-asthma drugs having airway smooth muscle cells mannose receptor ligand structural or functional properties.
There is a need in the art for improved methods of therapy to treat asthma by blocking the onset of airway smooth muscle cells proliferation and the resultant airway hyperreactivity. The prior art is deficient in the lack of drugs to specifically block mannose receptors on airway smooth muscle cells and of methods to treat asthma using these blockers. Specifically, the prior art is deficient in methods of using mannan as a therapeutic in the treatment of asthma. The present invention fulfills this longstanding need and desire in the art.
TABLE 1Comparison of the Properties of Mannose Receptor Family MembersReceptorMøMRASM-MREndo180sPLA2RDEC205Mass (kD)165-180175180180205Amino acid35100 99 36 33Identity (%)Ca++ bindingYesYesYesNoNo(Ctype)Clathrin coatedYesYesYesYesYespitsEndogenous LigandYesYesYesYesNoKnownMannose bindingYesYesYesNoNoGIcNAc bindingYesYesYesNoNokD (nM)10 50UnkUnkUnkFunctionScavengeMitogenicOsteogenicclearpresentHost defenseCollagensPLA2antigenuptake