Role Of Receptor-mediated-endocytosis In Oligochitosan Induced Macrophage Activation

Chitin (a liner homopolymer of β-1,4-linked N-acetylglucosamine) and chitosan (the partial deacetylated product of chitin) are the second most abundant carbohydrate polymers present in the exoskeleton of crustaceans and insects and in the cell walls of fungi and yeast. Because of their renewable, nontoxic, biocompatable and biodegradable properties, chitin and chitosan have received much attention as functional biopolymers for diverse applications, especially in pharmaceutics, foods and cosmetics . These properties have been revealed not only be depend upon their chemical structure but also the molecular size. To be effectively absorbed in human body, they should be converted into their oligomers. Oligochitosans can be obtained by either chemical or enzymatic hydrolysis of chitosan. They are composed of 3-10 N-acetyl-glucosamine (NAGA) or glucosamine residues. They have low molecular weights, short chains and are soluble in neutral aqueous solutions with lower viscosity. Oligochitosans have also been proved to possess versatile functional properties such as antitumor, antifungal and antimicrobial activities, to have potential as sustained release carriers for drugs, to accelerate wound healing, and to enhance the in vitro infectivity of adenovirus to mammalian cells.In the mechanisms by which chitosan derivatives mediate the differentimmunostimulating activities, activation of macrophages is thought to be important. There are many findings suggested that chitosan derivatives can up regulate the production of tumor necrosis factor -alpha (TNF-α ), interleukin-1 (IL-1) and colony-stimulating factor (CSF) by macrophages. Our previous study also indicated that oligochitosan was able to enhance Nitric Oxide (NO), TNF-α and IL-1 β release in RAW264.7 cells and NF- κ B was involved in the course of activation. However, the mechanism by which oligochitosan interacts and activates macrophages is not very clear. Previous studies indicated that binding of NAGA to the specific receptors was a prerequisite for enhancing macrophages activation and mannose receptor (MR) might be involved in this interaction. In this paper, we further studied the role of MR in oligochitosan-induced RAW264.7 activation. Our observation demonstrated that oligochitosan could bind to macrophage membrane and be internalized gradually. Internalization was temperature dependent, low temperature could completely check the uptake process. Competition studies and Ca~2+- dependent experiments both indicated the involvement of MR in oligochitosan uptake. Further characteration of the uptake showed that both membrane MR and recycling MR were responsible for oligochitosan uptake. MR mediated pinocytosis was the mechanism under which oligochitosan was internalized and that MR played an important role both in oligochitosan uptake and macrophage activation.