Identification of a novel mechanism for endotoxin-mediated down-modulationof CC chemokine receptor expression

In the present study, we explored the molecular mechanisms by which bacteri al endotoxin (LPS) mediates the down-regulation of CCR2 receptors on human monocytes. We found that LPS induced a marked reduction in CCR2 cell surfac e protein levels which was blocked by pretreatment with the tyrosine kinase inhibitors genistein and herbimycin A. The effector mechanism underlying L PS-induced CCR2 down-modulation appears to involve the enzymatic activity o f proteinases since Western blot analysis of LPS-stimulated monocytes revea led the degradation of a 38-kDa species corresponding to the CCR2B monomer. In RBL cells expressing the CCR2B-green fluorescent protein (GFP) fusion c hemokine receptor, LPS stimulated the internalization and degradation of CC R2. The serine proteinase inhibitor N-alpha-p-tosyl-L-lysine chloromethyl k etone blocked LPS-induced down-modulation of CCR2 in monocytes and CCR2B-GF P in RBL cells. This work describes a previously uncharacterized mechanism for CC chemokine receptor down-modulation that is dependent upon tyrosine k inase activation and serine proteinase-mediated receptor degradation and ma y provide further insight into the mechanisms of leukocyte regulation durin g immunological and inflammatory responses.