Expression and function of megalin, an endocytic receptor in proximal tubule cells (PTCs), are reduced in diabetic nephropathy, involved in the development of proteinuria/albuminuria. Lipopolysaccharide (LPS) is chronically increased in diabetic sera, by the mechanism called metabolic endotoxemia. We investigated low-level LPS-mediated signaling that regulates megalin expression in immortalized rat PTCs (IRPTCs). Incubation of the cells with LPS (10 ng/ml) for 48 h suppressed megalin protein expression and its endocytic function. TNF-alpha mRNA expression was increased by LPS treatment, and knockdown of the mRNA with siRNA inhibited LPS-mediated downregulation of megalin mRNA expression at the 24-h time point. Incubation of IRPTCs with exogenous TNF-alpha also suppressed megalin mRNA and protein expression at the 24- and 48-h time points, respectively. MEK1 inhibitor PD98059 competed partially but significantly TNF-alpha-mediated downregulation of megalin mRNA expression. Collectively, low-level LPS-mediated TNF-alpha-ERK1/2 signaling pathway is involved in downregulation of megalin expression in IRPTCs. (C) 2011 Elsevier Inc. All rights reserved.