During endotoxemia, the ATP-dependent drug efflux pump P-glycoprotein (Abcb1/P-gp) is upregulated in kidney proximal tubule epithelial cells. The signaling pathway through which lipopolysaccharide (LPS) or tumor necrosis factor-alpha (TNF-alpha) regulates P-gp expression and activity was investigated further in the present study. Exposure of rat kidney proximal tubule cells to TNF-alpha alone or TNF-alpha and LPS increased P-gp gene and protein expression levels and efflux activity, suggesting de novo P-gp synthesis. Upon exposure to TNF-alpha in combination with LPS, P-gp activity in renal proximal tubule cells is increased under influence of nitric oxide (NO) produced by inducible NO synthase. Upon exposure to TNF-alpha alone, P-gp upregulation seems to involve TLR4 activation and nuclear factor kappaB (NF-kappaB) translocation, a pathway that is likely independent of NO. These findings indicate that at least two pathways regulate P-gp expression in the kidney during endotoxemia.