Time‐dependent tumor necrosis factor (TNF)‐α signaling to P‐glycoprotein (P‐gp) in rat brain capillaries

Abstract

The ATP-driven drug efflux pump, P-gp, is a critical, selective element of the blood-brain barrier and a primary impediment to CNS pharmacotherapy. We recently demonstrated rapid and reversible inactivation of P-gp in rat brain capillaries signaled through the brain’s innate immune response (LPS and TNF-α) and the endothelin (ET) B-type receptor (Hartz et al, Mol Pharm 66:387, 2004, and in press). Neither tight junctional permeability nor p-gp expression were altered. Here we show here that P-gp activity and expression changed in a complex manner in capillaries exposed continuously to low levels of TNF-α or ET-1. The initial reduction in transport activity with unaltered expression was followed by a 2-3 h plateau and then by increased activity and expression; after 6 h, both had increased to twice control levels. Signaling involved the following sequence:1) TNF-α binding to TNF-R1, 2) big-ET release and cleavage, 3) ET binding to ETA and ETB receptors, 4) NOS activation, 5) PKC activation, 6) NF-κB activation and translocation from cytoplasm to nucleus, and 7) increased P-gp expression and function. These results imply a tightening of the selective blood-brain barrier with chronic, low level inflammation. Moreover, by implicating NF-κB, they raise the possibility that other effectors acting through this transcription factor may also similarly induce the selective component of the barrier.