Angiotensin II (Ang II) exerts powerful proinflammatory and growth effects on the development of Ang II-induced hypertensive glomerulosclerosis and tubulo-interstitial fibrosis. The proinflammatory and growth actions of Ang II are primarily mediated by activation of cell surface type 1 receptors (AT(1)) and the transcription factor nuclear factor-kappaB (NF-kappaB). However, binding of cell surface receptors by extracellular Ang II also induces receptor-mediated endocytosis of the agonist-receptor complex in renal cells. The purpose of the present study was to determine whether AT(1) receptor-mediated endocytosis of extracellular Ang II is required for Ang II-induced NF-kappaB activation and subsequent proliferation of rabbit renal proximal tubule cells. Expression of AT(1) (primarily AT(1a) or human AT(1)) receptors in these cells was confirmed by Western blot, showing that transfection of a human AT(1) receptor-specific 20-25 nucleotide siRNA knocked down more than 70% of AT(1) receptor protein (P < 0.01). Stimulation of proximal tubule cells by Ang II (1 nM) induced fourfold increases in NF-kappaB activity (P < 0.01). The Ang II-increased NF-kappaB activity was significantly attenuated by coadministration of losartan (10 microM), an AT(1) receptor-selective blocker, or colchicine (1 microM), a selective cytoskeleton microtubule inhibitor known to block receptor-mediated endocytosis (P < 0.01). Furthermore, Ang II significantly increased (3)H-thymidine incorporation (>55%, P < 0.01), an index of cell proliferation and DNA synthesis, and the effect was also attenuated by coadministration of losartan and colchicine (P < 0.01). Our results therefore suggest that AT(1) receptor-mediated endocytosis of extracellular Ang II may be required for Ang II-induced NF-kappaB activation and subsequent cell proliferation in renal proximal tubule cells.