Reactive Oxygen Species Stimulate p44/42 Mitogen-Activated Protein Kinase and Induce p27Kip1

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Abstract
. Angiotensin II (AngII) induces G1 phase arrest and hypertrophy of cultured renal proximal tubular cells. In previous studies, it was shown that these effects depend on oxygen radical-mediated induction of p27Kip1, an inhibitor of cyclin-dependent kinases. The present study was undertaken to investigate whether mitogen-activated protein (MAP) kinases serve as signaling intermediates between AngII-induced oxidative stress and induction of p27Kip1. AngII (10-7 M) induces a biphasic phosphorylation pattern of p44/42 MAP kinase with an early phosphorylation after 2 min and a later, second phosphorylation peak after prolong incubation (12 h) in cultured proximal tubular cells from two different species (MCT and LLC-PK1 cells). Total protein expression of MAP kinase was not changed by AngII. These phosphorylation patterns of p44/42 MAP kinase caused activation of the enzyme, as detected by phosphorylated MAP substrate Elk-1 after immuno-precipitation of MAP kinase. Exogenous H2O2 also stimulates a biphasic phosphorylation of p44/42 MAP kinase. The flavoprotein inhibitor diphenylene iodinium, as well as the antioxidant N-acetylcysteine, prevented AngII-induced p44/42 MAP kinase phosphorylation, indicating involvement of reactive oxygen species generated by membrane-bound NAD(P)H oxidase. The MAP kinase kinase inhibitor PD98059 completely inhibits AngII-induced p27Kip1 expression and 3[H]leucine incorporation into proteins as a previously established marker of cell hypertrophy. PD98059 did not attenuate AngII-stimulated intracellular synthesis of oxygen radicals. Transient transfection with p44/42 MAP kinase antisense, but not sense, phosphorothioate-modified oligonucleotides also prevented AngII-induced MAP kinase phosphorylation, p27Kip1 expression, and cell hypertrophy. Furthermore, induction of p27Kip1 by H2O2 was also abolished in the presence of PD98059. Although AngII induces phosphorylation of the stress-activated p38 MAP kinase, inhibition of this enzyme with SB203580 failed to attenuate induced p27Kip1 expression and hypertrophy. These data provide evidence that AngII- mediated oxygen stress leads to the phosphorylation of p44/42 MAP kinase in proximal tubular cells. Activation of this enzyme is essential for p27Kip1 expression, G1 phase arrest, and hypertrophy of proximal tubular cells. These findings may lead to new concepts concerning interference of the development of proximal tubular hypertrophy, which may eventually turn into a maladaptive process in vivo leading ultimately to tubular atrophy and tubulointerstitial fibrosis.

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