Effect of EGF on H₂O₂-induced inhibition of α-MG uptake in renal proximal tubule cells: Involvement of MAPK and AA release

Both oxidative stress and epidermal growth factor (EGF) contribute to the initiation and progression of renal proximal tubular dysfunction under pathophysiologic conditions. Thus, this study was performed (1) to examine both the individual, and the combined effects of H₂O₂ and EGF on α-methyl-D-glucopyranoside uptake (α-MG uptake) in the primary cultured renal proximal tubule cells (PTCs), and (2) to elucidate the involvement of p44/42 mitogen activated protein kinase (MAPK) and phospholipase A₂ in mediating these actions. Both H₂O₂ and EGF inhibited α-MG uptake individually, while the combination of H ₂O₂ and EGF further potentiated the inhibitory effect on α-MG uptake, which was elicited by each agent. H₂O₂ not only caused a rapid increase in the phosphorylation of p44/42 MAPK, but also promoted the translocation of cytosolic phospholipase A₂ (cPLA ₂) from the cytosolic to particulate fraction, and stimulated cellular [³H]-arachidonic acid (AA) release. EGF similarly activates phosphorylation of p44/42 MAPK and stimulates [³H]-AA release. When PTCs were exposed to 100 μM H₂O₂ and 50 ng/ml EGF simultaneously, a further increase in the phosphorylation of p44/42 MAPK, of [³H]-AA release, and of prostaglandin E₂ (PGE₂) production was elicited as compared with the effects of each individual agonist alone. Moreover, the additive phosphorylation of p44/42 MAPK, [ ³H]-AA release, and PGE₂ production by H₂O ₂ and EGF was almost completely inhibited by the p44/42 MAPK inhibitor, PD 98059. In conclusion, these results are consistent with the hypothesis that under conditions of oxidative stress, the H₂O ₂-induced inhibition of α-MG uptake in the renal proximal tubule is mediated through a modulation of the EGF signaling pathway, promoting further phosphorylation of p44/42 MAPK, activation of PLA₂.