Hydrogen Peroxide Stimulates Phospholipase A₂-Mediated Arachidonic Acid Release in Cultured Intestinal Epithelial Cells (INT 407)

Abstract

The mechanisms by which hydrogen peroxide and, for comparison, 4-beta-phorbol-12-myristate-13-acetate (PMA) stimulate release of radiolabeled arachidonic acid (¹⁴C-AA) in cultured intestinal epithelial cells (INT 407) were investigated. Both hydrogen peroxide and PMA caused a rapid (3 min) and dose-related intracellular release of free ¹⁴C-AA, followed by a dose- and time-dependent release of ¹⁴C-AA into the extracellular medium, but hydrogen peroxide was about 50,000 times less effective than PMA in releasing ¹⁴C-AA. No ¹⁴C-AA was released on stimulation with 4-alpha-phorbol-12,13-di-decanoate (PDD), a phorbol ester that does not activate protein kinase C. The ¹⁴C-AA release was reduced by the phospholipase A₂ inhibitors nordihydroguaiaretic acid and 4-bromophenacyl bromide and by the calmodulin/protein kinase C inhibitor trifluoperazine and the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). However, H-7 was less effective than the other inhibitors in reducing the hydrogen peroxide-stimulated ¹⁴C-AA release. The hydrogen peroxide-stimulated, but not the PMA-stimulated, rapid (3 min) ¹⁴C-AA release was associated with an increased influx of extracellular calcium. Stimulation of the cells with PMA resulted in phosphorylation of a cellular protein of about 32 kDa, whereas no phosphorylation of this protein was detected after stimulation with hydrogen peroxide. Taken together, these findings indicate that (i) both PMA and hydrogen peroxide may stimulate phospholipase A₂-mediated AA release from human intestinal epithelial cells; (ii) this stimulation is brought about via protein kinase C and calmodulin-mediated events; (iii) PMA-stimulated ¹⁴C-AA release is associated with phosphorylation of a 32-kDa protein, possibly lipocortin, whereas the hydrogen peroxide-stimulated release is not; and (iv) calmodulin is more important for the hydrogen peroxide-stimulated ¹⁴C-AA release than is protein kinase C. The possibility that hydrogen peroxide-evoked AA release may contribute to the mucosal abnormality in Crohn's disease is discussed.

Key Words:

• Arachidonic acid
• hydrogen peroxide
• intestinal epithelial cells
• phospholipase A₂ activation
• 4-beta-phorbol-12 myristate-13-acetate