Mitochondrial H2O2 limits U937 cell survival to peroxynitrite by promoting ERK1/2 dephosphorylation

Abstract

Sequential activation of cytosolic phospholipase A2 (cPLA2) and 5-lipoxygenase (5-LO), critically regulated by extracellular signal-regulated kinase 1 and 2 (ERK1/2)-dependent phosphorylation, mediates U937 cell survival to peroxynitrite. In contrast, a limiting factor is represented by the parallel mitochondrial formation of H2O2 leading to suppression of the survival signaling. We now report that the inhibitory effects of H2O2 are at the level of ERK1/2 phosphorylation and involve activation of orthovanadate-sensitive phosphotyrosine protein phosphatase(s). Under these conditions, the otherwise stimulatory effects of peroxynitrite on ERK1/2 phosphorylation are concealed by phosphatase-dependent dephosphorylation and the activities of cPLA2 and 5-LO are significantly reduced or suppressed, respectively. The ensuing inhibition of downstream events preventing mitochondrial permeability transition rapidly leads these cells to death. Thus, endogenous H2O2 limits U937 cell survival to peroxynitrite via activation of phosphotyrosine protein phosphatase(s) promoting upstream inhibition of the survival signaling critically regulated by the extent of ERK1/2 phosphorylation.

Mitochondrial ascorbic acid is responsible for enhanced susceptibility of U937 cells to the toxic effects of peroxynitrite

“…A second important consideration supporting the above hypothesis is that mitochondria are central for the effects of peroxynitrite, a notion once again established in our laboratory in the same cell line herein used, as well as in other cell types belonging to the monocyte‐macrophage lineage. Our previous work showed that the cytogenotoxic properties of peroxynitrite are strictly dependent on early events associated with the mitochondrial formation of reactive oxygen species . In particular, peroxynitrite triggered O 2 − formation in the mitochondrial respiratory chain, an event followed by dismutation of the latter to diffusible H 2 O 2 , in turn responsible for a variety of effects, such as DNA single‐strand breakage and upstream inhibition of the survival signaling, a determinant of the ensuing lethal response .…”

Peroxynitrite damages U937 cell DNA via the intermediate formation of mitochondrial oxidants

“…macrophages), surviving to peroxynitrite regardless from the damage accumulated (20,26). DNA damage mediated by peroxynitrite in these cells under inflammatory conditions is indeed of importance in carcinogenesis (27).…”

“…In this direction, it is important to point out that these cells are resistant to peroxynitrite as they are able to respond with a signaling pathway, sequentially involving cytosolic phospholipase A 2 , 5-lipoxygenase and PKCa, promoting Bad phosphorylation (11,(20)(21)(22)(23). In other words, these cells, as well as cells belonging to the monocyte/macrophage lineage, including human monocytes and macrophages (23)(24)(25)(26), while committed to mitochondrial permeability transition by peroxynitrite, nevertheless survive by promoting the cytosolic localization of Bad, an event creating optimal conditions for the cytoprotective effects of Bcl-2/BclX L .…”