Effect of nitric oxide on endoplasmic reticulum calcium homeostasis, protein synthesis and energy metabolism

It has been suggested that nitric oxide (NO) may contribute to ischemia-induced cell injury. However, the mechanisms underlying NO toxicity have not yet been fully elucidated. In the present study, we investigated the effect of NO on the level of endoplasmic reticulum (ER) calcium stores, on ER Ca2+ pump activity, on protein synthesis, on concentrations of high-energy phosphates, and on gadd153 mRNA levels. Primary neuronal cells were exposed to the NO-donor (+/-)-S-Nitroso-N-acetylpenicillamine (SNAP) for 1 h, 2 h, 6 h or 24 h. The level of ER calcium stores was evaluated by measuring the increase in cytoplasmic calcium activity induced by exposing cells to thapsigargin, an irreversible inhibitor of ER Ca(2+)-ATPase; the activity of ER Ca(2+)-ATPase was determined by measuring a phosphorylated intermediate; SNAP-induced changes in gadd153 expression were evaluated by quantitative PCR; SNAP-induced changes in protein synthesis were investigated by measuring the incorporation of L-[4,5-3H]leucine into proteins, and changes in the levels of ATP, ADP, AMP were measured by HPLC. Exposing cells to SNAP for 1 h to 2 h induced a marked depletion of ER calcium stores through an inhibition of ER Ca(2+)-ATPase (to 58% of control), and a concentration-dependent suppression of protein synthesis which was reversed in the presence of hemoglobin, suggesting NO-related effects. ATP levels and adenylate energy charge were significantly decreased only when cells were exposed to the highest SNAP concentration for 6 h or 24 h, excluding significant effects of NO on the energy state of cells in the acute state, i.e. when ER calcium stores were already completely depleted and protein synthesis severely suppressed. In light of the regulatory role of ER calcium homeostasis in the control of protein synthesis, the results imply that the suppression of protein synthesis resulted from NO-induced inhibition of ER Ca(2+)-ATPase and depletion of ER calcium stores, and that NO-induced disturbances of energy metabolism are secondary to the effect of NO on ER calcium homeostasis. It is, therefore, concluded that ER calcium stores are a primary target of NO-toxicity.