Translational repression of inducible NO synthase in macrophages by l-arginine depletion is not associated with an increased phosphorylation of eIF2α

In mouse inflammatory macrophages the cytokine-mediated expression of inducible nitric oxide synthase (iNOS) is regulated by the availability of the substrate l-arginine. Following arginine starvation the levels of iNOS mRNA remain unimpaired, whereas the translation of iNOS protein is strikingly downregulated. In the present study we addressed the question, whether arginine-deficient macrophages follow the canonical integrated stress response (ISR) that in other cell types depleted of amino acids was characterized by the accumulation of phosphorylated (i.e. inactive) eukaryotic translation initiation factor-2α (eIF2α), the attenuation of global protein synthesis and the induction of certain stress response target genes. Unexpectedly, resting as well as stimulated inflammatory macrophages constitutively exhibited high levels of phosphorylated eIF2α, which was not further increased upon l-arginine starvation. At the same time, macrophages deprived of l-arginine showed a significant upregulation of the mRNA levels of ISR genes. From these data we conclude that l-arginine deficiency blocks the translation of iNOS and elicits a stress response in macrophages, both of which, however, do not result from an enhanced phosphorylation of eIF2α. Alternative modes of translational repression of iNOS need to be considered.