T he stress response and stress proteins confer protection against diverse forms of cellular and tissue injury, including acute lung injury. The stress response can inhibit nonstress protein gene expression, therefore transcriptional inhibition of proinflammatory responses could be a mechanism of protection against acute lung injury. To explore this possibility, we determined the effects of the stress response on nuclear translocation of the transcription factor NF-kappaB, an important regulator of proinflammatory gene expression. In A549 cells induction of the stress response decreased tumor necrosis factor-alpha (TNF-alpha)-mediated NF-kappaB nuclear translocation. TNF-alpha initiates NF-kappaB nuclear translocation by causing dissociation of the inhibitory protein I-kappaBalpha from NF-kappaB and rapid degradation of I-kappaBalpha. Prior induction of the stress response inhibited TNF-alpha-mediated dissociation of I-kappaBalpha from NF-kappaB and subsequent degradation of I-kappaBalpha. Induction of the stress response also increased expression of I-kappaBalpha. We conclude that the stress response affects NFkappaB-mediated gene regulation by two independent mechanisms. The stress response stabilizes I-kappaBalpha and induces expression of I-kappaBalpha. The composite result of these two effects is to decrease NF-kappaB nuclear translocation. We speculate that the protective effect of the stress response against acute lung injury involves a similar effect on the I-kappaB/NF-kappaB pathway.