RAPID PROTEOLYSIS OF I-KAPPA-B-ALPHA IS NECESSARY FOR ACTIVATION OF TRANSCRIPTION FACTOR NF-KAPPA-B

INDUCIBLE gene expression in eukaryotes is mainly controlled by the activity of transcriptional activator proteins, such as NF-kappaB (refs 1-3), a factor activated upon treatment of cells with phorbol esters, lipopolysaccharide4, interleukin-1 and tumour necrosis factor-alpha5. Activation of NF-kappaB involves release of the inhibitory subunit IkappaB from a cytoplasmic complex with the DNA-binding subunits Rel-A (formerly p65) and p50 (refs 6, 7). Cell-free experiments have suggested that protein kinase C and other kinases transfer phosphoryl groups onto IkappaB causing release of IkappaB and subsequent activation of NF-kappaB8-10. Here we report that IkappaB-alpha (formerly MAD-3)11 is degraded in cells after stimulation with phorbol ester, interleukin-1, lipopolysaccharide and tumour necrosis factor-alpha, an event coincident with the appearance of active NF-kappaB. Treatment of cells with various protease inhibitors or an antioxidant completely prevented the inducible decay of IkappaB-alpha as well as the activation of NF-kappaB. Our findings suggest that the activation of NF-kappaB relies on an inducible degradation of IkappaB-alpha through a cytoplasmic, chymotrypsin-like protease. In intact cells, phosphorylation of IkappaB-alpha is apparently not sufficient for activation of NF-kappaB.