The IκB proteins are important in the regulation of the NF-κB/Rel group of transcription factors which are pivotal in the inflammatory response. IκB-α is itself upregulated by activation of NF-κB and is postulated to be part of a negative feedback loop. This role of IκB-α has been challenged, however, by recent evidence that demonstrates (1) continued activation of NF-κB in mesangial and endothelial cells despite the resynthesis of IκB-α protein and (2) that inhibition of the transactivating activity of NF-κB by corticosteroids can be dissociated from a rise in IκB-α protein. We investigated the role of IκB-α in mesangial cells using a phosphorothioate antisense oligonucleotide directed against the translational start point of IκB-α. If IκB-α does function as a negative feedback inhibitor in these cells, then reducing IκB-α levels should lead to an increase in NF-κB activity. We first demonstrated that IκB-α protein resynthesis following stimulation could be specifically reduced. We then showed that NF-κB DNA binding was not increased with antisense treatment following stimulation. Finally, NF- κB-dependent gene signalling after stimulation (determined through an NF-κB luciferase reporter and upregulation of the mRNA of known NF-κB-responsive genes MCP-1 and IκB-α) was reduced rather than increased. These data suggest that IκB-α does not form a negative autoregulatory loop for NF-κB in mesangial cells and may actually reduce NF-κB activity. This may have relevance to therapies directed at inhibition of NF-κB activity in mesangial cell diseases.