Regulation of nuclear factor κB by corticosteroids in rat mesangial cells

Abstract

Nuclear factor κB (NF-KB) is one of the most important proinflammatory transcription factors. The anti-inflammatory activity of steroids in leukocytes is partly due to inhibition of signaling by NF-κB, but it is not known whether steroids inhibit NF-κB in kidney cells. Since the mesangial cell is important in several kidney diseases, especially mesangial proliferative glomerulonephritis, the aims of this study were: (1) to define the mechanism of NF-κB activation in rat glomerular mesangial cells; and (2) to determine whether steroids inhibit activation of NF-κB in these cells. Electrophoretic mobility shift assays (EMSA) showed that interleukin-1β and tumor necrosis factor-α activated NF-κB from 15 min to 48 h after stimulation. Supershift EMSA demonstrated that p65 and p50 were the predominant subunits involved. Degradation of the inhibitory subunit IκB-a was first observed 15 min after stimulation by Western blot, was maximal at 15 to 30 min (>90% by densitometry), and had returned to near normal levels at 90 min. In contrast, IκB-β was maximally degraded at 60 to 120 min and was still reduced at 48 h (<50% of the untreated level). Although treatment of mesangial cells with dexamethasone increased IκB-α mRNA by 1.92x and protein by 1.45x over controls, pretreatment did not inhibit degradation of IκB-α or -β in response to stimulation, or prevent the increase in NF-κB binding activity shown by EMSA. However, dexamethasone significantly inhibited the increase in monocyte chemoattractant protein-1 mRNA seen after stimulation with interleukin 1β, although this was not complete. It did not reduce transcription of an NF-KB reporter. In comparison, the pyrrolidine derivative of dithiocarnamate (PDTC), a known inhibitor of NF-KB, prevented the increase in NF-KB binding activity and significantly reduced transcription of the NF-KB reporter. These studies suggest that steroids can partially inhibit transcriptional activation by NF-KB in mesangial cells but not through an increase in IκB-a protein alone. Their effect must occur at the promoter and may be restricted to some NF-KB-responsive genes. Therapies that block NF-κB more effectively than steroids in mesangial cells, therefore, may be useful in the treatment of mesangial proliferative glomerulonephritis.