Butyrate suppression of colonocyte NF-κB activation and cellular proteasome activity

Butyrate is derived from the microbial metabolism of dietary fiber in the colon where it plays an important role in linking colonocyte turnover and differentiation to luminal content. In addition, butyrate appears to have both anti-inflammatory and cancer chemopreventive activities. Using confocal microscopy and cell fractionation studies, butyrate pretreatment of a human colon cell line (HT-29 cells) inhibited the tumor necrosis factor-alpha (TNF-alpha)-induced nuclear translocation of the proinflammatory transcription factor NF-kappaB. Butyrate inhibited NF-kappaB DNA binding within 30 min of TNF-alpha stimulation, consistent with an inhibition of nuclear translocation. I kappaB.NF-kappaB complexes extracted from butyrate-treated cells were relatively resistant to in vitro dissociation by deoxycholate, suggesting a change in cellular I kappaB composition. Butyrate treatment increased p100 expression, an I kappaB that was not degraded upon TNF-alpha treatment. Butyrate also reduced the extent of TNF-alpha -induced I kappaB-alpha degradation and enhanced the presence of ubiquitin-conjugated I kappaB-alpha. The suppression of I kappaB-alpha degradation corresponded with a reduction in cellular proteasome activity as determined by in vitro proteasome assays and the increased presence of ubiquitin-conjugated proteins. The butyrate suppression of I kappaB-alpha degradation and proteasome activity may derive from its ability to inhibit histone deacetylases since the specific deacetylase inhibitor trichostatin A had similar effects. These results suggest a potential mechanism for the anti-inflammatory activity of butyrate and demonstrate the interplay between short chain fatty acids and cellular proteasome activity.