TNFα is a central mediator of inflammation and is critical for host response to infection and injury. TNFα biosynthesis is controlled by transcriptional and post-transcriptional mechanisms allowing for rapid, transient production. Tristetraprolin (TTP) is an AU-Rich Element binding protein that regulates the stability of TNFα mRNA. Previous work has demonstrated that TTP function is regulated by activation of both the p38 and ERK kinase pathways and that TTP protein is rapidly degraded by a proteasomal mechanism. Using a novel screen for TTP interacting proteins, we identified Cullin 4B (CUL4B), a scaffolding component of the CRL family of ubiquitin E3 ligases. shRNA knockdown of Cul4B causes a significant reduction in TNFα protein and mRNA in resting and LPS stimulated (100 ng/ml 90 min) mouse macrophage RAW264.7 cells. Cul4B knockdown also reduces TTP protein levels. Moreover, the TNFα half life was reduced from t1/2=68.7 min to t1/2=38.9 min. TNF-3’UTR luciferase assays utilizing wild-type and mutant TTP, which cannot be phosphorylated by p38, indicate that Cul4B operates independent of p38 activity. Based on the data to date, we conclude that Cul4B acts to inhibit TTP protein turnover and in turn limit TTP mediated decay of the TNFα mRNA. Thus, the loss of Cul4B enhanced both TTP protein and TNFα mRNA turnover. Further studies are ongoing to establish the mechanism through which Cul4B operates.
- Copyright © 2011 by The American Association of Immunologists, Inc.