LAUSR

A20, also referred to as tumor necrosis factor alpha (TNFα)‐induced protein 3 (TNFAIP3), is an ubiquitin‐editing enzyme whose expression is enhanced by NF‐κB activation, and plays an important role in silencing NF‐κB activity. Another well‐known role for A20 is to protect cells from TNFα‐induced apoptosis. Depletion of NF‐κB in differentiating U937 monocytic leukemia cells is known to cause apoptotic cell death; however, much remains to be explored about the molecules that are expressed in an NF‐κB‐dependent manner and which support monocyte‐macrophage differentiation. Using the monocytic cell line THP‐1, and peripheral blood monocytes, we show here a sustained increase in A20 expression during monocyte‐macrophage differentiation, which coincided with high NF‐κB‐dependent transcriptional activity. Depletion of NF‐κB by stable expression of a super‐repressor form of IκBα in THP‐1 cells caused remarkable cell death during phorbol 12‐myristate 13‐acetate (PMA)‐induced differentiation. A20 expression in these cells did not alter this NF‐κB suppression, but was sufficient to protect the cells and restore the cell surface expression of a differentiation marker (CD11b) and phagocytic activity. Mutational analyses revealed that this A20 activity requires the carboxy‐terminal zinc‐finger domain, but not its deubiquitinase activity. Based on these findings, we conclude that A20, when ectopically expressed, can support both survival and differentiation of THP‐1 cells in the absence of sustained NF‐κB activity.