Role of the E1A Rb-binding domain in repression of the NF-κB-dependent defense against tumor necrosis factor-α

The adenoviral E1A oncogene sensitizes mammalian cells to tumor necrosis factor-α (TNF-α), in part by repressing the nuclear factor-κ B (NF-κB)-dependent defense against this cytokine. Other E1A activities involve binding to either p300/cyclic AMP response element-binding protein (CBP) or retinoblastoma (Rb)-family proteins, but the roles of E1A interactions with these transcriptional regulators in sensitizing cells to TNF-α are unclear. E1A expression did not block upstream events in TNF-α-induced activation of NF-κB in NIH 3T3 cells, including degradation of IκB-α, nuclear translocation of NF-κB subunits, and their dimeric binding to κB sequences in the nucleus. However, E1A markedly repressed NF-κB-dependent transcription and sensitized cells to TNF-αinduced apoptosis. These E1A effects were selective for κB-dependent transcription and for the function of the NF-κB p65/RelA subunit. A four amino acid E1A deletion that eliminates binding to Rb-family proteins blocked both repression of TNF-α-induced transcription and sensitization to apoptosis. In contrast, mutations that eliminate E1A binding to p300/CBP (coactivators of p65/RelA) did not affect either E1A activity. These data suggest that E1A-Rb-binding blocks the NF-κB-dependent activation response to TNF-α by altering the function of p65/RelA at a stage after formation of the transcription factor-enhancer complex. These observations also open questions about the general role of Rb-family proteins in modulation of NF-κB-dependent transcription.