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-alpha (TNF-alpha), in part by repressing the nuclear factor-kappaB (NF-kappaB)-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-alpha are unclear. E1A expression did not block upstream events in TNF-alpha-incluced activation of NF-kappaB in NIH 3T3 cells, including degradation of IkappaB-alpha, nuclear translocation of NF-kappaB subunits, and their dimeric binding to kappaB sequences in the nucleus. However, E1A markedly repressed NF-kappaB-dependent transcription and sensitized cells to TNF-alpha-induced apoptosis. These E1A effects were selective for kappaB-dependent transcription and for the function of the NF-kappaB p65/RelA subunit. A four amino acid E1A deletion that eliminates binding to Rb-family proteins blocked both repression of TNF-alpha-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-kappaB-dependent activation response to TNF-alpha 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-kappaB-dependent transcription.