INTERFERONS AND INTERLEUKIN-6 SUPPRESS THE DNA-BINDING ACTIVITY OF E2F IN GROWTH-SENSITIVE HEMATOPOIETIC-CELLS

Transcription factor E2F binds to cellular promoters of certain growth - and cell cycle-controlling genes and forms distinct heteromeric comp lexes with other nuclear proteins. We show here that alpha and beta in terferons (alpha, beta) and interleukin-6 abolished the E2F-containing DNA-binding complexes in Daudi Burkitt lymphoma cells and in M1 myelo blastic cells, which responded to the cytokines by suppression of c-my c transcription. Time kinetics studies showed that the abolishment of E2F complexes coincided with reduction of c-myc expression and that bo th molecular events preceded the cell cycle block in G0/G1 phase. In c ontrast, the pattern of E2F complexes remained unchanged in an interfe ron-treated growth-resistant Daudi cell mutant that displayed relaxed regulation of c-myc. All of the DNA-binding E2F complexes, including t hose containing the retinoblastoma protein (pRB), cyclin A-p33cdk2, an d the free forms of E2F, were reduced by interferons or interleukin-6. Their abolishment was unperturbed by pharmacological treatments that alleviated the cyclin A and pRB responses to interferon. Thus, changes in cyclin A expression and pRB phosphorylation are not primary events that influence the pattern of E2F responses to cytokines. Addition of EDTA to cell extracts of interferon-treated Daudi cells restored the DNA-binding activity of E2F, resulting in the appearance of a single E 2F complex that exclusively contained pRB. It is suggested that the re gulation of E2F by growth-inhibitory cytokines that induce cell cycle exit takes place at the level of the DNA-binding activity, and by that mean it differs basically from the phase-specific regulation of E2F i n cycling cells.