A DOMINANT-NEGATIVE MUTANT OF AN IFN REGULATORY FACTOR FAMILY PROTEININHIBITS BOTH TYPE-I AND TYPE-II IFN-STIMULATED GENE-EXPRESSION AND ANTIPROLIFERATIVE ACTIVITY OF IFNS

Type I (alpha, beta) and type II (gamma) IFNs elicit antiproliferative and antiviral activities through two distinct transcription pathways involving 1) IRF family proteins and ISGF3, and 2) STAT1. We have empl oyed a dominant negative strategy to study the role of IRF family prot eins in eliciting the biologic activities of IFN, A truncated IRT: pro tein retaining the DNA-binding domain (DBD) of ICSBP (a member of the IRF family) was stably transfected into U937 monocytic cells, Clones e xpressing DBD had markedly reduced ISRE-binding activity and were defe ctive in expressing several type I IFN-inducible genes, STAT1 was one such type I IFN-inducible gene whose expression was also inhibited in DBD clones, As a result, the expression of several IFN-gamma-inducible genes was also inhibited in these clones, indicating functional coupl ing of the type I and type II IFN transcription pathways. Furthermore, DBD clones grew more slowly than control clones and were refractory t o antiproliferative effects of both types of IFNs, We found that IFN t reatment of U937 cells leads to a G1 arrest and an increase in underph osphorylated retinoblastoma gene product, However, IFN treatment did n ot change the cell cycle profile, nor retinoblastoma gene product phos phorylation state in DBD clones, These data indicate that expression o f DBD disrupts cell cycle regulatory mechanisms, Combined with the pre viously noted failure of DBD clones to elicit antiviral activity, the present work shows that IRF family proteins play an integral part in g rowth control activities of IFNs.