BIPHASIC TRANSCRIPTIONAL REGULATION OF THE INTERFERON REGULATORY FACTOR-I GENE BY PROLACTIN - INVOLVEMENT OF GAMMA-INTERFERON-ACTIVATED SEQUENCE AND STAT-RELATED PROTEINS

Stimulation of quiescent Nb2 T cells by PRL leads to the rapid transcriptional activation of a T cell activation gene, interferon regulatory factor-1 (IRF-1). IRF-1 is induced twice by PRL in a single cell cycle, first during G1 at 30-60 min and again over early S phase at 10-12 h. By nuclear run-on transcription analysis of IRF-1 promoter-chloramphenicolacetyl transferase (CAT) constructs, the -1.7 kilobase (kb) 5'-flanking IRF-1 DNA was shown to contain elements that mediate both G1 and S phase expression. The -200 bp IRF-1 promoter DHA contains elements that respond to G1 PRL stimulation in a protein synthesis independent manner, suggesting the involvement of pre-existing factors. Further promoter deletion analysis delineated a minimal PRL responsive region between -112 and -205 bp. Within this region is a Gamma Interferon Activated Sequence or GAS, consisting of two inverted GAAA motifs (-123/-113), which confers PRL-inducible expression to a reporter gene, suggesting that GAS can function as a PRL responsive element. Further, GAS exhibits binding with nuclear proteins in a PRL-inducible, cell cycle-dependent manner. One of these proteins appears to be related to the emerging family of Signal Transducer and Activator of Transcription or Stat factors. These studies suggest that the GAS site and Stat-like proteins participate in PRL receptor signal transduction to regulate the biphasic expression of the IRF-1 gene in PRL-stimulated T cells.