Receptor to nucleus signaling by prolactin and interleukin 2 via activation of latent DNA-binding factors

The mechanism of action of prolactin (PRL), a lactogenic and immunoregulatory hormone, has remained undetermined despite its critical role in development. This study identifies a DNA-binding factor induced by PRL that appears to mediate a signal from the cell surface receptor to specific gene expression in the nucleus. PRL stimulates the proliferation of Nb2 T- lymphoma cells and activates transcription of the interferon-regulatory factor 1 (IRF-1) gene. Within minutes of PRL stimulation, a PRL-induced factor (PRLIF) is activated and binds to a target site in the promoter of the IRF-1 gene. The PRLIF-binding site contains an inverted GAAA repeat that is also functional in the hormone-responsive β-casein gene. The PRL-receptor complex signals tyrosine phosphorylation of JAK2, a nonreceptor tyrosine kinase, which may lead to activation of PRLIF. T-cell proliferation and transcriptional activation of the IRF-1 gene is also induced by the cytokine interleukin 2 (IL-2). This report demonstrates the rapid activation of an IL- 2 nuclear-activated factor that recognizes the same GAAA inverted repeat in the IRF-1 promoter. PRLIF and IL-2 nuclear-activated factor are newly identified factors that appear to serve fundamental roles in the signal transduction pathways of PRL and IL-2, respectively, leading to the transcriptional regulation of responsive genes.