Prolactin Activates Stati But Does Not Antagonize Stati Activation and Growth Inhibition by Type I Interferons in Human Breast Cancer Cells1

Type I interferons (IFNa and IFN/3) are presently used in the adjuvant treatment of several human cancers. However, these cytokines have dem onstrated only modest success in breast cancer therapy, and research efforts have focused on improving their efficacy. Recent progress in understanding the molecular mechanisms underlying the antiproliferative effects of IFNs has identified the cytoplasmic transcription factor Stati as a critical mediator. It is, therefore, possible that IFN-induced growth inhibition of mammary epithelial cells is counteracted by other cytokines that also use Stati. One such candidate IFN-antagonist with particular relevance to breast cancer is the mammotropic hormone prolactin (PRL). The main goal of this study was to examine whether PRL would interfere with type I IFN (IFNo/ß) signal transduction by competing for limited cytoplasmic Stai factors. A second aim was to test whether pretreatment of mammary tumor cell lines with IFNy could enhance the effect of IFNo/ß. By analyzing the effect of PRL on IFNo/ß-induced tyrosine phosphorylation of Stat proteins and their binding to IFN-regulated genes, we now report that costimulation of PRL receptors did not interfere with IFNo/ß signals in several human breast cancer cell lines, including T47D, MCF-7, and BT-20. Specifically, PRL did not affect IFNo/ß-induced tyrosine phosphorylation or heterodimerization of Stati and Stat2 in any cell line. Instead, IFNa/ßand PRL-induced tyrosine phosphorylation of Stati was additive and occurred without evidence of competition for limited concentrations of cytoplasmic Stati. A similar additive relationship was observed on IFNo/ßand PRL-induced Stat3 tyrosine phosphorylation. Furthermore, electrophoretic mobility shift as says showed that type I IFNs induced predominantly Stati-Stati or Slat 1-StaO heteromeric complexes with various IFN-response elements of IFN-stimulâted genes, whereas PRL induced Stati homodimers. Despite significant mutual use of Stats by IFNs and PRL, these results indicated a high degree of signaling specificity in the two receptor systems, and that cytoplasmic levels of Stat proteins were not limiting. Similarly, PRL did not interfere with the growth-inhibitory effect of IFNß.On the other hand, the study indicated that pretreatment of human breast cancer cell lines with IFNy enhanced the growth-inhibitory action of type I IFNs, suggesting a possible avenue for improving the effect of type I IFNs in the treatment of breast cancer patients.