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Prolactin Activates Stat1 But Does Not Antagonize Stat1 Activation and Growth Inhibition by Type I Interferons in Human Breast Cancer Cells¹

Department of Pathology, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814

Type I interferons (IFN and IFNß) are presently used in the adjuvant treatment of several human cancers. However, these cytokines have demonstrated 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 Stat1 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 Stat1. 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 (IFN/ß) signal transduction by competing for limited cytoplasmic Stat factors. A second aim was to test whether pretreatment of mammary tumor cell lines with IFN could enhance the effect of IFN/ß. By analyzing the effect of PRL on IFN/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 IFN/ß signals in several human breast cancer cell lines, including T47D, MCF-7, and BT-20. Specifically, PRL did not affect IFN/ß-induced tyrosine phosphorylation or heterodimerization of Stat1 and Stat2 in any cell line. Instead, IFN/ß- and PRL-induced tyrosine phosphorylation of Stat1 was additive and occurred without evidence of competition for limited concentrations of cytoplasmic Stat1. A similar additive relationship was observed on IFN/ß- and PRL-induced Stat3 tyrosine phosphorylation. Furthermore, electrophoretic mobility shift assays showed that type I IFNs induced predominantly Stat1-Stat2 or Stat1-Stat3 heteromeric complexes with various IFN-response elements of IFN-stimulated genes, whereas PRL induced Stat1 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 IFN 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.

¹ Supported by United States Department of Defense Grant DAMD17-94-J-4349, NIH Grant RO1 DK52013-01A1, and Uniformed Services University of the Health Sciences Grant RO74HF.

² To whom requests for reprints should be addressed, at Department of Pathology, Uniformed Services University of the Health Sciences School of Medicine, Bethesda, MD 20814. Phone: (301) 295-3801; Fax: (301) 295-1640.

Received 11/ 7/97. Accepted 3/ 4/98.

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