Signal transducer and activator of transcription 3 activates CCAAT enhancer-binding protein δ gene transcription in G(o) growth-arrested mouse mammary epithelial cells and in involuting mouse mammary gland

Abstract

The CCAAT enhancer-binding protein (C/EBP) family of transcription factors is implicated in the regulation of cell proliferation and differentiation in a variety of tissues. C/EBPδ is involved in regulating G(o) growth arrest and apoptosis of mouse mammary epithelial cells. This study shows that activation of signal transducer and activator of transcription 3 (Stat3), but not activation of Stat1 or Stat5, occurs concurrently with G(o) growth arrest of HC11 mouse mammary epithelial cells, but not NIH 3T3 fibroblasts. Promoter analysis demonstrates that the C/EBPδ promoter fragment involved in transcriptional activation during G(o) growth arrest contains a Stat3 binding site and that mutation of this site eliminates the G(o) growth arrest inducibility of the C/EBPδ promoter. Overexpression of Stat3 increases C/EBPδ promoter activity during G(o) growth arrest of HC11 cells, whereas dominant negative Stat3 decreases C/EBPδ promoter activity under the same conditions. Neither Stat3 overexpression nor dominant negative Stat3 expression influences C/EBPδ promoter activity in growing HC11 cells or G(o) growth-arrested NIH3T3 cells, demonstrating that the effect is specific to G(o) growth arrest of mammary epithelial cells. Band shift assays and anti-body interference assays demonstrate specific binding of Stat3 to the acute phase response element in the C/EBPδ promoter in G(o) growth-arrested HC11 cell extracts and 24 h involuting mouse mammary gland extracts. These data indicate that Stat3 activates C/EBPδ transcription in G(o) growth-arrested mouse mammary epithelial cells and binds to the C/EBPδ promoter during involution. An autocrine mechanism of Stat3 activation is proposed.