Wnt-1-Inducing Factor-1: a Novel G/C Box-Binding Transcription Factor Regulating the Expression of Wnt-1 during Neuroectodermal Differentiation

Abstract

The Wnt-1 proto-oncogene is essential for proper development of the midbrain and is expressed in a spatially and temporally restricted manner during central nervous system development in mice. In vitro, the gene is specifically transcribed during the retinoic acid (RA)-induced neuroectodermal differentiation of the P19 line of embryonal carcinoma cells. The P19 cells differentiate into neurons, astrocytes, and fibroblast-like cells when treated with RA. Treatment of the cells with dimethyl sulfoxide leads to differentiation along mesodermal lineages, including skeletal and cardiac muscle. We have used the P19 cell line to study the Wnt-1 promoter and identify and characterize the transcription factor(s) that regulates the differentiation-specific transcription of Wnt-1 in RA-treated P19 cultures. Transient-transfection assays have revealed that a 230-bp region comprising positions -278 to -47 of the 5' upstream Wnt-1 sequence was sufficient to direct RA-specific transcription. This promoter fragment was shown to contain a binding site for a nuclear factor that was not detected in undifferentiated P19 stem cells or their dimethyl sulfoxide-treated derivatives but was induced in differentiating RA-treated cells. This factor was termed Wnt-1-inducing factor-1 (WiF-1). DNase I footprinting analysis has identified the G/C-rich WiF-1 binding site, and UV cross-linking studies have shown that WiF-1 is a protein with an M_r of 65,000. WiF-1 binding activity was also detected in postpubertal mouse testis, the only tissue that expresses Wnt-1 in adults. Site-directed mutations that inhibited WiF-1 binding to the Wnt-1 promoter concomitantly abolished the activity of the promoter in RA-treated P19 cells. The active WiF-1 protein was purified by DNA affinity chromatography. Our data suggest that WiF-1 is a novel G/C box-binding transcription factor and support a physiological role for WiF-1 in the developmentally regulated expression of Wnt-1.