V-erbA and c-erbA proteins enhance transcriptional activation by c-jun

The ability of different transcription factors to interact with one another is an important means by which the eucaryotic cell can integrate separate growth and differentiation signals into a coherent response. We report here an analysis of the interactions of transcription factors that are also the products of oncogenes: the erbA, jun and fos proteins. We demonstrate that the c-jun polypeptide can functionally interact with the c-erbA protein (thyroid hormone receptor) to yield an enhanced activity greater than that of either factor individually. Although the avian erythroblastosis v-erbA allele is generally thought to act as a transcriptional repressor in vertebrate cells, we also report the existence of promoter contexts where v-erbA, as well as c-erbA, can serve as 'co-activators' of c-jun function. V-erbA appears to augment retinoic acid receptor function in the same context. Our results suggest that v-erbA may have unanticipated positive effect on transcription in the neoplastic cell in addition to the repressor functions previously characterized.