Stat transcription factors in mammary gland development and tumorigenesis.

Two members of the Stat family of transcription factors play a vital role in mouse mammary gland development. Stat5a was originally described as a regulator of milk protein gene expression and was subsequently shown to be essential for mammary development and lactogenesis. In contrast, Stat3 is an essential mediator of apoptosis and post-lactational regression. Other members of the Stat family may have specific, but as yet undemonstrated, functions in mammary development. However, since Stat1 activity is regulated during mammary development in a pattern different from Stats 3 and 5, this factor too may have a functional role. Although both Stat4 and Stat6 are expressed in mammary tissue, it seems unlikely that they will have a significant function as each of these Stats is activated in response to a limited number of cytokines. Given the essential regulatory roles of Stat signaling molecules in mammary development, it was not surprising to discover that constitutively activated Stat factors are a feature of human breast cancers. Sustained Stat activity has also been described in a variety of tumors including leukemias. The cause of this sustained activation is not clear but probably involves mutation of one of the many Stat regulatory proteins or dysregulation of other signaling pathways which modulate Stat activity. It is now important to understand the mechanism of constitutive Stat activity and to develop strategies which will abrogate aberrant Stat signaling in tumors in vivo.