Stat5 transcription factors in human breast cancer: Prognosis, tamoxifen response prediction and gene regulation
Stat5 transcription factors are primary mediators of prolactin signaling in mammary epithelia with key roles in mammary gland development, as well as differentiation and milk production during pregnancy and lactation. However, the role of Stat5 in human breast cancer has been controversial. While animal models suggest a tumor promoting role of Stat5, evaluation of human clinical breast cancer specimens supports a model where Stat5 suppresses progression of breast cancer. The latter is further supported by in vitro mechanistic studies identifying a pro-differentiation and anti-invasion role of Stat5 in human breast cancer cell lines. Stat5a and Stat5b are two highly homologous transcription factors encoded by separate genes and their involvement in breast cancer has not been studied independently. We determined that levels of nuclear localized and tyrosine phosphorylated Stat5a/b, considered the transcriptionally active form, as well as nuclear Stat5a and Stat5b separately, were favorable prognostic markers in two independent clinical materials analyzed by two in situ detection methods including quantitative immunoprofiling. Nuclear Stat5a and nuclear Stat5b also predicted patient responsiveness to anti-estrogen therapy, which is of great clinical importance since many estrogen receptor-positive breast cancer patients fail to respond to anti-estrogen therapy and identifying these patients remains a significant challenge. However, our data also suggested that Stat5a and Stat5b function differently in human breast cancer. Nuclear Stat5a protein was frequently lost during breast cancer progression, while nuclear Stat5b protein remained unchanged. Stat5a and Stat5b modulated gene profiles in breast cancer that were distinct with limited overlap, which was unexpected considering their high degree of homology and generally similar associations to clinical outcome. Furthermore, prolactin and growth hormone were primary activators of Stat5a/b in luminal breast cancer cell lines, whereas EGF-family ligands activated Stat5a/b in basal-like breast cancer cell lines. Finally, we generated a novel constitutively active Stat5a construct incorporating Ser-Ala substitutions of proline-directed serine phosphorylation sites, S725 and S779, within the transactivation domain of Stat5a to facilitate functional studies of the biological effects of Stat5a in human breast cancer in vitro and in vivo. These studies identified Stat5a and Stat5b as potentially important clinical markers of breast cancer progression and prognosis.