Transgenic mouse models of mammary tumorigenesis and analyses of human breast tumour samples have indicated a role for Src proteins in the tumorigenic process. The downstream effectors of Src function in mammary epithelial cells are less well understood. STAT proteins constitute a family of transcription factors whose activation by cytokine and non-cytokine receptors leads to tyrosine phosphorylation, dimerization and translocation from the cytoplasm to the nucleus. In the nucleus they activate the transcription of specific genes by binding to consensus DNA elements. STATs 1 and 3 can be activated by both cytokine and non-cytokine receptors, and bind as homodimers or heterodimers to viral simian sarcoma virus (sis)-inducible elements such as that found in the c-fospromoter. Here we report that one of the downstream effectors of Src function in mammary epithelial cells is STAT3. We demonstrate that v-srcexpression in mammary epithelial cells induces Tyr-705 phosphorylation, nuclear translocation and DNA binding of STAT3. Furthermore, we demonstrate that v-srccan induce STAT3-dependent transcription. These observations are the first direct evidence that v-srccan regulate transcription through the activation of STAT proteins, and add a further level of complexity to the understanding of the mode of action of v-src.

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