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Oncogenic transformation of fibroblasts by the src oncogene has long been known to cause an increase in the size of cell-surface protein-bound oligosaccharides, owing primarily to increased N-glycan branching mediated by increased beta-1,6-N-acetylglucosaminyltransferase V (GnT V) activity. The src-responsive element of the GnT V promoter was localized to Ets-binding sites and the promoter was transcriptionally stimulated by both ets-1 and ets-2 expression [Buckhaults, Chen, Fregien and Pierce (1997) J. Biol. Chem. 272, 19575-19581; Kang, Saito, Ihara, Miyoshi, Koyama, Sheng and Taniguchi (1996) J. Biol. Chem. 271, 26706-26712]. Because GnT V action requires the prior action of beta-1,2-N-acetylglucosaminyltransferase II (GnT II) and the human GnT II promoter contains four putative Ets-binding sites [Chen, Zhou, Tan and Schachter (1998) Glycoconj. J. 15, 301-308], GnT II might also be under oncogenic control via Ets transcription factors. We now report that co-transfection into HepG2 or COS-1 cells of either ets-1 or ets-2 expression plasmids together with chimaeric GnT II promoter-chloramphenicol acetyltransferase plasmids results in a 2-4-fold stimulation of promoter activity. Mobility-shift assays and South-Western blots localized the functional Ets-binding site to one of the four putative sites on the GnT II promoter. The GnT II promoter, unlike the GnT V promoter, is not activated by either src or neu. Therefore although both promoters are stimulated by a member of the Ets family of transcription factors, the functional role of this Ets transcriptional control seems to be different for the two genes.

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-fos promoter. Here we report that one of the downstream effectors of Src function in mammary epithelial cells is STAT3. We demonstrate that v-src expression in mammary epithelial cells induces Tyr-705 phosphorylation, nuclear translocation and DNA binding of STAT3. Furthermore, we demonstrate that v-src can induce STAT3-dependent transcription. These observations are the first direct evidence that v-src can 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.