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Constitutively activated STAT3 is involved in the formation of multiple types of tumors including breast cancer. We examined the effects of Stat3 protein knockdown by RNAi using a di-cistronic lentivirus shRNA delivery system on the growth of mammary tumors in BALB/c mice induced by the 4TI cell line. A single exposure of 4T1 cells to shRNA/STAT3 lentivirus transduced 75% of the cells with GFP within 96 hr. In cells selected for GFP expression, neither Stat3 protein nor phosphotyrosine Stat3 was detected. Tumor formation induced by injecting 4T1 cells into the mammary fat pad was blocked by expression of the shRNA for STAT3 whereas all mice injected with 4T1 cells expressing only GFP efficiently formed tumors. C-Myc expression was reduced 75% in cells expressing greatly reduced levels of Stat3 compared to the GFP control. Of interest, the level of activated Src, which is known to activate Stat-3, was virtually eliminated but the level of the Src protein itself remained the same. Importantly, expression of Twist protein, a metastatic regulator, was eliminated in STAT3 knockdown cells. Invasion activity was of STAT3 knockdown cells was strongly inhibited. However, the proliferation rate of cells in Stat3 knockdown cells was similar to that of the GFP control; the cell cycle was also not affected. We conclude from these studies that activated Stat3 protein plays a critical role in the induction of breast tumors induced by 4T1 cells by enhancing the expression of several important genes including c-Myc and the metastatic regulator Twist. These studies suggest that stable expression of siRNA for STAT3 has potential as a therapeutic strategy for breast cancer.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]