Introductory sentence:

Increased activity of signal transducer and activator of transcription 3 (STAT3) is common in human malignancies, including colorectal cancers. Recently, we reported that STAT3 expression correlated with resistance to 5-fluorouracil (5-FU) based chemoradiotherapy. This is of considerable clinical relevance, because a large proportion of rectal cancers are resistant to preoperative multimodal treatment. We therefore examined whether STAT3 contributes to resistance to chemoradiotherapy.

Experimental procedures:

STAT3 mRNA and protein expression levels were determined in 12 colorectal cancers cell lines. STAT3 was inhibited using two different siRNAs and a small-molecular inhibitor (STATTIC) in the cell lines SW480 and SW837. Successful RNAi-mediated silencing of STAT3 or inhibition of phosphoSTAT3(Tyr705) was detected by Western blot and reduction of transcription factor activity was measured by a luciferase reporter assay. Additionally, we established doxycycline-inducible stable shRNA single cell populations and a non-silencing shRNA (shNEG) in SW480. To test the influence of STAT3 knock down or inhibition, clonogenic survival assays were performed. Therefore, RNAi or inhibitor treated cells were exposed to chemoradiotherapy using 3µM 5-FU and X-ray-irradiation at 1, 2, 4, 6, and 8 Gy. Finally, we tested the effect of a chemoradiotherapy combined with STATTIC treatment in a SW837 xenograft model in NMRI nude mice. To verify the sensitizing effect of STATTIC, tumor growth was recorded and growth delay assays were performed.


STAT3 was overexpressed in resistant cells at mRNA and protein level. siRNA transfected SW480, SW837, and SW480shRNA single cell clones showed a significant reduction of STAT3 protein and transcription factor activity after 96 hours. STATTIC inhibition led to a decreased phosphorylation of STAT3 after 1 hour. The silencing/inhibition resulted in a significantly increased chemoradiosensitivity with dose-reduction factors of 1.3 to 2.5 at a surviving fraction of 0.37. In vivo, additional STAT3 inhibition during chemoradiotherapy led to a profound chemoradiosensitization effect and a significant tumor growth delay in STATTIC treated mice. Survival of these mice was also enhanced, if compared to the control group.


STAT3 is highly overexpressed in resistant colorectal cancer cells, and silencing or inhibition of STAT3 leads to a significantly increased chemoradiosensitivity in vitro and in vivo. This highlights the potential relevance of STAT3 for mediating treatment resistance and provides a first proof of concept that STAT3 represents a novel molecular target in rectal cancer to sensitize a priori resistant colorectal tumor cells to chemoradiotherapy.

Citation Format: Melanie Spitzner, Birte Roesler, Christian Bielfeld, Carolin Herzberg, Georg Emons, Jochen Gaedcke, Margret Rave-Fränk, Tim Beißbarth, Thomas Ried, B. Michael Ghadimi, Marian Grade. Targeting STAT3 in vitro and in vivo reveals a novel therapeutic strategy to sensitize colorectal cancer cells to chemoradiotherapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 855. doi:10.1158/1538-7445.AM2014-855