STAT3 inhibition sensitizes colorectal cancer to chemoradiotherapy in vitro and in vivo

Int J Cancer. 2014 Feb 15;134(4):997-1007. doi: 10.1002/ijc.28429. Epub 2013 Sep 3.


Increased activity of signal transducer and activator of transcription 3 (STAT3) is common in human malignancies, including colorectal cancers (CRCs). We have recently reported that STAT3 gene expression correlates with resistance of CRC cell lines to 5-fluorouracil (5-FU)-based chemoradiotherapy (CT/RT). This is of considerable clinical importance, because a large proportion of rectal cancers are resistant to preoperative multimodal treatment. To test whether STAT3 contributes to CT/RT-resistance, we first confirmed that STAT3 protein expression correlated positively with increasing resistance. While STAT3 was not constitutively active, stimulation with interleukin-6 (IL-6) resulted in remarkably higher expression levels of phosphorylated STAT3 in CT/RT-resistant cell lines. A similar result was observed when we determined IL-6-induced expression levels of phosphorylated STAT3 following irradiation. Next, STAT3 was inhibited in SW480 and SW837 using siRNA, shRNA and the small-molecule inhibitor STATTIC. Successful silencing and inhibition of phosphorylation was confirmed using Western blot analysis and a luciferase reporter assay. RNAi-mediated silencing as well as STATTIC treatment resulted in significantly decreased clonogenic survival following exposure to 3 µM of 5-FU and irradiation in a dose-dependent manner, with dose-modifying factors of 1.3-2.5 at a surviving fraction of 0.37. Finally, STAT3 inhibition led to a profound CT/RT-sensitization in a subcutaneous xenograft model, with a significantly delayed tumor regrowth in STATTIC-treated mice compared with control animals. These results highlight a potential role of STAT3 in mediating treatment resistance and provide first proof of concept that STAT3 represents a promising novel molecular target for sensitizing resistant rectal cancers to CT/RT.

Keywords: STAT3; chemoradiotherapy-resistance; chemoradiotherapy-sensitization; molecular target; rectal cancer.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antimetabolites, Antineoplastic / pharmacology
  • Apoptosis*
  • Blotting, Western
  • Cell Proliferation
  • Chemoradiotherapy*
  • Colorectal Neoplasms / metabolism
  • Colorectal Neoplasms / pathology
  • Colorectal Neoplasms / therapy*
  • Drug Resistance, Neoplasm*
  • Female
  • Fluorouracil / pharmacology*
  • Humans
  • Immunoenzyme Techniques
  • In Vitro Techniques
  • Mice
  • Mice, Nude
  • RNA, Messenger / genetics
  • RNA, Small Interfering / genetics
  • Real-Time Polymerase Chain Reaction
  • Reverse Transcriptase Polymerase Chain Reaction
  • STAT3 Transcription Factor / antagonists & inhibitors*
  • STAT3 Transcription Factor / metabolism
  • Tumor Cells, Cultured
  • Xenograft Model Antitumor Assays


  • Antimetabolites, Antineoplastic
  • RNA, Messenger
  • RNA, Small Interfering
  • STAT3 Transcription Factor
  • Fluorouracil