Targeting the endoplasmic reticulum mediates radiation sensitivity in colorectal cancer

Radiotherapy is an established treatment modality for early and locally advanced rectal cancer as part of short course radiotherapy and long course chemoradiotherapy. The unfolded protein response (UPR) is a cellular stress response pathway often activated in human solid tumours which has been implicated in resistance to both chemotherapy and radiotherapy. This research has investigated whether the UPR pathway is upregulated in ex-vivo samples of human colorectal cancer and characterised the interaction between radiotherapy and UPR activation in two human colorectal cancer cell lines in vitro.

In vitro UPR expression was determined in response to clinical doses of radiotherapy in both the human colorectal adenocarcinoma (HT-29) cell line and a radio-resistant clone (HT-29R) using western blotting and quantitative polymerase chain reaction. The UPR was induced using a glucose deprivation culture technique before irradiation and radiosensitivity assessed using a clonogenic assay. Ex-vivo human colorectal cancer tissue was immuno-histochemically analysed for expression of the UPR marker glucose regulated protein 78 (GRP-78).

The UPR was strongly up regulated in ex-vivo human colorectal tumours with 36 of 50 (72.0%) specimens demonstrating moderate to strong staining for the classic UPR marker GRP-78. In vitro, therapeutic doses of radiotherapy did not induce UPR activation in either radiosensitive or radioresistant cell lines. UPR induction caused significant radiosensitisation of the radioresistant cell line (HT-29R SF2Gy=0.90 S.E.M. +/-0.08; HT-29RLG SF2Gy=0.69 S.E.M. +/-0.050).

This suggests that UPR induction agents may be potentially useful response modifying agents in patients undergoing therapy for colorectal cancer.