The unfolded protein response: a novel therapeutic target for poor prognostic BRAF mutant colorectal cancer


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Sandra Van Schaeybroeck1,Nicholas Forsythe1,Alaa Refaat1,Arman Javadi1,Hajrah Khawaja1,Jessica-Ann Weir1,Heba Emam1,Wendy Allen1,Frank Burkamp2,Vlad Popovici3,Puthen Jithesh4,Claudio Isella5,Melissa Labonte1,Ian Mills1,Patrick Johnston1
1Queen's University Belfast,2Almac Discovery Laboratories,3Masarykova Univerzita,4Sidra Medical and Research Centre,5University of Torino School of Medicine

Abstract

Background

BRAFV600E mutations occur in ~10% of colorectal cancer (CRC), are associated with poor survival and have limited responses to BRAF/MEK inhibition with or without EGFR inhibition. There is an unmet need to understand the biology of poor prognostic BRAFMT CRC.

Method

We have used differential gene expression and pathway analyses of untreated stage II and stage III BRAFMT (discovery set: n=31; validation set: n=26) CRC and an siRNA screen to characterize the biology underpinning the BRAFMT subgroup with poorest outcome.

Results

These analyses identified the unfolded protein response (UPR) as a novel and druggable pathway associated with the BRAFMT CRC subgroup with poorest outcome. We also found that oncogenic BRAF drives the endoplasmic reticulum (ER) stress pathway activation through MEK/ERK. Furthermore, inhibition of GRP78, the master regulator of the UPR, using siRNA or small molecule inhibition, resulted in acute ER stress and apoptosis, in particular in BRAFMT CRC cells. In addition, dual targeting of protein degradation using combined Carfilzomib (proteasome inhibitor) and ACY-1215 (HDAC6-selective inhibitor) treatment resulted in marked accumulation of protein aggregates, acute ER stress, apoptosis and therapeutic efficacy in BRAFMT in vitro and xenograft models. Mechanistically, we found that the apoptosis following combined Carfilzomib/ACY-1215 treatment is mediated through increased CHOP expression.

Conclusion

Taken together, our findings indicate that oncogenic BRAF induces chronic ER stress and that inducers of acute ER stress could be a novel treatment strategy for poor prognostic BRAFMT CRC.