Elucidating the role of Gremlin1 in colorectal cancer

Abstract

Gremlin1 (GREM1) is a secreted bone morphogenetic protein (BMP) antagonist, involved in many biological processes, including organogenesis and tissue differentiation. Consequently, GREM1 dysregulation is implicated in many human diseases such as diabetic nephropathy, pulmonary hypertension and cancer. Many reports link high GREM1 expression with increased carcinogenesis and worse patient survival in a range of cancers, as well as increased tumour cell proliferation, migration and invasion. Several reports identify high GREM1 mRNA in colorectal cancer, with GREM1 playing a vital role in maintaining the architecture of the colonic crypt. However, despite these numerous reports, an in-depth understanding into the molecular mechanisms by which GREM1 regulates these processes remains elusive.

Using a series of in silico and in situ methodologies, we identified that GREM1 expression is significantly higher in the colorectal cancer consensus molecular subtype-4 and correlates with reduced patient survival. Furthermore, we identified a novel paracrine signalling circuit, involving the secretion of GREM1 from cancer associated fibroblasts and uptake by surrounding colonic epithelial cells. We extended these findings to show that non-GREM1 expressing cells were capable of taking up GREM1 protein via both clathrin- and calveolar-mediated endocytosis. No specific localisation of GREM1 to distinct organelles was identified, but perinuclear and endoplasmic reticulum staining were evident in some cells. Potential mechanisms of GREM1-mediated carcinogenesis were then interrogated, with our data demonstrating that GREM1 promotes colony formation in HCT116 cells. The most widely reported non-canonical signalling mechanism of GREM1 is VEGFR2 activation. Despite exhaustive efforts in a multitude of cell lines, we could not demonstrate GREM1-mediated VEGFR2 activation. Computational genomics was used to compare gene expression changes in the intestines of wild-type versus Villin1-Grem1 transgenic mice. Our results predicted a variety of cell signalling relationships and pathways, with an emphasis on GREM1-mediated effects on immune cell pathways. GREM1 was also investigated as a potential therapeutic target for colorectal cancer, with a computational approach supported by in vitro studies identifying Simvastatin to be a promising candidate for the inhibition of GREM1 action in colorectal cancer. In addition, novel small molecule inhibitors of GREM1 were also characterised for their potential as anti-cancer agents.

Overall, this thesis provides novel insights into the mechanisms of GREM1 signalling in colorectal cancer, and the possible downstream pathways involved. The data presented suggest many new and interesting research directions to pursue in the future and identifies the potential of GREM1 as a novel therapeutic target and predictive biomarker in colorectal cancer.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Derek Brazil (Supervisor), Philip Dunne (Supervisor) & Mark Lawler (Supervisor)