Colorectal cancer is the second most commonly diagnosed cancer worldwide, and may develop sporadically or as a result of chronic inflammation (colitis-associated cancer). Macrophages are a major component of the tumour micro-environment in colorectal cancer and may be broadly classified into two groups: M1 (classically-activated) and M2 (alternatively-activated) macrophages. Haematopoietic Cell Kinase (Hck) is a myeloid-specific Src family kinase involved in the alternative polarisation of macrophages, which correlate with a poor prognosis for human colorectal cancer.
We investigated the role of Hck in tumourigenesis using two mouse models of colorectal cancer and show that aberrant Hck activation promotes tumourigenesis by polarising macrophages towards a tumour-promoting phenotype. Firstly, Hck mutant mice that express a constitutively active form of Hck (HckF/F) were subjected to the azoxymethane/dextran sodium sulphate (AOM/DSS) model of colitis-associated cancer. To model sporadic colorectal cancer, HckF/F mice were administered a single weekly injection of AOM for six consecutive weeks. In both tumour models, HckF/F mice exhibited increased tumour frequency and multiplicity compared to wild-type (WT) animals, in addition to mucosal invasion which was absent in WT mice. Western blotting and immunohistochemical analysis revealed a significant increase in tumour proliferation in HckF/F mutants compared to WT animals, consistent with an increased abundance of activated Stat3, Erk, Akt and S6 cytoplasmic signalling proteins. While the total number of macrophages remained unaffected, qPCR analysis on purified tumour macrophages showed a two-to-three fold increase in alternatively-activated Il4, Il10, Il13, Arg1 and Ym1 mRNAs in HckF/F mice compared to WT animals. Furthermore, reciprocal adoptive transfer of HckF/F bone-marrow into WT recipients was sufficient to increase tumour burden and promote alternative macrophage polarisation.
Collectively, these results demonstrate that aberrant Hck activation promotes colorectal tumourigenesis by increasing epithelial proliferation and the polarisation of alternatively-activated macrophages.