The five-year survival rate for patients with metastatic colorectal cancer (CRC) is less than 15%, necessitating the need to develop new therapeutic strategies for this disease. Loss of differentiation is associated with worse overall survival in CRC, suggesting strategies aimed at re-inducing differentiation may provide clinical benefit. Histone deacetylase inhibitors (HDACi) induce features of differentiation in CRC cells; however these effects are incomplete. Mutations in the MAPK pathway are associated with differentiation loss in CRC and MAPK inhibitors (MAPKi) induce expression of the CDX2 transcription factor, an established driver of colonic cell differentiation. These findings suggested that combining HDACi and MAPKi may induce a more complete differentiation program.
The aim of this study therefore was to determine the effect of combining an HDACi with a MAPKi on CRC differentiation.
Combination treatment of HT29 and T84 CRC cells with an HDACi and a MAPKi led to induction of a broader set of differentiation markers compared to either agent alone. Furthermore, induction of the differentiation markers Cadherin 17 (CDH17) and Keratin 20 (KRT20) were further enhanced upon combination treatment. HDACi and MAPKi treatment also additively enhanced expression of CDX2, and knockdown of CDX2 attenuated the differentiation-inducing effects of the combination, establishing a direct role for CDX2 in mediating these effects. Concomitant with differentiation induction, HDACi and MAPKi treatment synergistically induced apoptosis. To validate these findings in vivo, HT29 cells grown as xenografts and were treated with HDACi and MAPKi alone and in combination. Tumour size was significantly reduced in mice treated with the combination compared to mice treated with vehicle or either agent alone. Consistent with in vitro findings, immunostaining of resected tumours demonstrated enhanced expression of CDH17, KRT20 and CDX2 expression in mice treated with the combination. Collectively, this study highlights a novel drug combination strategy to re-induce differentiation for the treatment of CRC.