Elevated HOXB9 expression promotes differentiation and predicts a favourable outcome in colon adenocarcinoma patients

Background: Little is known about the tumour suppressive proteins and the underlying mechanisms that suppress colon cancer progression. Homeodomain-containing transcription factor HOXB9 plays an important role in embryogenesis and cancer development. We here aim to uncover the potential role of HOXB9 in the regulation of colon adenocarcinoma progression including epithelial-to-mesenchymal transition. Methods: HOXB9 expression in colon adenocarcinoma cells and patients was analysed by western blot and immunohistochemistry separately. Correlation between HOXB9 expressions with patients' survival was assessed by Kaplan-Meier analysis. HOXB9-regulated target gene expression was determined by RNA sequencing in HOXB9-overexpressing colon adenocarcinoma cells. Results: Elevated HOXB9 expression was identified in well-differentiated colon adenocarcinoma patients and was associated with a better overall patients' survival. Overexpression of HOXB9 inhibited colon adenocarcinoma cell growth, migration, invasion in vitro and tumour growth, liver as well as lung metastases in nude mice; whereas silencing HOXB9 promoted these functions. HOXB9 promoted colon adenocarcinoma differentiation via a mechanism that stimulates mesenchymal-to-epithelial transition, involving downregulation of EMT-promoting transcriptional factors including Snail, Twist, FOXC2 and ZEB1 and upregulation of epithelial proteins including E-cadherin, claudins-1, -4, -7, occludin and ZO-1. Conclusions: HOXB9 is a novel tumour suppressor that inhibits colon adenocarcinoma progression by inducing differentiation. Elevated expression of HOXB9 predicts a longer survival in colon adenocarcinoma patients.