Gene regulatory and phenotypic effects of cal- citriol and canonical WNT in human colon fibroblasts

Abstract

Vitamin D3 (cholecalciferol) is synthesized in the skin by action of solar UV radiation or incorporated via diet, and is the inactive precursor of 1α,25-dihydroxyvitamin D3 (calcitriol), a major pleiotropic hormone in the human organism. By binding and activation of vitamin D receptor (VDR), a member of the superfamily of nuclear receptors, calcitriol regulates the expression of hundreds of genes in a tissue- and cell-type specific manner by transcriptional and posttranscriptional mechanisms. Human WNT factors are a family of 19 members of secreted proteins that regulate crucial processes in many tissues and organs during development and adult life. Some (canonical) WNTs act by modulating the transcriptional activity of β-catenin whereas other (non-canonical) WNTs affect different signal transducers (Ca2+, Rho, JNK...) independently of β-catenin. Colorectal cancer (CRC) is one of the most important neoplasias worldwide in terms of incidence and mortality. The key role of the constitutive activation of the WNT/β-catenin signaling pathway promoting CRC and the protective effect of VDR agonists, in part by antagonizing the WNT/β-catenin pathway, have been widely described. However, the effects of calcitriol and canonical WNTs on stromal fibroblasts, which are important players in CRC with protumorigenic action, remain mostly unknown. We have studied the effect of calcitriol and WNT3A on the human CCD-18Co colonic fibroblast cell line and on primary colon fibroblasts isolated from CRC patients. Data from global transcriptomic analyses by RNA-sequencing and also functional assays (cell proliferation, migration...) indicate that calcitriol and WNT3A exert a wide regulatory action on the gene expression and phenotype of colonic fibroblasts. Remarkably, a complex interplay exists between the two agents: while calcitriol and WNT3A act cooperatively in some effects, antagonistic actions are found in others. Our results show that calcitriol and WNT3A are important modulators of human colon fibroblasts, with potential implications in colon homeostasis and neoplastic transformation.