Differential modulation of hepatitis C virus replication and innate immune pathways by synthetic calcitriol analogs

Introduction:

Vitamin D signaling is involved in infectious and non-infectious liver diseases, yet the natural vitamin D metabolites are suboptimal therapeutic agents. In the present study, we therefore aimed to explore the potential and mechanism of selected calcitriol analogs to regulate the hepatocellular transcriptome and to inhibit hepatitis C virus (HCV) in comparison with calcitriol.

Methods:

Human hepatoma cell lines and primary human macrophages were stimulated with calcitriol and selected calcitriol analogs. The effect of calcitriol and its derivatives on hepatocellular gene expression and vitamin D receptor (VDR) signaling as well as the replication of HCV were assessed by quantitative PCR, microarray analyses and in silico analyses of ligand-VDR complexes.

Results:

The structurally related vitamin D analogs calcipotriol and tacalcitiol, but not calcitriol itself, suppressed HCV replication in a VDR-dependent manner. The lead compound calcipotriol synergistically enhanced the antiviral effects of interferon-α and of the direct-acting antivirals daclatasvir and lomibuvir. Bioinformatic analyses indicate that calcipotriol induced conformational changes upon VDR binding that were distinct from those induced by calcitriol, which affect key residues (H397, F422) of the VDR charge clamp, a decisive regulatory element of the VDR for interaction with co-activators and repressors. As a consequence, calcipotriol induced stronger regulatory actions on the hepatocellular transcriptome, including expression of antimicrobial peptides such as cathelicidin and hepcidin in comparison to calcitriol. These differential actions of calcipotriol versus calcitriol were confirmed in primary macrophages, which are prime targets of vitamin D-mediated innate immunity.

Conclusions:

Subtle structural differences of calcipotriol versus calcitriol result in distinctive interactions with decisive regulatory residues of the VDR, which appear to translate into a superior clinical potential of calcipotriol to execute important non-classical vitamin D effects such as inhibition of HCV replication.