1,25-dihydroxyvitamin D(3) (1,25D) regulates gene expression by signaling through the nuclear
vitamin D receptor (VDR)
transcription factor and exhibits
calcium homeostatic, anticancer, and immunomodulatory properties.
Histone deacetylase inhibitors (HDACis) alter nuclear and cytoplasmic
protein acetylation, modify gene expression, and have potential for treatment of
cancer and other indications. The function of
nuclear receptor ligands, including 1,25D, can be enhanced in combination with HDACi. We designed
triciferol, a hybrid molecule in which the 1,25D side chain was replaced with the dienyl
hydroxamic acid of HDACi
trichostatin A.
Triciferol binds directly to the VDR, and functions as an agonist with 1,25D-like potency on several 1,25D target genes. Moreover, unlike 1,25D,
triciferol induces marked
tubulin hyperacetylation, and augments
histone acetylation at concentrations that largely overlap those where VDR agonism is observed.
Triciferol also exhibits more efficacious antiproliferative and cytotoxic activities than 1,25D in four
cancer cell models in vitro. The bifunctionality of
triciferol is notable because (i) the HDACi activity is generated by modifying the 1,25D side chain without resorting to linker technology and (ii) 1,25D and HDACi have sympathetic, but very distinct biochemical targets; the hydrophobic VDR
ligand binding domain and the active sites of HDACs, which are
zinc metalloenzymes. These studies demonstrate the feasibility of combining HDAC inhibition with
nuclear receptor agonism to enhance their therapeutic potential.