Thyroid cancer resistance to vitamin D receptor activation is associated with 24-hydroxylase levels but not the ff FokI polymorphism.
BACKGROUND
The vitamin D receptor (VDR) has been studied as a novel target for cancer therapy in many tissue types as VDR ligands decrease cell proliferation in vitro and decrease tumor growth in vivo in sensitive cells. The objective of this study was to analyze the response to VDR agonist therapy in a panel of validated thyroid cancer cells and assess genetic markers predicting sensitivity and resistance to calcitriol and the noncalcemic analog DP006.


METHODS
Thyroid cancer cell lines were analyzed for VDR and RXR protein by Western blot. Cell growth after VDR agonist treatment (calcitriol or DP006) was assessed after 6 days of treatment by viable cell assay. To investigate calcitriol/DP006 resistance in VDR-expressing cells, the VDR was sequenced and 1-α and 24-hydroxylase mRNA expression was assessed.


RESULTS
VDR protein was variably expressed in the thyroid cancer cell lines and its presence was not sufficient for decreased viable cell count in response to calcitriol or DP006. The most sensitive cells (TPC1) have an ff FokI VDR polymorphism and the most resistant cells (HTh7 and 8505C) have an FF FokI VDR. This is a unique finding given that the balance of the literature of VDR polymorphisms describes an association of the ff FokI polymorphism with cancer risk and/or decreased VDR transactivation. 1-α and 24-hydroxylase mRNA expression before and after VDR agonist therapy was examined. 1-α-Hydroxylase levels did not change after calcitriol treatment. However, we found that higher baseline 24-hydroxylase levels and/or lower stimulation of 24-hydroxylase levels after calcitriol treatment were associated with relative resistance to calcitriol/DP006.


CONCLUSIONS
The VDR represents a novel therapeutic target in poorly differentiated thyroid cancer; however, the efficacy of VDR agonist therapy to decrease viable thyroid cancer cell count cannot be predicted solely on the presence of the VDR. The FF FokI VDR genotype and high baseline 24-hydroxylase levels were associated with relative resistance to calcitriol and DP006. Therefore, identifiable markers of sensitivity or resistance to VDR agonist therapy may allow for a personalized use of these agents in poorly differentiated thyroid cancer.
Sharma
Vibha
V
Division of Endocrinology, Metabolism, and Diabetes, Department of Medicine, University of Colorado School of Medicine, 12801 E. 17th Ave., Aurora, CO 80045, USA.
Fretwell
Deborah
D
Crees
Zachary
Z
Kerege
Anna
A
Klopper
Joshua P
JP
D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism; D001483:Base Sequence; D002117:Calcitriol / Q000627:therapeutic use*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D015252:Deoxyribonucleases, Type II Site-Specific / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D006801:Humans; D011110:Polymorphism, Genetic; D018167:Receptors, Calcitriol / Q000819:agonists* / Q000187:drug effects / Q000235:genetics; D047488:Retinoid X Receptors / Q000235:genetics; D013250:Steroid Hydroxylases / Q000378:metabolism; D013964:Thyroid Neoplasms / Q000188:drug therapy* / Q000235:genetics; D014807:Vitamin D / Q000031:analogs & derivatives / Q000627:therapeutic use; D065668:Vitamin D3 24-Hydroxylase
D018167:Receptors, Calcitriol; D047488:Retinoid X Receptors; D014807:Vitamin D; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; C035042:endodeoxyribonuclease FokI; D015252:Deoxyribonucleases, Type II Site-Specific; D002117:Calcitriol