Marked Increase in CYP24A1 Gene Expression in Human Papillary Thyroid Cancer

Dear Editor:

The active metabolite of vitamin D₃, 1,25-dihydroxyvitamin D₃ (1,25-D₃) inhibits cell growth and induces cell differentiation and apoptosis in numerous tumors, such as colon, breast, and prostate cancers (1 –3). However, the anticancer effect of 1,25-D₃ cannot always be seen in a clinical setting. In case of colon cancers, a marked increase in the expression of the 1,25-D₃ inactivating enzyme, the 24-hydroxylase (CYP24A1), has been observed (4). The role of 1,25-D₃ in the development of thyroid carcinomas has not yet been established. However, Sharma et al. (5) observed the correlation of high baseline CYP24A1 and relatively low stimulation after calcitriol treatment (compared with sensitive cells) with lack of growth inhibition in numerous thyroid cancer cell lines. We have also examined the gene expression of the inactivating CYP24A1 and the activating 1-alpha-hydroxylase (CYP27B1) enzyme in human thyroid cancers.

To date, gene expression analysis of thyroid samples (both normal and papillary tumor tissue of the same patient) was carried out in six unrelated, consecutive, Hungarian, Caucasian patients at our Thyroid Clinic. All surgically removed thyroid tissue samples underwent histological examination, and papillary tumor was identified in all cases. The study was approved by the Regional Committee of Science and Research Ethics, Semmelweis University (SOTE-TUKEB 1160-0/2010-1018EKU), and all patients gave written informed consent.

Total RNA was isolated from each sample with Roche High Pure Total RNA Isolation kit. Five hundred nanograms of total RNA was reverse-transcribed to cDNA. The expression differences of selected genes were analyzed by Taqman probe-based quantitative real-time reverse transcriptase–polymerase chain reaction. Relative quantification was carried out from collected data (threshold cycle numbers) by Applied Biosystems 7500 System SDS software 1.3.

CYP24A1 mRNA expression was markedly increased in all but one papillary cancer compared with that of normal thyroid tissue, sometimes reaching 300-fold elevation (Supplementary Fig. S1; Supplementary Data are available online at www.liebertonline.com/thy). No significant alteration was seen in CYP27B1 gene activity between neoplastic and normal tissues. There was no significant difference in serum 25-OH-vitamin D₃ concentrations among patients (mean: 28.03 ng/mL, range: 22.7–31.9 ng/mL).

In this letter, we report marked increases in the 1,25-D₃-neutralizing CYP24A1 gene expression in human papillary thyroid cancer. The tumor cell growth-inhibiting role of vitamin D₃ has been extensively studied in different malignancies (1 –3). The interest in the association of vitamin-D₃ serum levels with various cancer incidences has dramatically increased. However, only very limited data are available on the relationship between serum vitamin D₃ concentrations and malignant thyroid conditions (6,7). The pilot study of Laney et al. showed levels of vitamin D and rates of vitamin D deficiency to be similar between patients with thyroid nodules, thyroid cancer in remission, and active thyroid cancer (8). However, the results of Stepien et al. revealed significantly lower calcitriol concentrations in patients with papillary, follicular, and anaplastic thyroid cancers (7). This finding could be attributable to higher CYP24A1 levels cleaving calcitriol, as there were no differences in serum 25-OH-vitamin D₃. Earlier, calcitriol had also been shown to attenuate thyrotropin-stimulated growth and iodide uptake by rat thyroid cells (FRTL-5) (9,10).

Papillary cancer is the most common thyroid malignancy. The role of vitamin D₃ in the prevention or development of this disorder has to be yet determined. The increase in CYP24A1 expression in this cancer type could be a “protective” mechanism against the well-known anticancer effect of calcitriol. The observation of Sharma et al. (5) regarding increased expression of CYP24A1 in thyroid cancer cell lines corroborates this notion. One of the cell lines they used was human papillary thyroid carcinoma cell line, whereas the other ones showed the features of human anaplastic thyroid carcinoma. Also, the most resistant cell lines to calcitriol had the highest baseline levels of CYP24A1. In our study at this stage, the number of examined tumor tissues does not allow drawing of statistically significant conclusion about the correlation of tumor histopathological stage or aggressiveness with CYP24A1 levels.

Khadzkou et al. demonstrated enhanced 1-alpha-hydroxylase expression but concluded that this elevation did not show large differences in CYP27B1 expression in papillary thyroid cancer and its metastasis compared with normal thyroid tissue (11). Similarly to our observation in papillary cancer in vivo, Sharma et al. reported that CYP27B1 level does not change in response to calcitriol in the different thyroid cancer cell lines (5).

If the observed increase in CYP24A1 expression proves to be true on a larger sample size of human thyroid cancers, the use of higher doses of vitamin D₃ and/or the development of CYP24A1 inhibitors or 24-hydroxylase-resistant vitamin D analogs could open a new approach to the effective treatment of thyroid cancers.