Tacalcitol increases the sensitivity of colorectal cancer cells to 5-fluorouracil by downregulating the thymidylate synthase

5-Fluorouracil (5-FU) is an anticancer drug that is most frequently used to treat colorectal cancer (CRC) patients, but unfortunately it shows limited efficacy. We recently demonstrated that vitamin D analogs (VDAs), particularly tacalcitol (coded as PRI-2191), potentiate its anticancer activity in an in vivo mouse and human CRC model. The purpose of this study was to explain the mechanism underlying the enhancement of 5-FU efficacy by PRI-2191 towards human HT-29 CRC cells. We showed that PRI-2191 induces the CDKN1A (gene encoding p21Waf1/Cip1) expression directly through vitamin D receptor (VDR) in a p53-independent manner and thus decreases the thymidylate synthase expression both at the mRNA and protein level. It is the main mechanism by which PRI-2191 improves the anticancer efficacy of 5-FU towards HT-29 cells. Additionally, we indicated that the VDR also participates in 5-FU mechanism of action. 5-FU significantly increased TYMS (gene encoding thymidylate synthase (TS)) and BIRC5 (gene encoding survivin) level in HT-29 cells with silenced VDR. Furthermore, PRI-2191 induced E-cadherin and ZO-1 expression and thus reduced the level of BIRC5 in HT-29 cells. The induction of E-cadherin expression may also contribute to the reduction of c-Myc level and consequently the downregulation of TS. Our results also indicate that calcium-sensing receptor (CaSR) plays a role in the activity of PRI-2191 but has no influence on the 5-FU mechanism of action. In conclusion, we suggest that both VDR and CaSR might be useful as molecular markers for predicting treatment outcomes and identifying the CRC patient subgroups who might benefit from 5-FU-based chemotherapy combined with vitamin D analog.