Hypovitaminosis D exacerbates the DNA damage load in human uterine fibroids, which is ameliorated by vitamin D3 treatment.

Uterine fibroids (UFs) are the most common benign neoplastic threat to women's health and associated with DNA damage and genomic instability. Hypovitaminosis D is a known risk factor for UFs, especially among African Americans. Vitamin D3 has been shown to effectively inhibit UF phenotype, but its mechanisms remain unclear. We hypothesize that Vitamin D3 ameliorates UFs by recovering the damaged DNA repair system, thus inhibits tumor progression. We compared the DNA damage status and Vitamin D receptor (VDR) expression between normal myometrial and UF primary cells. Unrepaired DNA double-strand breaks (DSBs) accumulated but VDR expression decreased in UFs. The RNA and protein levels of key DNA repair members belonging to DNA DSB sensors (MRE11, NBS1, RAD50), mediators and effectors (CHECK2, BRCA1, RAD51) were downregulated in UFs compared with myometrial cells. VDR KD induced DSB accumulation and DNA damage response (DDR) defects in myometrial cells. Using the DNA damage PCR array, the expression of many additional DNA repair genes was downregulated in VDR KD cells. Treatment of UF cells with Vitamin D3 (100 nM) significantly decreased DNA damage and restored DDR concomitant with VDR induction. Notably, the PCR array demonstrated that among 75 downregulated genes after VDR KD, 67 (89.3%) were upregulated after vitamin D3 treatment. These studies demonstrate a novel link between DNA damage and the vitamin D3/VDR axis in UFs. Vitamin D3 suppresses the UF phenotype through orchestrated targeting at multiple molecules in DNA repair pathways, thus offering novel mechanistic insights into the clinical effectiveness of vitamin D3 on UFs.