Abstract 4655: Plant polyphenols inhibit cellular 24-hydroxylase ( CYP24A1 ) expression and elevate serum 25-hydroxyvitamin D levels

1α,25-dihydroxyvitamin D3 (1,25D) is known to regulate various cellular functions, including cell proliferation and differentiation. Both 1,25D and its precursor, 25-hydroxyvitamin D3 [25(OH)D], are catabolized by 24-hydroxylase encoded by the CYP24A1 gene, which is one of the most responsive vitamin D receptor (VDR) target genes, and a is candidate oncogene frequently overexpressed in various malignancies. Therefore, compounds which can inhibit CYP24A1 expression and/or activity, thereby reducing degradation of vitamin D derivatives, may potentiate their anticancer effects. We have previously shown that the plant polyphenols carnosic acid, curcumin and silibinin synergistically enhance differentiation of acute myeloid leukemia (AML) cells induced by near physiologic concentrations of 1,25D. This was associated with the transcription factor Nrf2-assisted elevation of both VDR and the retinoid X receptor alpha (RXRα) protein levels. Here, we demonstrate that despite the latter effect the polyphenols markedly inhibited 1,25D-induced CYP24A1 mRNA expression in AML cell lines but not in osteoblast-like cells. Furthermore, while potentiating 1,25D-induced transactivation of the direct repeat-3 (DR3)-type consensus vitamin D response element (VDRE) in AML cells, as measured by the luciferase (Luc) reporter gene assay, the polyphenols strongly inhibited 1,25D-dependent activation of the CYP24-Luc reporter construct containing an early (-298 bp) promoter region with two critical DR3-type VDREs (VDRE1 and VDRE2). In contrast, the polyphenols failed to inhibit and even enhanced 1,25D-induced transactivation of CYP24-Luc transfected into osteoblastic cells. Inactivating mutations of the VDRE1 or VDRE2 sequence in this construct did not abolish the inhibitory effect of polyphenols. However, overexpression of Nrf2 resulted in reduced CYP24-Luc activation by 1,25D, while dominant-negative Nrf2 had an opposite effect. Importantly, feeding of healthy Balb/c mice with standard rodent chow supplemented with carnosic acid-rich rosemary extract resulted in a marked dose- and time-dependent increase in serum levels of 25(OH)D, as compared to control animals. This was associated with a significant decrease in CYP24A1 mRNA levels in the kidneys of these mice. Collectively, the above data suggest that by downregulating CYP24A1 in a tissue-dependent manner, plant polyphenols may potentiate therapeutic and preventive effects of vitamin D derivatives. These findings may also contrib