Differential Regulation of TauT by Calcitriol and Retinoic Acid via VDR/RXR in LLC-PK1 and MCF-7 Cells - Advances in Experimental Medicine and Biology - page 284

level via binding of the VDR/RXR complex to AP1 sites of the TauT promoter.
Another factor that differentially regulates TauT in these cell lines is p53.
It is possible that these interactions between vitamins A and D and taurine include
the operation of an enteral feedback loop. Taurine-conjugated bile salts increase
vitamin A and D intestinal absorption, and following metabolic conversion of these
vitamins, stimulation of the VDR/RXR receptor complex and binding to the pro-
moter region of TauT increases or reduces synthesis of the TauT transporter protein,
which, in turn, alters uptake of taurine into the cell. We speculate that in hepatic
cells, taurine is more available to conjugate with bile acids necessary for fat-soluble
vitamin absorption and the cycle begins anew. The negative aspects of this loop are
not as yet identified, but the findings in the MCF-7 cell line demonstrate that down-
regulation of TauT can occur.
Acknowledgements The authors wish to thank Andrea Patters for insightful comments and sug-
gestions. This work was supported by the Le Bonheur Chair of Excellence in Pediatrics.
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