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 272

Abbreviations

TauT Taurine transporter gene

VDR Vitamin D receptor

RXR Retinoic acid receptor

27.1 Introduction

Taurine is necessary to enhance the intestinal absorption of fat-soluble vitamins,
including vitamins A and D (Zamboni et al. 1993 ). Taurine, along with glycine, is
used to conjugate bile acids and promote lipid solubility across intestinal epithelial
surfaces (Hepner et al. 1973 ). We have recently shown that a contributing factor in
the pathogenesis of rickets in hand-reared polar bear cubs is a reduction in dietary
intake of taurine when an artificial cow milk-based formula is fed to the pups as a
milk replacer (Chesney et al. 2009 ). The milk of free-ranging, lactating polar bear
sows contains up to 80-fold higher taurine content than cow milk formula, probably
because of the taurine-rich marine seal and whale diet of the pregnant sow (Hedberg
et al. 2011 ). This interaction between intracellular taurine and the fat-soluble vita-
mins led us to speculate that these vitamins might be critical for taurine handling.
Accordingly, we asked whether these vitamins influenced the synthesis or activity
of the taurine transporter protein (TauT). This article focuses upon vitamin D and its
possible role in taurine transport.
Vitamin D deficiency in man is associated with aminoaciduria, including tauri-
nuria (Brodehl et al. 1971 ; Fraser et al. 1967 ; Scriver 1974 ) . This aminoaciduria has
been ascribed to the effect of secondary hyperparathyroidism (Chesney and Harrison
1975 ; Scriver 1974 ) because vitamin D deficiency is associated with elevated serum
values of parathyroid hormone (PTH). Urinary cAMP is also increased in vitamin
D deficiency and has been felt to possibly influence renal amino acid reabsorption,
potentially by interacting with renal proximal tubule cell membranes (Harrison
1979 ). However, when isolated renal brush border membranes were co-incubated
with varying concentrations of cAMP, there was no influence on taurine uptake by
vesicles (Dabbagh et al. 1989 ). In rats made vitamin D deficient, the uptake of tau-
rine into these brush border membrane vesicles, representing the uptake phase of
taurine accumulation by the renal proximal tubule, was diminished (Dabbagh et al.
1990 ). This vitamin D deficiency-induced reduction in taurine uptake was evident
despite the calcium and/or phosphate content of the diet and at quite different values
of PTH in rat serum. Put differently, reduced taurine uptake was independent of
PTH secretion and its ambient circulating levels. These results also suggest a role
for vitamin D in renal taurine handling.
What is vitamin D and how could it influence gene action and protein/peptide
synthesis? Vitamin D is a secosteroid compound that is present in certain foods
(bony fish, yeast) or that is formed within the skin (DeLuca 2008 ) . Regardless of