Mechanisms of Perturbed Vitamin A Mobilization from the Liver of Young Iron-Deficient Rats
Introduction: Iron deficiency (ID) is the most common nutritional deficiency worldwide and is notably prevalent among pregnant women and young children due to the demands imposed by growth and development. During fetal and child development, the signaling molecule retinoic acid (derived from dietary vitamin A) also plays a fundamental role in development. Multiple kinetic studies suggest that ID disrupts the movement of vitamin A from the liver to the circulation, thus interfering with its signaling capacity. However, the mechanism by which ID achieves this effect is unknown. This research project aims to identify the molecular mechanism underlying impaired hepatic vitamin A mobilization in ID rats, and tests the working hypothesis that ID inhibits hepatic secretion of Retinol binding protein (RBP). Methods: Three week old male and female rats were fed either a control diet or an iron-deficient diet (3 mg/kg) for six weeks. Hemoglobin, hematocrit and plasma were collected bi-weekly, and after six weeks on the diet the rats were euthanized and tissues were collected. Reverse-phase HPLC was used to calculate retinol and retinyl ester concentrations in plasma, liver, lung, and white adipose tissues. RNA was extracted from liver samples, and purified RNA was then reverse-transcribed into cDNA to be used for qPCR mRNA expression analysis. Plasma RBP concentrations were determined with an RBP ELISA Kit. Results: Serial blood sampling throughout the experimental time course revealed a significant decline in plasma hemoglobin levels in male and female rats. HPLC data shows that in male ID rats plasma retinol was significantly decreased, while hepatic retinyl esters were increased. Surprisingly, this effect was not observed in female ID rats. ELISA revealed that there is a significant decrease in ID male plasma RBP when compared to the control. Using qPCR to quantify the expression of various genes in the vitamin A metabolic pathway, it was found that Dhrs3, Dhrs4, Rdh, Cyp26a1, Ttr, and Rarb expression decreased in ID. In contrast, Cyp26b1expression increased in ID. Conclusion: Our findings support the model that ID impairs hepatic vitamin A mobilization. In agreement with our hypothesis, we have shown for the first time that ID also lowers circulating RBP4 levels, suggesting the hepatic secretion of this protein is central to the effect of ID. Our qPCR results provide insight into the effect of ID on vitamin A-related gene expression in the liver, suggesting that hepatic retinoic acid signaling is decreased by ID. The effect of ID is most pronounced in male rats, suggesting that females can better compensate for the lack of iron in the diet. Mechanistic studies into the effect of ID on hepatic vitamin A metabolism and its sex-specific effects are on-going. Due to the high global co-occurrence of ID and vitamin A deficiency, it is important that further research on their relationship is conducted in order to identify how deficiencies in these nutrients interact to impair child development.