Animals in group 1 were continuously fed a VAD diet and served as controls to determine thoroughly baseline levels of ��,��-carotene and retinoids. Animals in groups 2 and 3 received diets supplemented with ��,��-carotene (0.15 mg/g). To analyze the effect of preformed retinoids on ��,��-carotene absorption, mice in group 3 were additionally gavaged weekly with dietary vitamin A (300 UI), i.e., VAS diet. After 15 wk, we sacrificed these mice and determined ��,��-carotene and retinoid levels in liver and blood (Fig. 4). In animals of group 1, retinoid stores in the liver were largely depleted (Fig. 4B), demonstrating that the nonsupplemented diet effectively induced systemic vitamin A deficiency in these mice. This deficiency was also evidenced by an accumulation of serum RBP4 in the liver (Fig. 4A).
Retinol bound to RBP4 is the major transported form of retinoid in the blood, and RBP4 is secreted from the liver in a vitamin A-dependent manner (29). In ��,��-carotene supplemented WT mice (group 2), vitamin A levels in the liver were significantly increased over baseline levels of animals subjected to dietary vitamin A deprivation (Fig. 4B). Accordingly, liver RBP4 levels were decreased significantly compared to animals of group 1 (Fig. 4A). As expected, ��,��-carotene supplementation of BCMO1-knockout mice did not result in vitamin A production. Livers of these animals lacked retinoids, and they showed elevated hepatic RBP4 levels similarly to the vitamin A-deficient animals of group 1 (Fig. 4A, B). Instead, this mouse mutant accumulated large amounts of ��,��-carotene in the liver and blood (Fig.
4C, D). Supplementation with a combination of vitamin A and ��,��-carotene (group 3) significantly increased liver retinoid levels in BCMO1-knockout mice (Fig. 4B). Vitamin A-replenishment of BCMO1-knockout mice in group 3 was also evidenced by a decrease in liver RBP4 levels (Fig. 4A). Most important, dietary vitamin A prevented high levels of ��,��-carotene accumulation in BCMO1-knockout mice. Even though these animals were supplemented with the same amount of ��,��-carotene as their littermates of group 2, ��,��-carotene levels in the blood and liver were 22- and 20-fold lower, respectively. Figure 4. Retinoids control intestinal ��,��-carotene absorption levels. A) Immunoblot analysis for RBP4 in liver protein extracts from control and BCMO1?/? mice.
B, C) Levels of total Carfilzomib vitamin A (all-trans-retinol and retinyl esters) … ��,��-Carotene accumulation is correlated with decreased ISX and increased SR-BI expression in BCMO1-knockout mice We observed that dietary vitamin A can prevent ��,��-carotene accumulation in BCMO1-deficient mice. This finding is likely explained by the RA dependent induction of ISX that repressed intestinal SR-BI expression (see above) required for ��,��-carotene absorption (12).