β-Apo-10′-carotenoids Modulate Placental Microsomal Triglyceride Transfer Protein Expression and Function to Optimize Transport of Intact β-Carotene to the Embryo

Abstract

␤-Carotene is an important source of vitamin A for the mammalian embryo, which depends on its adequate supply to achieve proper organogenesis. In mammalian tissues, ␤-carotene 15,15-oxygenase (BCO1) converts ␤-carotene to retinaldehyde, which is then oxidized to retinoic acid, the biologically active form of vitamin A that acts as a transcription factor ligand to regulate gene expression. ␤-Carotene can also be cleaved by ␤-carotene 9,10-oxygenase (BCO2) to form ␤-apo-10-carotenal, a precursor of retinoic acid and a transcriptional regulator per se. The mammalian embryo obtains ␤-carotene from the maternal circulation. However, the molecular mechanisms that enable its transfer across the maternal-fetal barrier are not understood. Given that ␤-carotene is transported in the adult bloodstream by lipoproteins and that the placenta acquires, assembles, and secretes lipoproteins, we hypothesized that the aforementioned process requires placental lipoprotein biosynthesis. Here we show that ␤-carotene availability regulates transcription and activity of placental microsomal triglyceride transfer protein as well as expression of placental apolipoprotein B, two key players in lipoprotein biosynthesis. We also show that ␤-apo-10-carotenal mediates the transcriptional regulation of microsomal triglyceride transfer protein via hepatic nuclear factor 4␣ and chicken ovalbumin upstream promoter transcription factor I/II. Our data provide the first in vivo evidence of the transcriptional regulatory activity of ␤-apocarotenoids and identify microsomal triglyceride transfer protein and its transcription factors as the targets of their action. This study demonstrates that ␤-carotene induces a feed-forward mechanism in the placenta to enhance the assimilation of ␤-carotene for proper embryogenesis.The importance of vitamin A as a critical modulator of mammalian embryonic development has been known for decades (1). This essential nutrient exerts its function mainly through its active form, retinoic acid. Retinoic acid binds to retinoic acid receptors and retinoid X receptors (RXRs) 3 and regulates, in a spatial and temporal manner, the transcription of numerous genes vital to development (2-6).The mammalian embryo obtains retinoids (vitamin A and its derivatives) and provitamin A carotenoids from the maternal bloodstream. Among dietary carotenoids, ␤-carotene (BC) is the main source of vitamin A for the majority of the world population (7). Intact BC from the maternal circulation crosses the placenta and reaches the developing embryo where the cytoplasmic ␤-carotene 15,15Ј-oxygenase (BCO1) cleaves BC symmetrically to yield retinaldehyde, which in turn is oxidized to retinoic acid (8, 9). Asymmetric cleavage of BC by ␤-carotene 9Ј,10Ј-oxygenase (BCO2) also occurs, generating ␤-ionone and ␤-apo-10Ј-carotenal (apo10AL) (9). The latter, as well as other ␤-apocarotenoids of various chain lengths generated from oxidative breakdown of BC in food and animal tissues (10), can be converted into one molecule of retinaldehyde by BCO1 (9, 11). H...

Low-Density Lipoprotein Receptor Contributes to β-Carotene Uptake in the Maternal Liver

“…Hepatic Bco2 mRNA levels showed only a trend towards an increase in Ldlr +/− dams upon β-carotene administration ( p = 0.06), but there was no treatment- or genotype-dependent difference in its transcription levels (Figure 3). Despite no unequivocal correlation having been established between trascription levels of the β-carotene cleavage enzymes and their activity, it is known that their transcription is regulated by carotenoid availability, as well as by retinoic acid (in the case of Bco1 ) [19]. Thus, changes in their mRNA expression may reflect homeostatic control of the flux of retinoids and carotenoids within tissues, responding to changes in β-carotene and vitamin A status.…”

“…These data suggest a potential competitive mechanism between β-carotene and retinyl esters for the incorporation into lipoprotein particles, at least in this model of impaired lipoprotein clearance. The mechanism of incorporation of retinoids and carotenoids into lipoprotein particles is currently unknown, although recent data from our lab strongly suggest that microsomal triglyceride transfer protein (MTP) may mediate this process in the case of β-carotene [19]. Interestingly, a single β-carotene administration results in increased maternal serum retinol concentrations in both Ldlr +/− and Ldlr −/− dams without changes in serum levels of RBP.…”

“…Further studies are needed to confirm this hypothesis and clarify the underlying molecular mechanisms. The hepatic transcription of Bco2 , induced by carotenoids in vivo [8,19,39], was also not affected by β-carotene availability in the liver of the Ldlr −/− dams. Our data suggest a potential upregulation of the asymmetric cleavage pathway in the liver of the Ldlr +/− dams.…”

“…β-carotene was administered to the dams by intraperitoneal (IP) injection at mid-gestation, following a protocol previously established in our laboratory to circumvent the high mouse intestinal BCO1 cleavage activity and yield detectable intact β-carotene in the maternal bloodstream without signs of embryonic vitamin A toxicity [8,19]. An emulsion was prepared by adding β-carotene (Type II, Sigma Aldrich) to a mixture of ethanol, cremophor (Sigma, St. Louis, MO, USA) and PBS (1:11:18 ratio), under yellow light with vortexing.…”

“…Gene expression changes were expressed as mRNA fold change from the control group ( Ldlr +/− + vehicle). Primers and amplicon size are as previously reported [8,19]. …”