Role of Lipoprotein Assembly in Maternal-Fetal Transport of Beta-Carotene

Embryonic vitamin A deficiency or excess have been linked to congenital defects in humans and animal models, owing to the defective transcriptional action of retinoic acid (RA), the active form of vitamin A, during embryogenesis. β-carotene (BC) is the most abundant dietary vitamin A precursor. In the embryo, BCO1 cleaves intact BC, from the maternal circulation, to generate retinaldehyde, which is then oxidized to RA. Asymmetric cleavage of BC by BCO2, generates β-apo-10'-carotenal (apo10AL) which can serve as a precursor of retinoids (vitamin A and its derivatives), but also antagonize RA action. Our data suggest that lipoproteins synthesized within placenta mediate the transfer of BC towards the fetus. We hypothesize that apo10AL, generated in placenta by BCO2 from BC, upregulates microsomal triglyceride transfer protein (Mttp; MTP) through HNF4α and/or COUP-TFs, by antagonizing the action of RA. We propose three Specific Aims to understand the molecular mechanisms of BC placental-fetal transfer under a normal maternal vitamin A status. Aim 1. To investigate in vivo the regulation of placental lipoprotein biosynthesis by apo10AL and RA. We will assess placental carotenoids and retinoids levels and metabolism, and lipoprotein biosynthesis in WT, Bco1-/- and Bco2-/- mice. Aim 1A: To define the molecular events related to carotenoid metabolism and lipoprotein production that occur in placenta after a single maternal administration of BC. Dams will be sacrificed at various time points post-BC administration (at 13.5 dpc, by IP injection). Aim 1B: To establish the effects of a single maternal BC administration at various times during gestation on the molecular events related to carotenoid metabolism and lipoprotein production. Dams will be administered BC at various times during gestation and sacrificed after 4 and 24 h. Aim 2. To establish that placenta MTP deficiency abrogates accumulation of embryonic BC after its administration to the mother. A mouse model with placental-specific inactivation of Mttp will be used. Dams will be sacrificed 4 and 24 h after BC administration at 13.5 dpc, and placentas analyzed as in Aim 1. Aim 3. To investigate in vitro the regulation of human placental lipoprotein biosynthesis by apo10AL and RA. In this Aim, we will expand our studies to human models. Aim 3A: To study the apo10AL-mediated regulation of MTP function in human placenta explants and Bewo cells. We will: (i) determine the time course and dose-dependent effect of BC, apo10AL and RA on carotenoid and retinoid metabolism and lipoprotein biosynthesis; (ii) inactivate MTP by siRNA or an MTP inhibitor and (iii) Bco1 and Bco2 by siRNA; (iv) study the competing effects of apo10AL and RA on MTP expression. Aim 3B: To identify the cis-elements in the Mttp promoter and the transcription factors required for apo10AL-mediated increase in Mttp transcription. Promoter reporter assays, transcription factors knockdown, and ChIP assays will establish how placental Mttp expression is regulated by apo10AL through HNF4a and/or COUP-TFs.