Genetic deletion of Bco2 and Isx establishes a golden mouse model for carotenoid research

Low plasma levels of carotenoids are associated with mortality and chronic disease states. Genetic studies in animals revealed that the tissue accumulation of these dietary pigments is associated with the genes encoding β-carotene oxygenase 2 (BCO2) and the scavenger receptor class B type 1 (SR-B1). Here we examined in mice how BCO2 and SR-B1 affect the metabolism of the model carotenoid zeaxanthin that serves as a macular pigment in the human retina.

We used mice with a lacZ reporter gene knock-in to determine Bco2 expression patterns in the small intestine. By genetic dissection, we studied the contribution of BCO2 and SR-B1 to zeaxanthin uptake homeostasis and tissue accumulation under different supply conditions (50 mg/kg and 250 mg/kg). We determined the metabolic profiles of zeaxanthin and its metabolites in different tissues by LC-MS using standard and chiral columns. An albino Isx^-/-/Bco2^-/- mouse homozygous for Tyr^c-2J was generated to study the effect of light on ocular zeaxanthin metabolites.

We demonstrate that BCO2 is highly expressed in enterocytes of the small intestine. Genetic deletion of Bco2 led to enhanced accumulation of zeaxanthin, indicating that the enzyme serves as a gatekeeper of zeaxanthin bioavailability. Relaxing the regulation of SR-B1 expression in enterocytes by genetic deletion of the transcription factor ISX further enhanced zeaxanthin accumulation in tissues. We observed that the absorption of zeaxanthin was dose-dependent and identified the jejunum as the major zeaxanthin-absorbing intestinal region. We further showed that zeaxanthin underwent oxidation to ε,ε-3,3'-carotene-dione in mouse tissues. We detected all three enantiomers of the zeaxanthin oxidation product whereas the parent zeaxanthin only existed as (3R, 3'R)-enantiomer in the diet. The ratio of oxidized to parent zeaxanthin varied between tissues and was dependent on the supplementation dose. We further showed in an albino Isx^-/-/Bco2^-/- mouse that supra-physiological supplementation doses (250 mg/kg) with zeaxanthin rapidly induced hypercarotenemia with a golden skin phenotype and that light stress increased the concentration of oxidized zeaxanthin in the eyes.

We established the biochemical basis of zeaxanthin metabolism in mice and showed that tissue factors and abiotic stress affect the metabolism and homeostasis of this dietary lipid.