Study identifies transporter protein responsible for delivering carotenoids to RPE cells

Retinal pigment epithelial cells appear to preferentially accumulate the xanthophylls lutein and zeaxanthin over beta-carotene by a process that is dependent on the protein SR-BI, according to a study.

"These data provide new mechanistic information about the preferential accumulation of the xanthophylls in the human eye and strongly suggest the participation of the transporter SR-BI," the study authors said. "Further investigations will be necessary to confirm this suggestion, since the human ARPE-19 cells may not fully recapitulate the properties of the RPE in vivo."

In order to examine the mechanisms through which carotenoids accumulate in RPE cells, Alexandrine During, PhD, and colleagues compared cellular uptakes after introducing lutein, zeaxanthin and beta-carotene into the human RPE cell line ARPE-19.

Once the ARPE-19 cells had fully differentiated, the investigators found that the RPE cells had absorbed approximately twice as much of the xanthophylls lutein and zeaxanthin than beta-carotene (P < .05).

At 9 weeks, cellular uptakes were 1.6% for beta-carotene, 2.5% for lutein and 3.2% for zeaxanthin, they noted.

When using specific, targeted antibodies against the lipid transporters cluster determinant 36 (CD36) and scavenger receptor class B type I (SR-BI), RPE cellular uptake of beta-carotene and zeaxanthin was significantly decreased by between 40% and 60% in the presence of anti-SR-BI but not with anti-CD36.

In addition, the investigators found that using small interfering RNA against SR-BI yielded a 90% inhibition of SR-BI protein expression, which resulted in a 51% reduction in beta-carotene and an 87% reduction in zeaxanthin, according to the study, published in the August issue of Journal of Lipid Research.