The Inhibition of Macrophage Foam Cell Formation by 9-Cis β-Carotene Is Driven by BCMO1 Activity

Abstract

Atherosclerosis is a major cause of morbidity and mortality in developed societies, and begins when activated endothelial cells recruit monocytes and T-cells from the bloodstream into the arterial wall. Macrophages that accumulate cholesterol and other fatty materials are transformed into foam cells. Several epidemiological studies have demonstrated that a diet rich in carotenoids is associated with a reduced risk of heart disease; while previous work in our laboratory has shown that the 9-cis β-carotene rich alga Dunaliella inhibits atherogenesis in mice. The effect of 9-cis β-carotene on macrophage foam cell formation has not yet been investigated. In the present work, we sought to study whether the 9-cis β-carotene isomer, isolated from the alga Dunaliella, can inhibit macrophage foam cell formation upon its conversion to retinoids. The 9-cis β-carotene and Dunaliella lipid extract inhibited foam cell formation in the RAW264.7 cell line, similar to 9-cis retinoic acid. Furthermore, dietary enrichment with the algal powder in mice resulted in carotenoid accumulation in the peritoneal macrophages and in the inhibition of foam cell formation ex-vivo and in-vivo. We also found that the β-carotene cleavage enzyme β-carotene 15,15’-monooxygenase (BCMO1) is expressed and active in macrophages. Finally, 9-cis β-carotene, as well as the Dunaliella extract, activated the nuclear receptor RXR in hepa1-6 cells. These results indicate that dietary carotenoids, such as 9-cis β-carotene, accumulate in macrophages and can be locally cleaved by endogenous BCMO1 to form 9-cis retinoic acid and other retinoids. Subsequently, these retinoids activate the nuclear receptor RXR that, along with additional nuclear receptors, can affect various metabolic pathways, including those involved in foam cell formation and atherosclerosis.

Host‐related factors explaining interindividual variability of carotenoid bioavailability and tissue concentrations in humans

“…), are vitamin A precursors, constituting the predominant source of vitamin A in most developing countries (up to 90% ) as well as in Western countries especially with respect to vegetarians. Recently, it has also been suggested that cis ‐carotenoids are even more beneficial for the prevention of atherosclerosis and T2D than their all‐ trans isomers . Finally, it is now acknowledged that lutein and zeaxanthin play a role in vision by improving contrast sensitivity and visual acuity and participate in the prevention of age‐related macular degeneration .…”

Retinol but not retinoic acid can enhance the glutathione level, in a manner similar to β‐carotene, in a murine cultured macrophage cell line

“…Likewise, it was also reported there that the mRNA for βcarotene-15,15′-monooxygenase (BCMO1), which catalyzes the production of retinoids from β-carotene or β-cryptoxanthin, was not detected in RAW264 cells. In contradiction to that, Zolberg et al reported that BCMO1 protein and its product retinol were detected in RAW264.7 cells after the incubation with 9-cis βcarotene (Zolberg Relevy et al, 2015). Another possibility is that both β-carotene and retinol may, at least in part, have almost the same effect on the enhancement of GSH synthesis in RAW264 cells.…”

“…In contradiction to that, Zolberg et al. reported that BCMO1 protein and its product retinol were detected in RAW264.7 cells after the incubation with 9‐cis β‐carotene (Zolberg Relevy et al., ). Another possibility is that both β‐carotene and retinol may, at least in part, have almost the same effect on the enhancement of GSH synthesis in RAW264 cells.…”

Carotenoids Regulate Endothelial Functions and Reduce the Risk of Cardiovascular Disease

“…Furthermore, carotenoids induce increases in intracellular glutathione levels by elevating the activity of glutamate-cysteine ligase, the rate limiting enzyme in GSH synthesis [56]. In addition, preventive effects of β-carotene are associated with the β-carotene cleavage enzyme β-carotene 15,15′-monooxygenase (BCMO1) [57]. In the human macrophage cell line THP-1, β-carotene inhibited 7-ketocholesterol (7KC)-induced apoptosis by reducing expression levels of p53, p21, and Bax and inducing expression of AKT, Bcl-2, and Bcl-xL.…”