Scavenger receptor class B type I-mediated uptake of serum cholesterol is essential for optimal adrenal glucocorticoid production

Abstract

Impaired scavenger receptor class B type I (SR-BI)-mediated uptake of HDL-cholesterol esters (HDL-CE) induces adrenal insufficiency in mice. Humans contain an alternative route of HDL-CE clearance, namely through the transfer by cholesteryl ester transfer protein (CETP) to apolipoprotein B lipoproteins for subsequent uptake via the LDL receptor. In this study, we determined whether CETP can compensate for loss of adrenal SR-BI. Transgenic expression of human CETP (CETP Tg) in SR-BI knockout (KO) mice increased adrenal HDL-CE clearance from 33-58% of the control value. SR-BI KO/CETP Tg and SR-BI KO mice displayed adrenal hypertrophy due to equally high plasma adrenocorticotropic hormone levels. Adrenal cholesterol levels and plasma corticosterone levels were 38-52% decreased in SR-BI KO mice with and without CETP expression. SR-BI KO/CETP Tg mice also failed to increase their corticosterone level after lipopolysaccharide challenge, leading to an identical >4-fold increased tumor necrosis factor-alpha response compared with controls. These data indicate that uptake of CE via other routes than SR-BI is not sufficient to generate the cholesterol pool needed for optimal adrenal steroidogenesis. In conclusion, we have shown that CETP-mediated transfer of HDL-CE is not able to reverse adrenal insufficiency in SR-BI knockout mice. Thus, SR-BI-mediated uptake of serum cholesterol is essential for optimal adrenal function.

Fig. 1.

A: Plasma total cholesterol levels in WT (white bars), control SR-BI KO mice (hatched bars), and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; black bars). B) The adrenal uptake of HDL-cholesteryl ether (CEt) in WT (white bars), control SR-BI KO mice (hatched bars), and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; black bars). Mice were injected intravenously with [³H]CEt-HDL, and after 24 h the amount of radioactivity in the adrenals was determined. Values are expressed as percentage of the injected dose per gram tissue (%ID/g) and are means ± SEM (n = 3). C: Distribution of [³H]CEt over the different lipoprotein fractions generated from pooled plasma of three mice at 8 h after injection of the [³H]CEt-HDL in control SR-BI KO mice (hatched bars) and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; black bars). * P < 0.05, ** P < 0.01, and *** P < 0.001 (one-way ANOVA).

Fig. 2.

A: Photograph of adrenals from WT, control SR-BI KO mice, and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg). B: Weight of the adrenals from WT (white bars), SR-BI KO mice (hatched bars), and SR-BI KO/CETP Tg mice (black bars). Values are means ± SEM (n = 3). * P < 0.05 (one-way ANOVA).

Fig. 3.

A: Oil red O neutral lipid staining on cryosections of adrenals from WT mice, SR-BI KO mice, and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg). Sections were counterstained with hematoxylin for nuclei. Note the clearly diminished lipid staining in the adrenal cortex of SR-BI KO mice, which is not reversed by expression of human CETP. B: Immunohistochemical staining for CETP on adrenal sections from SR-BI KO/CETP Tg mice. CETP protein expression is localized specifically in the zona glomerulosa of the adrenal cortex.

Fig. 4.

Plasma corticosterone levels in WT (white bars), control SR-BI KO mice (hatched bars) mice, and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; black bars) that were nonfasted (FED) or subjected to 16 h of fasting (FASTED). Values are means ± SEM (n = 4–15). Fasting induced a significant rise in plasma corticosterone levels (P < 0.001). ** P < 0.01 compared with fasted WT mice (two-way ANOVA).

Fig. 5.

Plasma ACTH levels (A) and adrenal relative LDL receptor (LDLR) mRNA expression levels (B) in WT (white bars), control SR-BI KO mice (hatched bars), and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; black bars) that were subjected to 16 h of fasting. Values are means ± SEM (n = 3–14). * P < 0.05 and ** P < 0.01 (one-way ANOVA).

Fig. 6.

LPS-induced TNF-α response (A) and corticosterone response (B) in WT (open circles), control SR-BI KO mice (closed circles), and SR-BI KO mice expressing human CETP (SR-BI KO/CETP Tg; closed triangles). Mice were injected intravenously with 50 μg/kg LPS. Values are means ± SEM (n = 4–6). The TNF-α and corticosterone responses in SR-BI KO/CETP and SR-BI KO mice were essentially the same and significantly different from those of WT mice (P < 0.001 for both; two-way ANOVA).