Very Low‐Density Lipoprotein (VLDL) Signals Through Sphingosine‐1‐Phosphate Receptor 1, Upon Binding to VLDL Receptor, to Induce Aldosterone Synthesis and Secretion in Human Adrenocortical Cells

Abstract

Hypertension is prevalent in 46% of the US adult population with 40% of cases being attributable to obesity. Obese individuals have high serum levels of very low‐density lipoprotein (VLDL) and sphingosine‐1‐phosphate (S1P). VLDL and S1P have been shown to stimulate aldosterone production in multiple zona glomerulosa cell models, with aldosterone thought to be the link between hypertension and obesity. S1P is transported in blood bound to lipoproteins such as VLDL, low‐density lipoprotein (LDL), and high‐density lipoprotein (HDL); the VLDL particle contains the highest S1P levels. S1P in HDL has been shown to promote interactions between the scavenger receptor class B, type I (SR‐BI) and S1P receptor 1 (S1PR1). We hypothesized that like HDL, VLDL will signal through S1PRs, upon binding to SR‐BI; therefore, VLDL‐induced aldosterone secretion will be inhibited by S1PR and SR‐BI antagonists. Human adrenocortical (HAC15) cells were treated with VLDL and/or an S1PR1 antagonist (Ex26) or an anti‐SR‐BI blocking antibody for 24 h. Steroidogenic gene expression and aldosterone secretion were monitored by qRT‐PCR and radioimmunoassay, respectively. Ex26 significantly inhibited VLDL‐induced increases in CYP11B2 (22‐fold) and StAR (1.5‐fold) expression and aldosterone secretion (5‐fold) by 43%, 10%, and 36%, respectively. Unexpectedly, the anti‐SR‐BI antibody enhanced VLDL‐induced increases in CYP11B2 (29‐fold) and NR4A1 (2.2‐fold) expression by 17% and 33%, respectively; no effect was observed on VLDL‐induced increases in StAR and NR4A2 expression. However, when the VLDL receptor was blocked with recombinant human LDL receptor related protein associated protein 1, VLDL‐induced increases in CYP11B2 (2.8‐fold) and NR4A2 (5.3‐fold) expression were significantly reduced by 42% and 67%, respectively. Our results, therefore, indicate that VLDL signals through S1PR1, upon binding to the VLDL receptor and not SR‐BI, to induce aldosterone synthesis and secretion. Our study warrants further investigation into VLDL‐induced steroidogenic signaling pathways which may lead to the identification of novel therapeutic targets like S1PR1 to potentially treat obesity‐associated hypertension.