Very Low‐Density Lipoprotein Signals through Scavenger Receptor Class B Type I And Sphingosine‐1‐Phosphate Receptor 1 to Induce Aldosterone Production, in part, via Lipin‐1 in Human Adrenocortical Cells

Abstract

Aldosterone is considered to be a link between hypertension and obesity; obese individuals have high serum levels of both very low-density lipoprotein (VLDL) and sphingosine-1-phosphate (S1P). S1P is transported in blood bound to lipoproteins such as VLDL, as well as low-density lipoprotein and high-density lipoprotein (HDL); the VLDL particle contains the highest levels of S1P. S1P in HDL has been shown to promote interactions between scavenger receptor class B type I (SR-BI) and S1P receptor 1 (S1PR1), suggesting the possibility that VLDL, which can also act through SR-BI, may stimulate S1PR1. VLDL has been shown to induce aldosterone production in multiple zona glomerulosa models, mediated in part by phospholipase D (PLD). PLD is an enzyme that hydrolyzes phosphatidylcholine to produce phosphatidic acid (PA), a lipid second messenger that can also be dephosphorylated by lipin-1 to yield diacylglycerol (DAG), yet another lipid second messenger. However, it is unclear which of the two lipid signals, PA or DAG, underlies PLD's mediation of aldosterone production. We hypothesized that upon binding to SR-BI, VLDL signals through S1PR1 to induce aldosterone production, in part, via lipin-1 mediated metabolism of PA to DAG. To determine whether VLDL functions through SR-BI and S1PR1, human adrenocortical (HAC15) cells were treated with VLDL and/or an S1PR1 antagonist (Ex26) for 24 h. The expression of steroidogenic genes and aldosterone production were monitored by qRT-PCR and radioimmunoassay, respectively. Ex26 inhibited VLDL-induced increases in CYP11B2 (22-fold) and StAR (1.5-fold) expression by 43% and 10%, respectively. In addition, the VLDL-induced 5-fold increase in aldosterone levels was significantly inhibited by Ex26 (36%). Our results suggest that VLDL signals by binding to SR-BI and activating S1PR1, since the S1PR1 antagonist reduced VLDL-induced aldosterone production. To assess the role of lipin-1 in VLDL-induced aldosterone production, lipin-1 was overexpressed (using an adenovirus) or inhibited (using propranolol) in HAC15 cells followed by treatment with or without VLDL for 24 h. While lipin-1 overexpression enhanced the VLDL-stimulated 55-fold increase in CYP11B2 expression by 75%, lipin-1 inhibition decreased the VLDL-stimulated 21-fold increase in CYP11B2 expression by 66%. Similarly, the VLDL-stimulated increase in aldosterone production was enhanced by lipin-1 overexpression (182%) and was decreased by lipin-1 inhibition (80%). Our results are suggestive of DAG being the key lipid signal since manipulating lipin-1 levels/activity affects VLDL-stimulated steroidogenic gene expression and ultimately, aldosterone production. Our study warrants further investigation into VLDL-stimulated steroidogenic signaling pathways which may lead to the identification of novel therapeutic targets, such as S1PR1, lipin-1, or their downstream pathways, to potentially treat obesity-associated hypertension.