No abstract

Aldosterone is considered to be a link between hypertension and obesity; obese individuals have high serum levels of very low-density lipoprotein (VLDL). VLDL has been shown to induce aldosterone production in multiple adrenal 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 to yield diacylglycerol (DAG), yet another lipid signal. However, it is unclear which of the two lipid second messengers, PA or DAG, underlies PLD’s mediation of aldosterone production. We hypothesized that the key signal produced by PLD (indirectly) is DAG such that PLD mediates VLDL-induced aldosterone production via lipin-mediated metabolism of PA to DAG. To assess the role of lipin 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. Lipin-1 overexpression enhanced the VLDL-stimulated increase in CYP11B2 expression (by 75%), lipin-1 inhibition decreased the VLDL-stimulated 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 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 lipin-1 and its downstream pathways, to potentially treat obesity-associated hypertension.

Primary aldosteronism (PA), in which plasma aldosterone levels are normal or elevated relative to suppressed plasma renin levels, is the most frequent cause of secondary hypertension. PA accounts for 5‐10% of hypertension cases and up to 20% in those with resistant hypertension. Aldosterone is a mineralocorticoid hormone responsible for maintaining fluid volume, electrolyte balance, and blood pressure homeostasis; it is tightly regulated by the renin‐angiotensin‐aldosterone system. Our laboratory has previously shown that phospholipase D2 (PLD2) mediates angiotensin II‐induced aldosterone synthesis and secretion in vitro. Therefore, we hypothesized that PLD2 loss would result in decreased aldosterone production in vivo. To test this hypothesis, 11‐week‐old male and female wild‐type and PLD2‐/‐ mice were fed a high salt (1% NaCl) or a low salt (0.02% NaCl; to raise endogenous serum angiotensin II levels) diet for 24 hours. Adrenal steroidogenic gene expression and serum aldosterone, corticosterone, and electrolytes were measured by qRT‐PCR, radioimmunoassay, ELISA, and atomic absorption spectrometry, respectively. The PLD2‐/‐ male mice exhibited increased StAR, Dab2, and CYP11B1 expression; further, PLD2 loss impaired low salt‐induced increases in StAR, Dab2, and CYP11B1 expression; however, PLD2 loss had no effect on CYP11B2 expression or serum aldosterone, Na+, K+, and corticosterone levels in these male mice. These data suggest that increased steroidogenic enzyme expression may compensate for PLD2 loss in order to maintain aldosterone secretion in the males. On the other hand, the PLD2‐/‐ female mice exhibited decreased StAR and CYP11B1 expression and serum Na+ levels as well as impaired low salt‐induced increases in StAR, CYP11B2, Dab2, and CYP11B1 expression. PLD2 loss had no effect on serum aldosterone, K+, or corticosterone levels in these female mice. It is possible that other compensatory mechanisms (not involving steroidogenic enzymes) might be involved in maintaining aldosterone secretion in the females. Together, our findings suggest a likely importance of PLD2 in mediating aldosterone production and steroidogenic enzyme gene expression, with differential effects in male and female mice.

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.

Indoleamine-2,3-dioxygenase (IDO) degrades the essential amino acid tryptophan resulting in tryptophan depletion and the accumulation of catabolites such as kynurenine. The expression/activity of IDO in various cells, including macrophages and dendritic cells, results in an inhibition of T-cell responses in a number of situations, such as toward allogeneic fetuses and tissue grafts. Psoriasis is an immune-mediated skin disease involving T cells; kynureninase and its generation of catabolites downstream of IDO are reported to play an important role in this disease. We hypothesized that mice lacking the IDO1 gene would exhibit a hyperactive immune response and an exacerbation of skin lesions in the imiquimod-induced mouse model of psoriasis. Littermate wild-type and IDO1-knockout mice were treated with imiquimod for 5 days, and the severity of psoriasiform skin lesions assessed using the psoriasis area and severity index (PASI), ear edema measured using a digital caliper, and thickness of the epidermis determined by histology. Expression of pro-inflammatory mediators and tryptophan-metabolizing enzymes was monitored using quantitative RT-PCR. Imiquimod increased ear edema, PASI scores, and epidermal thickness in both WT and IDO1 knockout mice; however, there were no differences observed between the 2 genotypes. There were also no differences in imiquimod’s induction of skin inflammatory mediators, indicating no effect of IDO1 gene loss in this psoriasis model. Although these data suggest a lack of involvement of IDO1 in psoriatic skin inflammation, other possible mechanisms, such as compensatory changes in other pathways and the involvement of the IDO2 isoform, must also be considered.