Twinkle, twinkle little StAR, how we wonder what you are

The Herbicide Atrazine Potentiates Angiotensin II-Induced Aldosterone Synthesis and Release From Adrenal Cells

Atrazine is one of the most commonly used pre-emergence and early post-emergence herbicides in the world. We have shown previously that atrazine does not directly stimulate the pituitary or adrenal to trigger hormone release but acts centrally to activate a stress-like activation of the hypothalamic-pituitary-adrenal axis. In doing so, atrazine treatment has been shown to cause adrenal morphology changes characteristic of repeated stress. In this study, adrenals from atrazine treated and stressed animals were directly compared after 4 days of atrazine treatment or restraint stress. Both atrazine and stressed animals displayed reduced adrenocortical zona glomerulosa thickness and aldosterone synthase (CYP11B2) expression, indicative of repeated adrenal stimulation by adrenocorticotropic hormone. To determine if reduced CYP11B2 expression resulted in attenuated aldosterone synthesis, stressed and atrazine treated animals were challenged with angiotensin II (Ang II). As predicted, stressed animals produced less aldosterone compared to control animals when stimulated. However, atrazine treated animals had higher circulating aldosterone concentrations compared to both stressed and control groups. Ang II-induced aldosterone release was also potentiated in atrazine pretreated human adrenocortical carcinoma cells (H295R). Atrazine pretreated did not alter the expression of the rate limiting steroidogenic StAR protein or angiotensin II receptor 1. Atrazine treated animals also presented with higher basal blood pressure than vehicle treated control animals suggesting sustained elevations in circulating aldosterone levels. Our results demonstrate that treatment with the widely used herbicide, atrazine, directly increases stimulated production of aldosterone in adrenocortical cells independent of expression changes to rate limiting steroidogenic enzymes.

CRISPR/Cas9‒Mediated Tspo Gene Mutations Lead to Reduced Mitochondrial Membrane Potential and Steroid Formation in MA-10 Mouse Tumor Leydig Cells

The outer mitochondrial membrane translocator protein (TSPO) binds cholesterol with high affinity and is involved in mediating its delivery into mitochondria, the rate-limiting step in hormone-induced steroidogenesis. Specific ligand binding to TSPO has been shown to initiate steroid formation. However, recent studies of the genetic deletion of Tspo have provided conflicting results. Here, we address and extend previous studies by examining the effects of Tspo-specific mutations on steroid formation in hormone- and cyclic adenosine monophosphate (cAMP)-responsive MA-10 cells, using the CRISPR/Cas9 system. Two mutant subcell lines, nG1 and G2G, each carrying a Tspo exon2-specific genome modification, and two control subcell lines, G1 and HH, each carrying a wild-type Tspo, were produced. In response to dibutyryl cAMP, the nG1 and G2G cells produced progesterone at levels significantly lower than those produced by the corresponding control cells G1 and HH. Neutral lipid homeostasis, which provides free cholesterol for steroid biosynthesis, was altered significantly in the Tspo mutant cells. Interestingly, the mitochondrial membrane potential (ΔΨm) of the Tspo mutant cells was significantly reduced compared with that of the control cells, likely because of TSPO interactions with the voltage-dependent anion channel and tubulin at the outer mitochondrial membrane. Steroidogenic acute regulatory protein (STAR) expression was induced in nG1 cells, suggesting that reduced TSPO affected STAR synthesis and/or processing. Taken together, these results provide further evidence for the critical role of TSPO in steroid biosynthesis and suggest that it may function at least in part via its regulation of ΔΨm and effects on STAR.

Mutated KCNJ5 activates the acute and chronic regulatory steps in aldosterone production

Somatic and germline mutations in the inward-rectifying K(+) channel (KCNJ5) are a common cause of primary aldosteronism (PA) in aldosterone-producing adenoma and familial hyperaldosteronism type III, respectively. Dysregulation of adrenal cell calcium signaling represents one mechanism for mutated KCNJ5 stimulation of aldosterone synthase (CYP11B2) expression and aldosterone production. However, the mechanisms stimulating acute and chronic production of aldosterone by mutant KCNJ5 have not been fully characterized. Herein, we defined the effects of the T158A KCNJ5 mutation (KCNJ5(T158A)) on acute and chronic regulation of aldosterone production using an adrenal cell line with a doxycycline-inducible KCNJ5(T158A) gene (HAC15-TRE-KCNJ5(T158A)). Doxycycline incubation caused a time-dependent increase in KCNJ5(T158A) and CYP11B2 mRNA and protein levels. Electrophysiological analyses confirm the loss of inward rectification and increased Na(+) permeability in KCNJ5(T158A)-expressing cells. KCNJ5(T158A) expression also led to the activation of CYP11B2 transcriptional regulators, NURR1 and ATF2. Acutely, KCNJ5(T158A) stimulated the expression of total and phosphorylated steroidogenic acute regulatory protein (StAR). KCNJ5(T158A) expression increased the synthesis of aldosterone and the hybrid steroids 18-hydroxycortisol and 18-oxocortisol, measured with liquid chromatography-tandem mass spectrometry (LC-MS/MS). All of these stimulatory effects of KCNJ5(T158A) were inhibited by the L-type Ca(2+) channel blocker, verapamil. Overall, KCNJ5(T158A)increases CYP11B2 expression and production of aldosterone, corticosterone and hybrid steroids by upregulating both acute and chronic regulatory events in aldosterone production, and verapamil blocks KCNJ5(T158A)-mediated pathways leading to aldosterone production.

Growth hormone-regulated mRNAs and miRNAs in chicken hepatocytes

Growth hormone (GH) is a key regulatory factor in animal growth, development and metabolism. Based on the expression level of the GH receptor, the chicken liver is a major target organ of GH, but the biological effects of GH on the chicken liver are not fully understood. In this work we identified mRNAs and miRNAs that are regulated by GH in primary hepatocytes from female chickens through RNA-seq, and analyzed the functional relevance of these mRNAs and miRNAs through GO enrichment analysis and miRNA target prediction. A total of 164 mRNAs were found to be differentially expressed between GH-treated and control chicken hepatocytes, of which 112 were up-regulated and 52 were down-regulated by GH. A total of 225 chicken miRNAs were identified by the RNA-Seq analysis. Among these miRNAs 16 were up-regulated and 1 miRNA was down-regulated by GH. The GH-regulated mRNAs were mainly involved in growth and metabolism. Most of the GH-upregulated or GH-downregulated miRNAs were predicted to target the GH-downregulated or GH-upregulated mRNAs, respectively, involved in lipid metabolism. This study reveals that GH regulates the expression of many mRNAs involved in metabolism in female chicken hepatocytes, which suggests that GH plays an important role in regulating liver metabolism in female chickens. The results of this study also support the hypothesis that GH regulates lipid metabolism in chicken liver in part by regulating the expression of miRNAs that target the mRNAs involved in lipid metabolism.

The role of mitochondrial DNA mutations in mammalian aging

Mitochondrial DNA (mtDNA) accumulates both base-substitution mutations and deletions with aging in several tissues in mammals. Here, we examine the evidence supporting a causative role for mtDNA mutations in mammalian aging. We describe and compare human diseases and mouse models associated with mitochondrial genome instability. We also discuss potential mechanisms for the generation of these mutations and the means by which they may mediate their pathological consequences. Strategies for slowing the accumulation and attenuating the effects of mtDNA mutations are discussed.

Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis

Cholesterol transport from the outer to the inner mitochondrial membrane is the rate-determining step in steroid and bile acid biosyntheses. Biochemical, pharmacological and molecular studies have demonstrated that the peripheral-type benzodiazepine receptor (PBR) is a five transmembrane domain mitochondrial protein involved in the regulation of cholesterol transport. PBR gene disruption in Leydig cells completely blocked cholesterol transport into mitochondria and steroid formation, while PBR expression in bacteria, devoid of endogenous PBR and cholesterol, induced cholesterol uptake and transport. Molecular modeling of PBR suggested that cholesterol might cross the membrane through the five helices of the receptor and that synthetic and endogenous ligands might bind to common sites in the cytoplasmic loops. A cholesterol recognition/interaction amino acid consensus (CRAC) sequence in the cytoplasmic carboxy-terminus of the PBR was identified by mutagenesis studies. In vitro reconstitution of PBR into proteoliposomes demonstrated that PBR binds both drug ligands and cholesterol with high affinity. In vivo polymeric forms of PBR were observed and polymer formation was reproduced in vitro, using recombinant PBR protein reconstituted into proteoliposomes, associated with an increase in drug ligand binding and reduction of cholesterol-binding capacity. This suggests that the various polymeric states of PBR might be part of a cycle mediating cholesterol uptake and release into the mitochondria, with PBR functioning as a cholesterol exchanger against steroid product(s) arising from cytochrome P450 action. Taking into account the widespread presence of PBR in many tissues, a more general role of PBR in intracellular cholesterol transport and compartmentalization might be considered.

Cardiotonic steroids: potential endogenous sodium pump ligands with diverse function

The highly conserved cardiotonic steroid (CS) binding site present on the ubiquitous membrane sodium pump, sodium, potassium-ATPase, appears to have been conserved by no force other than its capacity to bind CS: a family that includes plant-derived cardiac glycosides and putative endogenous vertebrate counterparts. Binding of ligand is inhibited by increased extracellular potassium. This implies functional coordination because inhibition of the sodium pump would be counterproductive when extracellular potassium is elevated. The interesting biology of the CS binding site continues to stimulate investigations into the identity of endogenous ligands, their role as pump regulators at the cellular level, and as mediators of body fluid balance and blood pressure regulation. In addition to inhibition of sodium and potassium transport, there is considerable recent evidence suggesting that the sodium pump may act as a cell signaling receptor activated by CS binding and responding by coordination of intracellular signaling pathways that can be dependent on and also independent of the reduction in transmembrane ion flux resulting directly from pump inhibition. This signaling may influence cell survival, growth, and differentiation. Recent insight into the biology of pump regulation by CS is reviewed.

Lipoproteins regulate expression of the steroidogenic acute regulatory protein (StAR) in mouse adrenocortical cells

The steroidogenic acute regulatory protein (StAR) is required for the movement of cholesterol from the outer to the inner mitochondrial membrane, the site of cholesterol side chain cleavage. Here we describe a novel form of regulation of StAR gene expression in steroidogenic cells. Treatment of Y-1 BS1 adrenocortical cells with either low density lipoprotein (LDL) or high density lipoprotein (HDL) increases expression of endogenous StAR mRNA and protein in a dose-dependent manner. Induction of StAR mRNA by lipoprotein requires basal cAMP-dependent protein kinase, since the inhibitor, R(p)-8-Br-cAMP, inhibited induction of StAR protein by LDL. Likewise, basal StAR expression or LDL induction of StAR protein was not detectable in Y-1 kin-8 cells which are deficient in cAMP-dependent protein kinase. Aminoglutethimide and ketoconazole were used to determine if side chain cleavage of lipoprotein-derived cholesterol is required for induction of StAR mRNA. Treatment with either drug alone induced StAR mRNA expression 1.5-3-fold, while induction of StAR in cells treated with either drug plus LDL, was equal to, or greater than, induction seen with either agent alone, suggesting that lipoprotein does not regulate StAR via generation of an oxysterol intermediate. Both LDL and HDL increased expression of a mouse -966 StAR promoter-reporter construct 1.5-2.5-fold, indicating that regulation occurs at the level of transcription. In contrast, neither lipoprotein was able to induce transcription from a -966 StAR promoter in which the steroidogenic factor-1 site at -135 was abolished, indicating that regulation of StAR transcription by lipoproteins requires steroidogenic factor-1. The regulation of StAR gene expression by lipoproteins may represent a positive feedback circuit which links cholesterol availability with steroidogenic output.