Endoplasmic Reticulum Stress Enhances Mitochondrial Metabolic Activity in Mammalian Adrenals and Gonads

Dry molten globule conformational state of CYP11A1 (SCC) regulates the first step of steroidogenesis in the mitochondrial matrix

Multiple metabolic events occur in mitochondria. Mitochondrial protein translocation from the cytoplasm across compartments depends on the amino acid sequence within the precursor. At the mitochondria associated-ER membrane, misfolding of a mitochondrial targeted protein prior to import ablates metabolism. CYP11A1, cytochrome P450 cholesterol side chain cleavage enzyme (SCC), is imported from the cytoplasm to mitochondrial matrix catalyzing cholesterol to pregnenolone, an essential step for metabolic processes and mammalian survival. Multiple steps regulate the availability of an actively folded SCC; however, the mechanism is unknown. We identified that a dry molten globule state of SCC exists in the matrix by capturing intermediate protein folding steps dictated by its C-terminus. The intermediate dry molten globule state in the mitochondrial matrix of living cells is stable with a limited network of interaction and is inactive. The dry molten globule is activated with hydrogen ions availability, triggering cleavage of cholesterol sidechain, and initiating steroidogenesis.

Mitochondria are midfield players in steroid synthesis

Steroids are important membrane components and signaling metabolites and thus are required for cellular homeostasis. All mammalian cells retain the ability to uptake and synthesize steroids. Dysregulation of steroid levels leads to profound effects on cellular function and organismal health. Hence it comes as no surprise that steroid synthesis is tightly regulated. It is well established that the main site for steroid synthesis and regulation is the endoplasmic reticulum. However, mitochondria are essential for: (1) cholesterol production (the precursor of all steroids) by exporting citrate and; (2) the products of steroidogenesis (such as mineralocorticoids and glucocorticoids). In this review, we describe the midfield player role of mitochondria in steroid synthesis and bring the idea of mitochondria actively participating in steroid synthesis regulation. A better understanding of the mitochondrial regulatory roles in steroid synthesis would open new avenues to targeted approaches aiming to control steroid levels.

Protocol for direct measurement of stability and activity of mitochondria electron transport chain complex II

Mitochondria electron transport chain (ETC) complex II is essential for steroid metabolism. Here, we present a protocol to measure the stability and activity of mitochondria ETC complex II. We first describe mitochondria isolation from cell lines and tissues. We then detail how to determine the stability of ETC complex II using isothermal calorimetry and quantification of steroidogenesis using activity assays in parallel. Finally, we describe the steps to perform radioimmunoassay (RIA) to confirm the activity of ETC complex II. For complete details on the use and execution of this protocol, please refer to Bose et al. (2020).1.

Absence of Gem1 (mammalian Miro/Rhot) mitigates alpha-synuclein toxicity in a yeast model of Parkinson's disease

Alpha-synuclein aggregation is a hallmark of Parkinson's disease (PD). Mutants A30P and A53T alpha-synuclein are known to exacerbate the toxicity of alpha-synuclein, which includes oxidative stress, mitochondrial and endoplasmic reticulum (ER) dysfunction. Saccharomyces cerevisiae (budding yeast) is a cellular model widely used to investigate mechanisms underlying neurodegenerative disorders, such as PD. In yeast, Gem1 (Miro/Rhot mammalian orthologue) coordinates mitochondrial dynamics and ER homeostasis, which is impaired in the presence of mutant alpha-synuclein and can lead to cell death. In this study, A30P or A53T alpha-synuclein were expressed in wild type or ΔGem (deletion of Gem1 gene) yeast strains. ΔGem cells presented decreased viability and increased mitochondrial H2O2 production and ER stress compared to wild type cells. However, in the presence of mutant alpha-synuclein, ΔGem cells showed increased growth compared to cells that do not express mutant alpha-synuclein. ΔGem cells expressing A53T alpha-synuclein also presented reduced ER stress and increased ability to deal with oxidative stress. Together, our results suggest that deletion of Gem1 activates pathways that strengthen cells against other stressful agents such as the presence of mutant alpha-synuclein.

Proteostasis and resilience: on the interphase between individual's and intracellular stress

A long proportion of the population is resilient to the negative consequences of stress. Glucocorticoids resulting from endocrine responses to stress are essential adaptive mediators, but also drive alterations to brain function, negatively impacting neuronal connectivity, synaptic plasticity, and memory-related processes. Recent evidence has indicated that organelle function and cellular stress responses are relevant determinant of vulnerability and resistance to environmental stress. At the molecular level, a fundamental mechanism of cellular stress adaptation is the maintenance of proteostasis, which also have key roles in sustaining basal neuronal function. Here, we discuss recent evidence suggesting that proteostasis unbalance at the level of the endoplasmic reticulum, the main site for protein folding in the cell, represents a possible mechanistic link between individuals and cellular stress.

Regulation of Estradiol Synthesis by Aromatase Interacting Partner in Breast (AIPB)

Estradiol is essential for the development of female sex characteristics and fertility. Postmenopausal women and breast cancer patients have high levels of estradiol. Aromatase catalyzes estradiol synthesis; however, the factors regulating aromatase activity are unknown. We identified a new 22-kDa protein, aromatase interacting partner in breast (AIPB), from the endoplasmic reticulum of human breast tissue. AIPB expression is reduced in tumorigenic breast and further reduced in triple-negative tumors. Like that of aromatase, AIPB expression is induced by nonsteroidal estrogen. We found that AIPB and aromatase interact in nontumorigenic and tumorigenic breast tissues and cells. In tumorigenic cells, conditional AIPB overexpression decreased estradiol, and blocking AIPB availability with an AIPB-binding antibody increased estradiol. Estradiol synthesis is highly increased in AIPB knockdown cells, suggesting that the newly identified AIPB protein is important for aromatase activity and a key modulator of estradiol synthesis. Thus, a change in AIPB protein expression may represent an early event in tumorigenesis and be predictive of an increased risk of developing breast cancer.

Distinctive functioning of STARD1 in the fetal Leydig cells compared to adult Leydig and adrenal cells. Impact of Hedgehog signaling via the primary cilium

STARD1 stimulates cholesterol transfer to mitochondrial CYP11A1 for conversion to pregnenolone. A cholesterol-binding START domain is guided by an N-terminal domain in a cell selective manner. Fetal and adult Leydig cells (FLC, ALC) show distinct Stard1 regulation. sm- FISH microscopy, which resolves individual molecules of Stard1 mRNA, shows uniformly high basal expression in each FLC. In ALC, in vivo, and cultured MA-10 cells, basal Stard1 expression is minimal. PKA activates loci asynchronously, with delayed splicing/export of 3.5 kb mRNA to mitochondria. After 60 min, ALC transition to an integrated mRNA delivery to mitochondria that is seen in FLC. Sertoli cells cooperate in Stard1 stimulation in FLC by delivering DHH to the primary cilium. There PTCH, SMO and cholesterol cooperate to release GLI3 to activate the Stard1 locus, probably by directing histone changes. ALC lack cilia. PKA then primes locus activation. FLC and ALC share similar SIK/CRTC/CREB regulation characterized for adrenal cells.

Mitochondrial function in development and disease

Mitochondria are organelles with vital functions in almost all eukaryotic cells. Often described as the cellular ‘powerhouses’ due to their essential role in aerobic oxidative phosphorylation, mitochondria perform many other essential functions beyond energy production. As signaling organelles, mitochondria communicate with the nucleus and other organelles to help maintain cellular homeostasis, allow cellular adaptation to diverse stresses, and help steer cell fate decisions during development. Mitochondria have taken center stage in the research of normal and pathological processes, including normal tissue homeostasis and metabolism, neurodegeneration, immunity and infectious diseases. The central role that mitochondria assume within cells is evidenced by the broad impact of mitochondrial diseases, caused by defects in either mitochondrial or nuclear genes encoding for mitochondrial proteins, on different organ systems. In this Review, we will provide the reader with a foundation of the mitochondrial ‘hardware’, the mitochondrion itself, with its specific dynamics, quality control mechanisms and cross-organelle communication, including its roles as a driver of an innate immune response, all with a focus on development, disease and aging. We will further discuss how mitochondrial DNA is inherited, how its mutation affects cell and organismal fitness, and current therapeutic approaches for mitochondrial diseases in both model organisms and humans.

Summary: Mitochondria have a plethora of functions beyond metabolism. This Review discusses the emerging and multifaceted roles of mitochondria in different model organisms and human disease biology.

Oxidative Stress and Endoplasmic Reticulum Stress in Rare Respiratory Diseases

Several studies have shown that some rare respiratory diseases, such as alpha-1 antitrypsin deficiency (AATD), idiopathic pulmonary fibrosis (IPF), cystic fibrosis (CF), and primary ciliary dyskinesia (PCD) present oxidative stress (OS) and endoplasmic reticulum (ER) stress. Their involvement in these pathologies and the use of antioxidants as therapeutic agents to minimize the effects of OS are discussed in this review.

A Novel Mitochondrial Complex of Aldosterone Synthase, Steroidogenic Acute Regulatory Protein, and Tom22 Synthesizes Aldosterone in the Rat Heart

Aldosterone, which regulates renal salt retention, is synthesized in adrenocortical mitochondria in response to angiotensin II. Excess aldosterone causes myocardial injury and heart failure, but potential intracardiac aldosterone synthesis has been controversial. We hypothesized that the stressed heart might produce aldosterone. We used blue native gel electrophoresis, immunoblotting, protein crosslinking, coimmunoprecipitations, and mass spectrometry to assess rat cardiac aldosterone synthesis. Chronic infusion of angiotensin II increased circulating corticosterone levels 350-fold and induced cardiac fibrosis. Angiotensin II doubled and telmisartan inhibited aldosterone synthesis by heart mitochondria and cardiac production of aldosterone synthase (P450c11AS). Heart aldosterone synthesis required P450c11AS, Tom22 (a mitochondrial translocase receptor), and the intramitochondrial form of the steroidogenic acute regulatory protein (StAR); protein crosslinking and coimmunoprecipitation studies showed that these three proteins form a 110-kDa complex. In steroidogenic cells, extramitochondrial (37-kDa) StAR promotes cholesterol movement from the outer to inner mitochondrial membrane where cholesterol side-chain cleavage enzyme (P450scc) converts cholesterol to pregnenolone, thus initiating steroidogenesis, but no function has previously been ascribed to intramitochondrial (30-kDa) StAR; our data indicate that intramitochondrial 30-kDa StAR is required for aldosterone synthesis in the heart, forming a trimolecular complex with Tom22 and P450c11AS. This is the first activity ascribed to intramitochondrial StAR, but how this promotes P450c11AS activity is unclear. The stressed heart did not express P450scc, suggesting that circulating corticosterone (rather than intracellular cholesterol) is the substrate for cardiac aldosterone synthesis. Thus, the stressed heart produced aldosterone using a previously undescribed intramitochondrial mechanism that involves P450c11AS, Tom22, and 30-kDa StAR. SIGNIFICANCE STATEMENT: Prior studies of potential cardiac aldosterone synthesis have been inconsistent. This study shows that the stressed rat heart produces aldosterone by a novel mechanism involving aldosterone synthase, Tom22, and intramitochondrial steroidogenic acute regulatory protein (StAR) apparently using circulating corticosterone as substrate. This study establishes that the stressed rat heart produces aldosterone and for the first time identifies a biological role for intramitochondrial 30-kDa StAR.

The effect of long-term dehydration and subsequent rehydration on markers of inflammation, oxidative stress and apoptosis in the camel kidney

Background

Dehydration has deleterious effects in many species, but camels tolerate long periods of water deprivation without serious health compromise. The kidney plays crucial role in water conservation, however, some reports point to elevated kidney function tests in dehydrated camels. In this work, we investigated the effects of dehydration and rehydration on kidney cortex and medulla with respect to pro-inflammatory markers, oxidative stress and apoptosis along with corresponding gene expression.

Results

The cytokines IL-1β and IL-18 levels were significantly elevated in the kidney cortex of dehydrated camel, possibly expressed by tubular epithelium, podocytes and/or mesangial cells. Elevation of IL-18 persisted after rehydration. Dehydration induced oxidative stress in kidney cortex evident by significant increases in MDA and GSH, but significant decreases in SOD and CAT. In the medulla, CAT decreased significantly, but MDA, GSH and SOD levels were not affected. Rehydration abolished the oxidative stress. In parallel with the increased levels of MDA, we observed increased levels of PTGS1 mRNA, in MDA synthesis pathway. GCLC mRNA expression level, involved in GSH synthesis, was upregulated in kidney cortex by rehydration. However, both SOD1 and SOD3 mRNA levels dropped, in parallel with SOD activity, in the cortex by dehydration. There were significant increases in caspases 3 and 9, p53 and PARP1, indicating apoptosis was triggered by intrinsic pathway. Expression of BCL2l1 mRNA levels, encoding for BCL-xL, was down regulated by dehydration in cortex. CASP3 expression level increased significantly in medulla by dehydration and continued after rehydration whereas TP53 expression increased in cortex by rehydration. Changes in caspase 8 and TNF-α were negligible to instigate extrinsic apoptotic trail. Generally, apoptotic markers were extremely variable after rehydration indicating that animals did not fully recover within three days.

Conclusions

Dehydration causes oxidative stress in kidney cortex and apoptosis in cortex and medulla. Kidney cortex and medulla were not homogeneous in all parameters investigated indicating different response to dehydration/rehydration. Some changes in tested parameters directly correlate with alteration in steady-state mRNA levels.

Histone deacetylases inhibitors as new potential drugs against Leishmania braziliensis, the main causative agent of new world tegumentary leishmaniasis

The protozoan parasite Leishmania braziliensis is a major causative agent of the neglected tropical diseases Cutaneous and Mucocutaneous Leishmaniases in the New World. There are no vaccines to prevent the infection and the treatment relies on few drugs that often display high toxicity and costs. Thus, chemotherapeutic alternatives are required. Histone Deacetylases (HDACs) are epigenetic enzymes involved in the control of chromatin structure. In this work, we tested an in-house library of 78 hydroxamic acid derivatives as putative inhibitors of L. braziliensis HDACs (HDACi). The compounds were evaluated in relation to the toxicity to the host cell macrophage and to the leishmanicidal effect against L. braziliensis during in vitro infection. Eight HDACi showed significant leishmanicidal effects and the top 5 compounds showed effective concentrations (EC₅₀) in the range of 4.38 to 10.21 μM and selectivity indexes (SI) from of 6 to 21.7. Analyses by Transmission Electron Microscopy (TEM) indicated induction of apoptotic cell death of L. braziliensis amastigotes with a necrotic phenotype. An altered chromatin condensation pattern and cellular disorganization of intracellular amastigotes was also observed. A tight connection between the mitochondrion and nuclear protrusions, presumably of endoplasmic reticulum origin, was found in parasites but not in the host cell. In flow cytometry (FC) analyses, HDACi promoted parasite cell cycle arrest in the G2-M phase and no changes were found in macrophages. In addition, the direct effect of HDACi against the promastigotes showed apoptosis as the main mechanism of cell death. The FC results corroborate the TEM analyses indicating that the HDACi lead to changes in the cell cycle and induction of apoptosis of L. braziliensis. The production of nitric oxide by the infected macrophages was not altered after treatment with the top 5 compounds. Taken together, our results evidenced new HDACi as promising agents for the development of new treatments for American Tegumentary Leishmaniasis caused by L. braziliensis.

The Impact of Sugar-Sweetened Beverage Consumption on the Liver: A Proteomics-based Analysis

Cardiometabolic complications such as the metabolic syndrome and Type 2 Diabetes Mellitus (T2DM) are major causes of global morbidity and mortality. As sugar-sweetened beverages (SSBs) are implicated in this process, this study aimed to obtain greater mechanistic insights. Male Wistar rats (~200 g) were gavaged with a local SSB every day for a period of six months while the control group was gavaged with an iso-volumetric amount of water. Experimental dosages were calculated according to the surface area-to-volume ratio and were equivalent to 125 mL/day (in human terms). A proteomic analysis was performed on isolated liver samples and thereafter, markers of endoplasmic reticulum (ER) stress, antioxidant/oxidant capacity, calcium regulation, and mitochondrial functionality were assessed. These data show that SSB consumption resulted in (a) the induction of mild hepatic ER stress; (b) altered hepatic mitochondrial dynamics; and (c) perturbed calcium handling across mitochondria-associated ER membranes. Despite significant changes in markers of ER stress, the antioxidant response and calcium handling (proteomics data), the liver is able to initiate adaptive responses to counteract such stressors. However, the mitochondrial data showed increased fission and decreased fusion that may put the organism at risk for developing insulin resistance and T2DM in the longer term.

Fumarate and oxidative stress synergize to promote stability of C/EBP homologous protein in the adipocyte

C/EBP homologous protein (CHOP) is a transcription factor that is elevated in adipose tissue across many models of diabetes and metabolic stress. Although increased CHOP levels are associated with the terminal response to endoplasmic reticulum stress and apoptosis, there is no evidence for CHOP mediated apoptosis in the adipose tissue during diabetes. CHOP protein levels increase in parallel with protein succination, a fumarate derived cysteine modification, in the adipocyte during metabolic stress. We investigated the factors contributing to sustained CHOP proteins levels in the adipocyte, with an emphasis on the regulation of CHOP protein turnover by metabolite-driven modification of Keap1 cysteines. CHOP protein stability was investigated in conditions of nutrient stress due to high glucose or elevated fumarate (fumarase knockdown model); where cysteine succination is specifically elevated. CHOP protein turnover is significantly reduced in models of elevated glucose and fumarate with a ~30% increase in CHOP stability (p > 0.01), in part due to decreased CHOP phosphorylation. Sustained CHOP levels occur in parallel with elevated heme-oxygenase-1, a production of increased Nrf2 transcriptional activity and Keap1 modification. While Keap1 is directly succinated in the presence of excess fumarate derived from genetic knockdown of fumarase (fumarate levels are elevated >20-fold), it is the oxidative modification of Keap1 that predominates in adipocytes matured in high glucose (fumarate increases 4-5 fold). Elevated fumarate indirectly regulates CHOP stability through the induction of oxidative stress. The antioxidant N-acetylcysteine (NAC) reduces fumarate levels, protein succination and CHOP levels in adipocytes matured in high glucose. Elevated CHOP does not contribute elevated apoptosis in adipocytes, but plays a redox-dependent role in decreasing the adipocyte secretion of interleukin-13, an anti-inflammatory chemokine. NAC treatment restores adipocyte IL-13 secretion, confirming the redox-dependent regulation of a potent anti-inflammatory eotaxin. This study demonstrates that physiological increases in the metabolite fumarate during high glucose exposure contributes to the presence of oxidative stress and sustained CHOP levels in the adipocyte during diabetes. The results reveal a novel metabolic link between mitochondrial metabolic stress and reduced anti-inflammatory adipocyte signaling as a consequence of reduced CHOP protein turnover.

Emerging roles of the unfolded protein response (UPR) in the nervous system: A link with adaptive behavior to environmental stress?

Stressors elicit a neuroendocrine response leading to increased levels of glucocorticoids, allowing the organism to adapt to environmental changes and maintain homeostasis. Glucocorticoids have a broad effect in the body, modifying the activity of the immune system, metabolism, and behavior through the activation of receptors in the limbic system. Chronic exposition to stressors operates as a risk factor for psychiatric diseases such as depression and posttraumatic stress disorder. Among the cellular alterations observed as a consequence of environmental stress, alterations to organelle function at the level of mitochondria and endoplasmic reticulum (ER) are emerging as possible factors contributing to neuronal dysfunction. ER proteostasis alterations elicit the unfolded protein response (UPR), a conserved signaling network that re-establish protein homeostasis. In addition, in the context of brain function, the UPR has been associated to neurodevelopment, synaptic plasticity and neuronal connectivity. Recent studies suggest a role of the UPR in the adaptive behavior to stress, suggesting a mechanistic link between environmental and cellular stress. Here, we revise recent evidence supporting an evolutionary connection between the neuroendocrine system and the UPR to modulate behavioral adaptive responses.

STARD1 Functions in Mitochondrial Cholesterol Metabolism and Nascent HDL Formation. Gene Expression and Molecular mRNA Imaging Show Novel Splicing and a 1:1 Mitochondrial Association

STARD1 moves cholesterol (CHOL) from the outer mitochondrial membrane (OMM) to the inner membrane (IMM) in steroidogenic cells. This activity is integrated into CHOL trafficking and synthesis homeostasis, involving uptake through SR-B1 and LDL receptors and distribution through endosomes, ER, and lipid droplets. In adrenal cells, STARD1 is imported into the mitochondrial matrix accompanied by delivery of several hundred CHOL molecules. This transfer limits CYP11A1-mediated generation of pregnenolone. CHOL transfer is coupled to translation of STARD1 mRNA at the OMM. In testis cells, slower CHOL trafficking seems to be limiting. STARD1 also functions in a slower process through ER OMM contacts. The START domain of STARD1 is utilized by a family of genes, which includes additional STARD (forms 3-6) and GRAMD1B proteins that transfer CHOL. STARD forms 2 and 7 deliver phosphatidylcholine. STARD1 and STARD7 target their respective activities to mitochondria, via N-terminal domains (NTD) of over 50 amino acids. The NTD is not essential for steroidogenesis but exerts tissue-selective enhancement (testis>>adrenal). Three conserved sites for cleavage by the mitochondrial processing protease (MPP) generate three forms, each potentially with specific functions, as demonstrated in STARD7. STARD1 is expressed in macrophage and cardiac repair fibroblasts. Additional functions include CHOL metabolism by CYP27A1 that directs activation of LXR and CHOL export processes. STARD1 generates 3.5- and 1.6-kb mRNA from alternative polyadenylation. The 3.5-kb form exclusively binds the PKA-induced regulator, TIS11b, which binds at conserved sites in the extended 3'UTR to control mRNA translation and turnover. STARD1 expression also exhibits a novel, slow splicing that delayed splicing delivery of mRNA to mitochondria. Stimulation of transcription by PKA is directed by suppression of SIK forms that activate a CRTC/CREB/CBP promoter complex. This process is critical to pulsatile hormonal activation in vivo. sm-FISH RNA imaging shows a flow of single STARD1 mRNA particles from asymmetric accumulations of primary transcripts at gene loci to 1:1 complex of 3.5-kb mRNA with peri-nuclear mitochondria. Adrenal cells are similar but distinguished from testis cells by appreciable basal expression prior to hormonal activation. This difference is conserved in culture and in vivo.

Deficient pregnenolone synthesis associated with congenital adrenal hyperplasia and organelle dysfunction

Steroid hormones are essential for the survival of all mammals. In adrenal glands and gonads, cytochrome P450 side chain cleavage enzyme (SCC or CYP11A1), catalyzes conversion of cholesterol to pregnenolone. We studied a patient with ambiguous genitalia by the absence of Müllerian ducts and the presence of an incompletely formed vagina, who had extremely high adrenocorticotropic hormone (ACTH) and reduced pregnenolone levels with enlarged adrenal glands. The testes revealed seminiferous tubules, stroma, rete testis with interstitial fibrosis and reduced number of germ cells. Electron microscopy showed that the patient's testicular mitochondrial size was small with little SCC expression within the mitochondria. The mitochondria were not close to the mitochondria-associated ER membrane (MAM), and cells were filled with the microfilaments. Our result revealed that absence of pregnenolone is associated with organelle stress, leading to altered protein organization that likely created steric hindrance in testicular cells. Learning points: Testes revealed seminiferous tubules, stroma, rete testis with interstitial fibrosis and reduced number of germ cells; Testicular mitochondrial size was small with little SCC expression within the mitochondria; Absence of pregnenolone is associated with organelle stress.

Inner Mitochondrial Translocase Tim50 Is Central in Adrenal and Testicular Steroid Synthesis

Adrenal and gonadal mitochondrial metabolic activity requires electrons from cofactors, cholesterol, and a substrate for rapid steroid synthesis, an essential requirement for mammalian survival. Substrate activity depends on its environment, which is regulated by chaperones and mitochondrial translocases. Cytochrome P450 side-chain cleavage enzyme (SCC or CYP11A1) catalyzes cholesterol to pregnenolone conversion, although its mechanism of action is not well understood. We find that SCC is directly imported into the mitochondrial matrix, where its N-terminal sequence is cleaved sequentially, after which it becomes activated following the second cleavage, which is dependent on the folding of the protein. Following integration of the SCC C terminus into the TIM23 complex, amino acids 141 to 146 interact with the intermembrane-exposed Tim50 protein, forming a large complex. The absence of Tim50 or its mutation reduced enzymatic activity. For the first time, we report that a protein activated at the matrix remains mostly unfolded and is transported back to the IMS to integrate with the TIM23 translocase complex and align with the Tim50 protein. Amino acid changes that suppress the association of Tim50 with SCC ablate metabolic activity. Thus, the TIM23 complex is the central regulator of metabolism guided by Tim50.

LUMAN/CREB3 Plays a Dual Role in Stress Responses as a Cofactor of the Glucocorticoid Receptor and a Regulator of Secretion

LUMAN/CREB3, originally identified through its interaction with a cell cycle regulator HCFC1, is a transcription factor involved in the unfolded protein response during endoplasmic reticulum stress. Previously using gene knockout mouse models, we have shown that LUMAN modulates the glucocorticoid (GC) response leading to enhanced glucocorticoid receptor (GR) activity and lower circulating GC levels. Consequently, the stress response is dysregulated, leading to a blunted stress response in the Luman-deficient mice. One question that remained was how LUMAN deficiency affected the stress response at the cellular level leading to the changes in the physiological stress response. Here, we found that LUMAN interacts with GR through a putative nuclear receptor box site and can activate GR in the absence of a ligand. Further investigation showed that, when activated, LUMAN binds to the glucocorticoid response element (GRE), increasing the activity of GR exponentially compared to GR-ligand binding alone. On the other hand, we also found that in the absence of LUMAN, cells were more sensitive to cellular stress, exhibiting decreased secretory capacity. Hence our current data suggest that LUMAN may function both as a transcriptional cofactor of GR and a hormone secretion regulator, and through this, plays a role in stress sensitivity and reactivity to stress.

Cholesterol signaling in single cells: lessons from STAR and sm-FISH

Cholesterol is an important regulator of cell signaling, both through direct impacts on cell membranes and through oxy-metabolites that activate specific receptors (steroids, hydroxy-cholesterols, bile acids). Cholesterol moves slowly through and between cell membranes with the assistance of specific binding proteins and transfer processes. The prototype cholesterol regulator is the Steroidogenesis Acute Regulatory (STAR), which moves cholesterol into mitochondria, where steroid synthesis is initiated by cytochrome P450 11A1 in multiple endocrine cell types. CYP27A1 generates hydroxyl cholesterol metabolites that activate LXR nuclear receptors to control cholesterol homeostatic and transport mechanisms. LXR regulation of cholesterol transport and storage as cholesterol ester droplets is shared by both steroid-producing cells and macrophage. This cholesterol signaling is crucial to brain neuron regulation by astrocytes and microglial macrophage, mediated by ApoE and sensitive to disruption by β-amyloid plaques. sm-FISH delivers appreciable insights into signaling in single cells, by resolving single RNA molecules as mRNA and by quantifying pre-mRNA at gene loci. sm-FISH has been applied to problems in physiology, embryo development and cancer biology, where single cell features have critical impacts. sm-FISH identifies novel features of STAR transcription in adrenal and testis cells, including asymmetric expression at individual gene loci, delayed splicing and 1:1 association of mRNA with mitochondria. This may represent a functional unit for the translation-dependent cholesterol transfer directed by STAR, which integrates into mitochondrial fusion dynamics. Similar cholesterol dynamics repeat with different players in the cycling of cholesterol between astrocytes and neurons in the brain, which may be abnormal in neurodegenerative diseases.

Mitochondria-Endoplasmic Reticulum Contact Sites Function as Immunometabolic Hubs that Orchestrate the Rapid Recall Response of Memory CD8+ T Cells

Glycolysis is linked to the rapid response of memory CD8⁺ T cells, but the molecular and subcellular structural elements enabling enhanced glucose metabolism in nascent activated memory CD8⁺ T cells are unknown. We found that rapid activation of protein kinase B (PKB or AKT) by mammalian target of rapamycin complex 2 (mTORC2) led to inhibition of glycogen synthase kinase 3β (GSK3β) at mitochondria-endoplasmic reticulum (ER) junctions. This enabled recruitment of hexokinase I (HK-I) to the voltage-dependent anion channel (VDAC) on mitochondria. Binding of HK-I to VDAC promoted respiration by facilitating metabolite flux into mitochondria. Glucose tracing pinpointed pyruvate oxidation in mitochondria, which was the metabolic requirement for rapid generation of interferon-γ (IFN-γ) in memory T cells. Subcellular organization of mTORC2-AKT-GSK3β at mitochondria-ER contact sites, promoting HK-I recruitment to VDAC, thus underpins the metabolic reprogramming needed for memory CD8⁺ T cells to rapidly acquire effector function.

Role of Heat Shock Factor 1 in Conserving Cholesterol Transportation in Leydig Cell Steroidogenesis via Steroidogenic Acute Regulatory Protein

Testicular testosterone synthesis begins with cholesterol transport into mitochondria via steroidogenic acute regulatory (StAR) protein in Leydig cells. Acute heat stress is known to obstruct testicular steroidogenesis by transcriptional repression of StAR. In contrast, chronic heat stress such as cryptorchidism or varicocele generally does not affect testicular steroidogenesis, suggesting that Leydig cells adapt to heat stress and retain their steroid synthesis ability. However, the mechanisms of the stress response in steroid-producing cells are unclear. We examined the relationship between the heat stress response and heat shock factor 1 (HSF1), which protects cells from proteotoxic stress by inducing heat shock protein as a molecular chaperone. The influences of HSF1 deficiency on cholesterol transport by StAR and the expression of steroidogenic enzymes under chronic heat stress were studied in testes of HSF1-knockout (HSF1KO) mice with experimental cryptorchidism. StAR protein in wild-type-cryptorchid mice was transiently decreased after induction of cryptorchidism and then gradually returned to basal levels. In contrast, StAR protein in HSF1KO mice continued to decrease and failed to recover, resulting in impaired serum testosterone. StAR messenger RNA was not decreased with cryptorchidism, indicating that posttranslational modification of StAR, not its transcription, was obstructed in cryptorchidism. Other steroidogenic enzymes, including CYP11A1, 3β-HSD, and CYP17A1, were not decreased. Lipid droplets were increased in the cytosol of HSF1KO-cryptorchid mice, suggesting dysfunctional cholesterol transportation. These findings provide insight into the role of HSF1 in Leydig cell steroidogenesis, suggesting that it maintains cholesterol transport by recovering StAR under chronic heat stress.

De novo disruption of promoter and exon 1 of STAR gene reveals essential role for gonadal development

SUMMARY

Cholesterol transport into the mitochondria is required for synthesis of the first steroid, pregnenolone. Cholesterol is transported by the steroidogenic acute regulatory protein (STAR), which acts at the outer mitochondrial membrane prior to its import. Mutations in the STAR protein result in lipoid congenital adrenal hyperplasia (CAH). Although the STAR protein consists of seven exons, biochemical analysis in nonsteroidogenic COS-1 cells showed that the first two were not essential for pregnenolone synthesis. Here, we present a patient with ambiguous genitalia, salt-lossing crisis within two weeks after birth and low cortisol levels. Sequence analysis of the STAR, including the exon-intron boundaries, showed the complete deletion of exon 1 as well as more than 50 nucleotides upstream of STAR promoter. Mitochondrial protein import with the translated protein through synthesis cassette of the mutant STAR lacking exon 1 showed protein translation, but it is less likely to have synthesized without a promoter in our patient. Thus, a full-length STAR gene is necessary for physiological mitochondrial cholesterol transport in vivo.

LEARNING POINTS

STAR exon 1 deletion caused lipoid CAH.Exon 1 substitution does not affect biochemical activity.StAR promoter is responsible for gonadal development.

ACTH Action on StAR Biology

Adrenocorticotropin hormone (ACTH) produced by the anterior pituitary stimulates glucocorticoid synthesis by the adrenal cortex. The first step in glucocorticoid synthesis is the delivery of cholesterol to the mitochondrial matrix where the first enzymatic reaction in the steroid hormone biosynthetic pathway occurs. A key response of adrenal cells to ACTH is activation of the cAMP-protein kinase A (PKA) signaling pathway. PKA activation results in an acute increase in expression and function of the Steroidogenic Acute Regulatory protein (StAR). StAR plays an essential role in steroidogenesis- it controls the hormone-dependent movement of cholesterol across the mitochondrial membranes. Currently StAR's mechanism of action remains a major unanswered question in the field. However, some insight may be gained from understanding the mechanism(s) controlling the PKA-dependent phosphorylation of StAR at S194/195 (mouse/human StAR), a modification that is required for function. This mini-review provides a background on StAR's biology with a focus on StAR phosphorylation. The model for StAR translation and phosphorylation at the outer mitochondrial membrane, the location for StAR function, is presented to highlight a unifying theme emerging from diverse studies.