Cholesterol accumulation, lipid droplet formation, and steroid production in Leydig cells: Role of translocator protein (18-kDa)

Loss of PBX1 function in Leydig cells causes testicular dysgenesis and male sterility

Leydig cells are essential components of testicular interstitial tissue and serve as a primary source of androgen in males. A functional deficiency in Leydig cells often causes severe reproductive disorders; however, the transcriptional programs underlying the fate decisions and steroidogenesis of these cells have not been fully defined. In this study, we report that the homeodomain transcription factor PBX1 is a master regulator of Leydig cell differentiation and testosterone production in mice. PBX1 was highly expressed in Leydig cells and peritubular myoid cells in the adult testis. Conditional deletion of Pbx1 in Leydig cells caused spermatogenic defects and complete sterility. Histological examinations revealed that Pbx1 deletion impaired testicular structure and led to disorganization of the seminiferous tubules. Single-cell RNA-seq analysis revealed that loss of Pbx1 function affected the fate decisions of progenitor Leydig cells and altered the transcription of genes associated with testosterone synthesis in the adult testis. Pbx1 directly regulates the transcription of genes that play important roles in steroidogenesis (Prlr, Nr2f2 and Nedd4). Further analysis demonstrated that deletion of Pbx1 leads to a significant decrease in testosterone levels, accompanied by increases in pregnenolone, androstenedione and luteinizing hormone. Collectively, our data revealed that PBX1 is indispensable for maintaining Leydig cell function. These findings provide insights into testicular dysgenesis and the regulation of hormone secretion in Leydig cells.

The human microglial surveillant phenotype is preserved by de novo neurosteroidogenesis through the control of cholesterol homeostasis: Crucial role of 18 kDa Translocator Protein

Neurodegenerative disease-associated microglia commonly exhibit harmful cholesterol accumulation that impairs their ability to resolve the neuroinflammatory response, contributing to disease onset and progression. Neurosteroids, whose levels have been often found significantly altered in brain diseases, are the most potent endogenous anti-inflammatory molecules exerting beneficial effects on activities of brain cells, including microglia. For the first time, the impact of neurosteroidogenesis on cholesterol homeostasis for the immune surveillance phenotype maintenance was investigated in a human microglia in vitro model. To enhance and inhibit neurosteroidogenesis, pharmacological stimulation and knock-down of 18 kDa Translocator Protein (TSPO), which is involved in the neurosteroidogenesis rate-limiting step, were used as experimental approaches, respectively. The obtained results point to an essential autocrine control of neurosteroidogenesis in orchestrating cholesterol trafficking in human microglia. TSPO pharmacological stimulation ensured cholesterol turnover by strengthening cholesterol efflux systems and preserving healthy immune surveillant phenotype. Conversely, TSPO knock-down induced an impairment of the controlled interplay among cholesterol synthesis, efflux, and metabolism mechanisms, leading to an excessive cholesterol accumulation and acquisition of a chronically activated dysfunctional phenotype. In this model, the exogenous neurosteroid administration restored proper the cholesterol clearance. The TSPO ability in promoting native neurosteroidogenesis opens the way to restore cholesterol homeostasis, and thus to maintain microglia proper functionality for the treatment of neuroinflammation-related brain diseases.

Animal models of male reproductive ageing to study testosterone production and spermatogenesis

Ageing is the time-dependent gradual decline of the functional characteristics in an organism. It has been shown that it results in the loss of reproductive health and fertility. The age-dependent decline of fertility is a potential issue as the parenthood age is increasing in Western countries, mostly due to socioeconomic factors. In comparison to women, for whom the consequences of ageing are well documented and general awareness of the population is extensively raised, the effects of ageing for male fertility and the consequences of advanced paternal age for the offspring have not been widely studied. Studies with humans are welcome but it is hard to implement relevant experimental approaches to unveil the molecular mechanisms by which ageing affects male reproductive potential. Animal models have thus been extensively used. These models are advantageous due to their reduced costs, general easy maintenance in laboratory facilities, rigorous manipulation tools, short lifespan, known genetic backgrounds, and reduced ethical constraints. Herein, we discuss animal models for the study of male reproductive ageing. The most well-known and studied reproductive ageing models are rodents and non-human primates. The data collected from these models, particularly studies on testicular ageing, steroidogenesis, and genetic and epigenetic changes in spermatogenesis are detailed. Notably, some species challenge the currently accepted ageing theories and the concept of senescence itself, which renders them interesting animal models for the study of male reproductive ageing.

Cellular sources of TSPO expression in healthy and diseased brain

The 18 kDa translocator protein (TSPO) is a highly conserved protein located in the outer mitochondrial membrane. TSPO binding, as measured with positron emission tomography (PET), is considered an in vivo marker of neuroinflammation. Indeed, TSPO expression is altered in neurodegenerative, neuroinflammatory, and neuropsychiatric diseases. In PET studies, the TSPO signal is often viewed as a marker of microglial cell activity. However, there is little evidence in support of a microglia-specific TSPO expression. This review describes the cellular sources and functions of TSPO in animal models of disease and human studies, in health, and in central nervous system diseases. A discussion of methods of analysis and of quantification of TSPO is also presented. Overall, it appears that the alterations of TSPO binding, their cellular underpinnings, and the functional significance of such alterations depend on many factors, notably the pathology or the animal model under study, the disease stage, and the involved brain regions. Thus, further studies are needed to fully determine how changes in TSPO binding occur at the cellular level with the ultimate goal of revealing potential therapeutic pathways.

Lipid profiling using Raman and a modified support vector machine algorithm

Lipid droplets are dynamic organelles that play important cellular roles. They are composed of a phospholipid membrane and a core of triglycerides and sterol esters. Fatty acids have important roles in phospholipid membrane formation, signaling, and synthesis of triglycerides as energy storage. Better non-invasive tools for profiling and measuring cellular lipids are needed. Here we demonstrate the potential of Raman spectroscopy to determine with high accuracy the composition changes of the fatty acids and cholesterol found in the lipid droplets of prostate cancer cells treated with various fatty acids. The methodology uses a modified least squares fitting (LSF) routine that uses highly discriminatory wavenumbers between the fatty acids present in the sample using a support vector machine algorithm. Using this new LSF routine, Raman micro-spectroscopy can become a better non-invasive tool for profiling and measuring fatty acids and cholesterol for cancer biology.

MITOCHONDRIAL CHOLESTEROL AND CANCER

Cholesterol is a crucial component of membrane bilayers that determines their physical and functional properties. Cells largely satisfy their need for cholesterol through the novo synthesis from acetyl-CoA and this demand is particularly critical for cancer cells to sustain dysregulated cell proliferation. However, the association between serum or tissue cholesterol levels and cancer development is not well established as epidemiologic data do not consistently support this link. While most preclinical studies focused on the role of total celular cholesterol, the specific contribution of the mitochondrial cholesterol pool to alterations in cancer cell biology has been less explored. Although low compared to other bilayers, the mitochondrial cholesterol content plays an important physiological function in the synthesis of steroid hormones in steroidogenic tissues or bile acids in the liver and controls mitochondrial function. In addition, mitochondrial cholesterol metabolism generates oxysterols, which in turn, regulate multiple pathways, including cholesterol and lipid metabolism as well as cell proliferation. In the present review, we summarize the regulation of mitochondrial cholesterol, including its role in mitochondrial routine performance, cell death and chemotherapy resistance, highlighting its potential contribution to cancer. Of particular relevance is hepatocellular carcinoma, whose incidence in Western countries had tripled in the past decades due to the obesity and type II diabetes epidemic. A better understanding of the role of mitochondrial cholesterol in cancer development may open up novel opportunities for cancer therapy.

Identification of Koumine as a Translocator Protein 18 kDa Positive Allosteric Modulator for the Treatment of Inflammatory and Neuropathic Pain

Koumine is an alkaloid that displays notable activity against inflammatory and neuropathic pain, but its therapeutic target and molecular mechanism still need further study. Translocator protein 18 kDa (TSPO) is a vital therapeutic target for pain treatment, and recent research implies that there may be allostery in TSPO. Our previous competitive binding assay hint that koumine may function as a TSPO positive allosteric modulator (PAM). Here, for the first time, we report the pharmacological characterization of koumine as a TSPO PAM. The results imply that koumine might be a high-affinity ligand of TSPO and that it likely acts as a PAM since it could delay the dissociation of ³H-PK11195 from TSPO. Importantly, the allostery was retained in vivo, as koumine augmented Ro5-4864-mediated analgesic and anti-inflammatory effects in several acute and chronic inflammatory and neuropathic pain models. Moreover, the positive allosteric modulatory effect of koumine on TSPO was further demonstrated in cell proliferation assays in T98G human glioblastoma cells. In summary, we have identified and characterized koumine as a TSPO PAM for the treatment of inflammatory and neuropathic pain. Our data lay a solid foundation for the use of the clinical candidate koumine to treat inflammatory and neuropathic pain, further demonstrate the allostery in TSPO, and provide the first proof of principle that TSPO PAM may be a novel avenue for the discovery of analgesics.

Impact of endocrine-disrupting chemicals on steroidogenesis and consequences on testicular function

Testicular steroidogenesis is a tightly regulated process that produces the androgens important for the development, maintenance and function of the male reproductive system. These androgens are also essential for overall health, and well-being. Disruptions in the ability of the testis to form steroids can result in developmental abnormalities, dysfunction, and infertility. Endocrine-disrupting chemicals (EDCs) can interfere with the intricate signaling and metabolizing networks that produce androgens and promote their dysfunction. These chemicals are found ubiquitously in our environment, as they are integral components of products that are used every day. The effects of EDCs, such as bisphenols, phthalates, and alkyl chemicals, have been studied independently, revealing deleterious effects; but the combined influence of these structures on steroidogenesis has yet to be completely elucidated. This manuscript presents an updated review on EDC mixtures and their impact on testicular function and fertility, highlighting new findings that illustrate the anti-androgenic capabilities of EDC mixtures.

Cholesterol was identified as a biomarker in human melanocytic nevi using DESI and DESI/PI mass spectrometry imaging

The rapid differentiation between diseased tissue and healthy normal tissue is of great importance for the intraoperative diagnosis. Herein, desorption electrospray ionization (DESI) and DESI/post-photoionization (DESI/PI) mass spectrometry imaging were combined to in situ visualize the distribution of biochemicals within the tissue regions of human melanocytic nevi under the ambient condition with a spatial resolution of around 200 μm. Plenty of polar and nonpolar lipids were found to be specifically distributed in melanocytic nevi with statistical significance and could be used to differentiate the healthy normal tissue and melanocytic nevi. Cholesterol was further confirmed to be a potential biomarker for melanocytic nevi diagnosis by multivariate statistical analysis and immunohistochemistry of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and translocator protein (TSPO) enzymes. This work provides a visual way for the diagnosis of human melanocytic nevi by lipid profiling, which benefits the understanding of the pathological mechanism of melanocytic nevi and provides a new insight to control melanin growth from the synthesis, transport, and metabolism of cholesterol.

Role of Constitutive STAR in Leydig Cells

Leydig cells contain significant amounts of constitutively produced steroidogenic acute regulatory protein (STAR; STARD1). Hormone-induced STAR plays an essential role in inducing the transfer of cholesterol into the mitochondria for hormone-dependent steroidogenesis. STAR acts at the outer mitochondrial membrane, where it interacts with a protein complex, which includes the translocator protein (TSPO). Mutations in STAR cause lipoid congenital adrenal hyperplasia (lipoid CAH), a disorder characterized by severe defects in adrenal and gonadal steroid production; in Leydig cells, the defects are seen mainly after the onset of hormone-dependent androgen formation. The function of constitutive STAR in Leydig cells is unknown. We generated STAR knockout (KO) MA-10 mouse tumor Leydig cells and showed that STAR KO cells failed to form progesterone in response to dibutyryl-cAMP and to TSPO drug ligands, but not to 22(R)-hydroxycholesterol, which is a membrane-permeable intermediate of the CYP11A1 reaction. Electron microscopy of STAR KO cells revealed that the number and size of lipid droplets were similar to those in wild-type (WT) MA-10 cells. However, the density of lipid droplets in STAR KO cells was drastically different than that seen in WT cells. We isolated the lipid droplets and analyzed their content by liquid chromatography-mass spectrometry. There was a significant increase in cholesteryl ester and phosphatidylcholine content in STAR KO cell lipid droplets, but the most abundant increase was in the amount of diacylglycerol (DAG); DAG 38:1 was the predominantly affected species. Lastly, we identified genes involved in DAG signaling and lipid metabolism which were differentially expressed between WT MA-10 and STAR KO cells. These results suggest that constitutive STAR in Leydig cells is involved in DAG accumulation in lipid droplets, in addition to cholesterol transport. The former event may affect cell functions mediated by DAG signaling.

Lack of adrenal TSPO/PBR expression in hamsters reinforces correlation to triglyceride metabolism

Despite being a highly conserved protein, the precise role of the mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), remains elusive. The void created by studies that overturned a presumptive model that described TSPO/PBR as a mitochondrial cholesterol transporter for steroidogenesis has been filled with evidence that it can affect mitochondrial metabolic functions across different model systems. We previously reported that TSPO/PBR deficient steroidogenic cells upregulate mitochondrial fatty acid oxidation and presented a strong positive correlation between TSPO/PBR expression and tissues active in triglyceride metabolism or lipid storage. Nevertheless, the highlighting of inconsistencies in prior work has provoked reprisals that threaten to stifle progress. One frequent factoid presented as being supportive of a cholesterol import function is that there are no steroid-synthesizing cell types without high TSPO/PBR expression. In this study, we examine the hamster adrenal gland that is devoid of lipid droplets in the cortex and largely relies on de novo cholesterol biosynthesis and uptake for steroidogenesis. We find that Tspo expression in the hamster adrenal is imperceptible compared to the mouse. This observation is consistent with a substantially low expression of Cpt1a in the hamster adrenal, indicating minimal mitochondrial fatty acid oxidation capacity compared to the mouse. These findings provide further reinforcement that the much sought-after mechanism of TSPO/PBR function remains correlated with the extent of cellular triglyceride metabolism. Thus, TSPO/PBR could have a homeostatic function relevant only to steroidogenic systems that manage triglycerides associated with lipid droplets.