REVIEW
The role of oxidative stress, glucocorticoid receptor and ARMC5 in lipid metabolism
2024 Volume 71 Issue 12 Pages 1097-1101
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2024 Volume 71 Issue 12 Pages 1097-1101
Lipid metabolism includes lipogenesis, lipolysis, and cholesterol metabolism and it exerts a wide range of biological effects. We previously found novel roles of adipocyte oxidative stress in diet-induced obesity, adipocyte glucocorticoid receptor in Cushing syndrome, and ARMC5 in adrenocortical cells. Using genetically modified mice in which oxidative stress was eliminated or augmented specifically in adipose tissues, we have been able to elucidate that obesity-induced oxidative stress inhibited healthy adipose expansion and ameliorated insulin sensitivity. Using adipocyte-specific glucocorticoid receptor knockout mice, we found that glucocorticoids also inhibited healthy adipose expansion and decreased insulin sensitivity. This was partly due to the transcriptional upregulation of ATGL. We identified ARMC5 as a novel ubiquitin E3 ligase of full-length SREBF, a master regulator of lipid metabolism. In adrenocortical cells, ARMC5 suppresses SREBF2 activity, and loss of ARMC5 may lead to cholesterol accumulation and the development of primary bilateral macronodular adrenal hyperplasia.
Lipid metabolism includes lipogenesis, lipolysis, and cholesterol metabolism, and it exerts a wide range of biological effects, such as insulin resistance, dyslipidemia, obesity, liver steatosis, neurological disorders, and tumorigenesis. Based on our recent findings, we have focused on the role of adipocyte oxidative stress, adipocyte glucocorticoid receptor and ARMC5 in adrenocortical cells in lipid metabolism, and introduced a brief summary and future perspectives.
Oxidative stress is caused by an imbalance between the production and elimination of reactive oxygen species (ROS) and plays various roles in the pathogenesis of various diseases. In 3T3-L1 adipocytes, oxidative stress reduces insulin-induced glucose uptake [1]; however, it is unclear whether oxidative stress is increased in white adipose tissue (WAT) in obesity and whether it has pathological roles in vivo. In 2004, our group demonstrated that oxidative stress in WAT, which we call Fat ROS, was increased in obese mice and was associated with the upregulation of NADPH oxidase and downregulation of antioxidants [2]. We also identified PPARγ-responsive element (PPRE) in the catalase gene, one of the antioxidants, in mice [3] and humans [4], which might explain downregulation of catalase in the WAT of obese mice.
To elucidate the pathological role of oxidative stress in vivo, we generated aP2-catalase/Sod1 double transgenic mice (aP2-dTg), in which Fat ROS were eliminated by overexpression of two antioxidants, catalase and Sod1, under the control of the aP2 promoter. Under a high-fat/high-sucrose diet (HF/HSD), the elimination of Fat ROS increases adipose mass in the subcutaneous WAT and decreases lipid deposition in the mesenteric WAT, brown adipose tissue (BAT), and the liver, leading to an improved systemic insulin sensitivity. As the aP2 gene was also expressed in macrophages, and catalase and Sod1 were overexpressed in adipose tissue macrophages in aP2-dTg [5], we also generated Adipoq-Gclc knockout mice, where Fat ROS was augmented by the knockout of the antioxidant, Gclc, under the control of the Adipoq promoter, whose activity was highly specific to adipocytes [6]. In agreement with the phenotype of aP2-dTg mice, augmentation of Fat ROS inhibited adipose expansion with ectopic lipid accumulation in the liver, and BAT with deteriorated insulin sensitivity. These findings highlight that obesity-induced Fat ROS inhibit the ability to store energy, which we call healthy adipose expansion, especially in the subcutaneous WAT, resulting in the overflow of energy and ectopic lipid accumulation in the peripheral tissues, leading to insulin resistance (Fig. 1).
The mechanism by which Fat ROS inhibit adipose tissue expansion is currently unclear. Fat ROS may directly diminish lipogenesis, as we observed the ROS-mediated downregulation of lipogenic genes in 3T3-L1 adipocytes [5]. Recently, Croft et al. reported that adipocyte-specific overexpression of mitochondria-targeted catalase exacerbates hyperglycemia [7]. As native catalase overexpressed in Fat ROS-eliminated mice should be localized in the peroxisome, the selective elimination of oxidative stress in specific cellular compartments might be necessary for the treatment of metabolic syndrome.
It is well known that glucocorticoids induce various abnormalities which resemble metabolic syndrome as a part of Cushing syndrome, such as obesity, liver steatosis, insulin resistance and dyslipidemia. The actions of glucocorticoids are mainly mediated by glucocorticoid receptor (GR). Several studies have demonstrated that the metabolic phenotypes of Cushing syndrome are mediated by GR in insulin-sensitive organs such as the liver [8], skeletal muscle [9, 10] and pancreatic beta cells [11].
In vitro studies indicated that adipocyte GR plays an important role in glucocorticoid-induced metabolic abnormalities. GR has been shown to inhibit glucose uptake [12], downregulate Irs1 expression [13] and stimulate both lipolysis [14, 15] and lipogenesis [16] in cultured adipocytes. Furthermore, glucocorticoid response elements (GREs) are located in the regulatory regions of many metabolism-related genes [16]. Past clinical studies have also suggested the involvement of adipocyte GR in Cushing syndrome [17-20]. For example, glucocorticoids have been shown to increase plasma free fatty acids (FFAs), and the expression of various metabolic genes is upregulated in the adipose tissue of patients with high levels of glucocorticoids [17-20].
In order to gain an insight into the role of GR in Cushing syndrome, we have recently developed adipocyte-specific GR knockout (AGRKO). Contrary to our initial hypothesis that adipocyte GR mediates obesity in Cushing syndrome, the AGRKO mice exhibited enhanced healthy adipose expansion and improved insulin sensitivity under normal chow with exogenous corticosterone in the drinking water. This was mediated by the upregulation of Atgl in adipocytes through a GRE in its promoter (Fig. 1) [21]. We also investigated adipose tissues from patients with Cushing syndrome and found that the expression of collagen genes was significantly downregulated. This is in accordance with the fact that the striae observed in Cushing syndrome result from decreased collagen in dermal fibroblasts.
Notably, seven laboratories, including our group, independently reported the phenotype of AGRKO mice between 2015 and 2019, with some controversies. In 2015, Kloet et al. reported a reduced fat mass in AGRKO mice with diet-induced obesity [22]. In 2016, Desarzens et al. [23] and Bose et al. [24] observed no changes in adipose tissue mass in AGRKO mice fed a high-calorie diet. Bose et al. reported increased fat mass and unchanged glucose tolerance in AGRKO mice treated with glucocorticoid [24]. In 2017, Mueller et al. reported the attenuation of aging- and high-fat diet-induced obesity and glucose intolerance in AGRKO mice [25], and Shen et al. reported that GR was important in insulin resistance due to exogenous steroids, but not due to high-fat feeding [26]. In 2019, Dalle et al. showed that adipocyte GR deficiency promotes adipose tissue expandability and insulin sensitivity under glucocorticoid excess [27]. The experimental conditions and phenotypes regarding AGRKO in these studies are summarized in Tables 1 and 2. Whether the observed discrepancies are due to slightly different experimental conditions should be clarified in future studies. Although we did not evaluate AGRKO mice under a high-fat diet, we feel that adipocyte GR suppresses healthy adipose expansion and augments insulin resistance under excess exogenous glucocorticoids, but adipocyte GR had relatively minor roles in diet-induced obesity.
| Author | Background | Treatment | Fat mass | Glucose tolerance |
|---|---|---|---|---|
| de Kloet et al. [22] | C57BL/6 × 129 | HFD for 42 days | Decrease | N.D. |
| Desarzens et al. [23] | C57BL/6 | HF/HSD for 15 weeks | No change | No change |
| Bose et al. [24] | C57BL/6 | HFD for 16 weeks | No change | No change |
| Mueller et al. [25] | C57BL/6 × FVB/N | HFD for 16 weeks | Decrease | Improve |
| Shen et al. [26] | N.D. | HFD for 14 weeks | No change | No change |
HFD; high-fat diet. HF/HSD; high-fat/high-sucrose diet. N.D.; not described.
| Author | Background | Treatment | Fat mass | Glucose tolerance |
|---|---|---|---|---|
| Bose et al. [24] | C57BL/6 | 10mg/kg dexamethasone for 2 weeks | Increase | No change |
| Shen et al. [26] | N.D. | 3mg/kg dexamethasone every other day for 2 months | No change | Improve |
| Dalle et al. [27] | C57BL/6J (CreERT2) | 100ug/mL corticosterone for 4 weeks | Increase | Improve |
| Hayashi et al. [21] | C57BL/6 | 50ug/mL corticosterone for 2 weeks | Increase | Improve |
N.D.; not described.
SREBF, also known as SREBP, is a master transcription factor involved in the lipid metabolism. SREBF1 mainly regulates lipogenesis, and SREBF2 mainly regulates cholesterol metabolism. SREBF is synthesized as inactivated full-length SREBF attached to the endoplasmic reticulum (ER). Under the depletion of the cellular cholesterol, SREBF is transported to the Golgi apparatus, where the N-terminal fragment is cleaved by two proteases, transported to the nucleus, and functions as a transcription factor as nuclear SREBF. To identify a novel regulator of SREBF, we recently performed affinity-capture mass spectrometry (MS) on differentiated 3T3-L1 adipocytes using nuclear SREBF1 as the bait. The top 10 genes that ranked high among 233 candidate genes were ribosome binding protein (GCN1), transcriptional coactivator (CREBBP and EP300), heat shock protein (HSPA2 and HSPA1A), DNA-dependent protein kinase (PRKDC) and tubulin (TUBA1B, TUBB5, TUBA1C, and TUBB4B); however, these proteins have well-known roles in regard to various other proteins other than SREBF1. In contrast, the top 11th gene, ARMC5, remains functionally unknown. Therefore, we anticipated that ARMC5 might specifically regulate SREBF1 and be a promising target for the treatment of the dysregulated lipid metabolism. During our study, another group reported that ARMC5 interacts with the CUL3-dependent ubiquitin ligase complex and ubiquitinates and degrades unidentified target proteins [28]. In agreement with these findings, our study revealed that ARMC5 is a part of E3 ubiquitin ligase of SREBF1. Importantly, as ARMC5 is localized mainly in the cytosol, ARMC5 can degrade full-length SREBF1 but not nuclear SREBF1 (Fig. 2) [29].
ARMC5 is the causal gene of primary bilateral macronodular adrenal hyperplasia (PBMAH), in which the adrenal cortex becomes hyperplastic and secretes glucocorticoids autonomously, leading to Cushing syndrome. The adrenal cortex is one of the tissues that abundantly expresses SREBF, probably because steroids are synthesized from cholesterol. Steroid synthesis consumes cellular cholesterol, thus resulting in the activation of SREBF2. In adrenocortical cells, ARMC5 has been shown to suppress SREBF2 activity, and the loss of ARMC5 stimulates cell growth through an excess cholesterol supply [29]. We are currently investigating the role of ARMC5 in adipocytes using adipocyte-specific ARMC5 knockout mice. As SREBF is important in adipose tissue, the liver and adrenocortical cells, ARMC5 may be a promising target for the treatment of metabolic syndrome.
Although lipid metabolism has been extensively studied for a long duration of time, several issues remain unsolved. We are particularly interested in the role of fatty acid desaturation and elongation in adipocyte biology, the differences in lipid metabolism between subcutaneous and mesenteric adipose tissues, and the precise role of ARMC5 in lipid metabolism in various organs.
The authors have declared that no conflict of interest exists.
