Developmental Characteristics and Representative Markers of BAT

Perirenal adipocytes exist as adipocytes in the prenatal phase and mature gradually after birth, a process known as whitening [25]. This differs from the typical subcutaneous white adipocyte maturation; adipocytes differentiate into adipocytes faster than subcutaneous adipocytes [25], and the activity of brown adipocytes in the perirenal region is similar to that of typical brown adipocytes around the scapula [26].

The cells of origin of brown adipocytes are present in the embryonic mesoderm, and adipocytes expressing myogenic factor 5 (MYF5) differentiate into brown adipocytes and myogenic cells, which then differentiate into muscle and fat, depending on the presence or absence of the PR/SET structural domain 16 (PRDM16) gene. Thus, brown adipocytes have the same developmental origin and functional relevance as muscle; therefore, the activation of brown adipocytes is possible for exercise [27]. Moreover, even adipocytes that do not express MYF5 can differentiate into beige cells when UCP1 is expressed [28].

The main marker of brown adipocytes is UCP1, which is involved in fatty acid oxidative thermogenesis by activating the uncoupled respiratory chain [29]. Secretory protein, acidic and cysteine-rich (SPARC), is an adipokine involved in the maintenance of brown adiposity, also known as bone connexin. Calsyntenin 3 (CLSTN3) is involved in multilocular expression and a large number of small droplets represent the histological features of brown adipocytes. Potassium double pore channel subfamily K member 3 (KCNK3) has a temperature-sensitive function. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and PRDM16 are brown fat transfer factors. PPARG coactivator 1 alpha (PPARGC1A) and Cbp/P300 interact with glutamate [E] and aspartate [D] carboxy-terminal rich domain 1 (CITED1) as transcriptional cofactors. Retinoid X receptor gamma (RXRγ) is a differentiation factor. In addition, Ebf3, Fbxo31, Lhx8, TBX1, ELOVL3, and CIDEA are typical brown adipocyte markers. Human-specific brown adipocyte markers are ACOT11, PYGM, and FABP3. HMGCS2 and CKMT1A/1B are increased in expression in brown adipocytes compared to white adipocytes [14,30]. Other brown/beige adipocytes and white adipocyte markers are shown in Table 1.

When UCP1 is expressed in white adipocytes, it transforms into a beige cell that is intermediate between white and brown adipocytes and exhibits a temperature-sensitive phenotype in response to various stimuli such as low temperature, drugs, or genetic factors [4]. When cells are transformed into beige cells, they express CD137, Tbx1 Tmem26, and Epsti1 [31], but the expression of leptin, peroxisome proliferator-activated receptor γ (PPARγ), HOXC8, and HOXC9 is decreased [14].

Main Stimulators for Activation of Brown Adipocytes

The main stimuli for brown adipocyte activation and beige fibrillation are hypothermia and drugs (Figure 1C) [32]. Hypothermia is the most effective inductor; when treated for long periods (2 hours per day for 6 weeks) or short periods (6 hours per day for 10 days), caloric expenditure increases, and body fat are significantly reduced [33]. A known mechanism of activation is non-chilling thermogenesis. The sympathetic nervous system is stimulated by cold and activates brown adipocytes, which produce fatty acids from hydrolyzed triglycerides and generate heat [34].

Browning of white adipocytes at low temperatures is caused by the activation of UCP1 [5,7]. As glucose and fatty acids are efficiently consumed to produce heat, this process is considered as a treatment for metabolic diseases. Therefore, ucp1 activation drugs are being investigated [4]; Mirabegron, a β3 antagonist, was initially approved for the treatment of overactive bladder, but it has been reported to increase energy expenditure by activating brown adipocytes [35]. The pungent capsaicin derivative activates temperature-related genes through the same receptors as white adipocyte browning [36]. Liraglutide, an antidiabetic drug, acts on the GLP -1 glucagon-like peptide-1 receptor and significantly reduces body weight in obese patients by increasing energy expenditure [37]. Codesoxycholic acid (CDCA) is a bile acid that induces brown adipocyte activation by enhancing mitochondrial respiration [38] and through the G protein-coupled receptor (TGR5) [39].

stimulates intracellular thyroid hormone activation of brown adipocytes. Bone morphogenetic protein 7 (BMP7) and BMP8b are important for the maturation of brown adipocytes, temperature sensitivity, and browning of white adipocytes. It was found that BMP8b is involved in weight loss through the activation of brown adiposity [40]. In overweight type 2 diabetic patients, fibroblast growth factor 21 (FGF21) mimetics showed a decrease in plasma lipids, an increase in blood lipocalin levels, and a significant reduction in body weight [4].

As a trial drug, 2,4-dinitrophenol, a drug similar to UCP1, was used as a weight loss drug in the 1930s but was discontinued due to hyperthermic death and adverse effects when patients took too high doses [16]. β3 antagonist CL316,243 also failed due to various drug receptors and poor oral activity.

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Other Factors for Activation of Brown Adipocytes

The browning of perirenal adipose tissue was significantly higher in women than in men when exposed to cold [7]. In immunohistochemical staining, 33% of perirenal adipocytes were positive for ucp1 in women compared to 7% in men [7]. In histological comparisons, lipid droplets were smaller in females than in males [7]. In females, the following processes are more active than in males: Cold-activated UCP1 expression increases heat production in mitochondria, which leads to increased energy expenditure and consequently to adipose tissue loss [41]. These sex-specific physiological differences are related to sex hormones. The relevant hormones are (1) Follicular hormone estradiol (E2), an estrogen that induces calorie production from brown fat by increasing the metabolic rate of interphase cells through E2 (adrenergic signaling is inhibited when α2-adrenergic receptors, a pathway that directly affects brown fat, are activated [7], and E2 induces calorie production from brown fat by inhibiting α 2-adrenergic receptor activation in brown adipocytes); (2) testosterone inhibits brown adipocyte activity by suppressing UCP1 [42]; (3) estrogen induces brown adipocyte activation and white adipocyte browning [7]; (4) gonadotropins and Y chromosome suppress UCP1 expression in brown adipocytes [43]; (5) UCP1 transcriptional and translational processes are regulated by sex [ 44] for epigenetic regulation.

In adults, 70-80% of perirenal fat consists of brown adipocytes [14], and brown adipose progenitor cells are distributed throughout the perirenal adipose tissue. In contrast, the distribution of inactive brown adipocytes varies by location, with an increase in inactive cells when close to the adrenal glands. Inactivated cells are expressed through the SPARC gene, which is a representative gene indicating the inactivation status [3]. Macrophages are a new cell type that mediates the browning of white adipocytes [45]; previously, it was known only as the catecholamine-secreting cell. size of BAT is inversely related to obesity and age [34], whereas white adipose tissue is positively related to [3]. the beige coloration of white adipocytes is significantly reduced after 40 years of age [46].

As environmental factors, diet and exercise are important for browning. Dietary compounds include capsaicin (and its capsaicin analogs), menthol, 6-isothiocyanate, allyl isothiocyanate, benzyl isothiocyanate, 3,5,40-trihydroxy-trans-stilbene (a polyphenol), curcumin, green tea catechins (e.g., epigallocatechin, epigallocatechin gallate, epicatechin), flavopiridol, fish oil plus all-trans retinoic acid, dietary methionine, fucoxanthin flavins, lignans, citrulline, bile acids, resveratrol, n-3 polyunsaturated fatty acids, linoleic acid, 5-methyl- 2-isopropyl phenol, β-apache, polyphenol-rich foods, and tefillin C, which has a thermogenic potential associated with UCP1 [51,52].

Physical exercise stimulates the central nervous system, especially specific neuronal populations, such as spiny mouse-associated protein (AgRP) and proopiomelanocortin (POMC) neurons. activation of POMC neurons stimulates browning, while AgRP neurons inhibit browning. Through the POMC neurons, insulin and leptin signaling are regulated. In leptin signaling, exercise stimulates JAK2 and STAT3 tyrosine phosphorylation to transcribe anorexia nervosa neuropeptides. In insulin signaling, exercise enhances the activation of IRS-1/2 and Akt and the phosphorylation of Fox01 and sequentially stops the transcription of anorexigenic neuropeptides.

The pharmacological products are PPAR-α agonists, adrenergic receptor stimulators, thyroid hormone administration agents, irisin and FGF21 inducers [52] and adenylate cyclase activators (e.g., forskolin) [54]. Bioinformatics has also been used to improve pharmacological efficiency.DNA microarrays are used to quantify gene expression, RNA sequencing is used to quantify RNA expression, and chromatin immunoprecipitation sequencing (ChIP-seq) is used to identify protein binding sites in DNA and detect histone modifications. For example, the gene expression profiles of white adipocytes in normal mice overexpressing EBF2 and transgenic mice were compared by RNA sequencing. Mice overexpressing EBF2 in white adipocytes exhibited a brown adipocyte genotype with reduced white adipocyte-specific gene expression compared to normal mice.

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Transformation of White Adipocytes into Beige Cells

In the resting state of the cell cycle, beige cells exhibit gene expression similar to that of white adipocytes but are stimulated by low temperature or UCP1 expression. Beige cells consume energy similar to brown adipocytes [4]. Due to the two-sided nature of beige cells, there are two hypotheses regarding the origin of beige cells: (1) the progenitor cell model: beige cells originate from specific progenitor cell populations that respond to stimuli such as low temperature or specific genetic regulation; (2) the interconversion model: beige cells originate from mature white adipocytes that undergo transdifferentiation after appropriate stimulation [47]. In addition, environmental temperature, genetic background, and local location are thought to have an effect on [4]. The concept of converting white adipocytes to beige cells is very useful in the treatment of metabolic diseases [4].

If white adipocytes can be converted into beige cells by the browning process, then histologically, a large number of small lipid droplets can be seen and genetically, the expression of UCP1 can be increased as a cell whose purpose is to change from energy storage to energy expenditure.

Reported white adipocyte browning inducers include sustained low-temperature exposure, transcriptional/epigenetic regulators, lifestyle/environmental factors, endocrine/hormonal, and natural/synthetic pharmacological products (Figure 1E). Temperature-sensitive factors reported include PGC-1α, PRDM16, MMPs, thyroid hormones, bile acids, natriuretic peptides, FGF-21, and cytokines. Hormones include irisin, tyrosine, and catecholamines. Muscle secretion of irisin during exercise promotes browning [48], thyroid hormones are involved in the secretion of irisin [49], and adjacent adrenal glands secrete catecholamines involved in anatomy [7]. The transfer regulators are PPARγ, PRDM16, PGC-1α, and early b-cell factor-2 (EBF2) [50].

Transplantation of Brown Adipocytes

Transplantation of brown adipocytes into diabetic or obese mice significantly reduced blood glucose levels, systemic inflammation, and serum adipokine [56] concentrations. When brown adipocytes were transplanted into IL-6-deficient mice, there was an increase in IL-6 concentration in vivo and an increase in insulin sensitivity in skeletal muscle and adipose tissue. This result suggests that IL-6 is secreted from the implant, and although IL-6 is a pro-inflammatory cytokine, it has a role in enhancing insulin sensitivity in skeletal muscle and adipose tissue [56]. Meanwhile, the expression of temperature-related genes was not altered, implying that transplanted brown adipocytes are insensitive to the temperature pathway [57]. To date, human transplantation of brown adipocytes has not been attempted because the safety of this approach has not been proven.

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Renal Pathological Aspect

As mentioned above, the benefits of perirenal adipose tissue are limited to healthy donor tissue. Because perirenal adipose tissue is in direct anatomical contact with the kidneys and adrenal glands, it can lead to various pathological abnormalities when obesity or other problems lead to an increase in body size [58].

The increase in perirenal adipose tissue volume implies an increase in white adipocytes (1) secretion of inflammatory adipokines, (2) an increase in free fatty acids, glucose, triglycerides, and uric acid, (3) a decrease in blood flow to the renal artery and renal parenchyma, (4) decrease in glomerular filtration rate, (5) increase in sodium reabsorption, and (6) stimulation of renin secretion leading to acute/chronic renal failure [59]. In addition, adipose afferent reflex, activation of the renin-angiotensin-aldosterone system, and elevated adipokines/cytokines are associated with hypertension, cardiovascular disease [60], atherosclerosis [61], and insulin resistance [62]. In addition, dormant brown adipocyte activation and pro-inflammatory cytokine synthesis are associated with tumor progression. Therefore, it is necessary to consider the pathological risk of perirenal adipose tissue obtained from unhealthy donors.

Conclusions

The perirenal adipose tissue contains a large number of brown adipocytes and there is a high conversion efficiency of beige cells from white adipocytes. Technically, we have identified the stimulating factors for inactive brown adipocytes, and browning factors have also been also identified. This research has found that adipocytes of the perirenal adipose tissue obtained from a healthy donor represent an effective human cell source with which to treat metabolic diseases through energy consumption, rather than being incinerated as medical waste.

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How can Cistanche extract benefit the kidneys?

In western medical theory, the main function of the kidney is the filter of the human body, metabolizing harmful substances in the human body through physical means. It is associated with kidney failure, nephritis, kidney cancer, low testosterone production, and so on. In short, damages the kidney organs. The mechanism of Cistanche treating these diseases can be summarized as follows: 1. Strong antioxidant capacity and inhibition of renal cell apoptosis. 2. The ability to promote cell proliferation and the recolonization of kidney cells.

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