Androgen receptor agonism promotes an osteogenic gene program in preadipocytes

An overview on androgen-mediated actions in skeletal muscle and adipose tissue

Androgens are a class of steroid hormones primarily associated with male sexual development and physiology, but exert pleiotropic effects in either sex. They have a crucial role in various physiological processes, including the regulation of skeletal muscle and adipose tissue homeostasis. The effects of androgens are mainly mediated through the androgen receptor (AR), a ligand-activated nuclear receptor expressed in both tissues. In skeletal muscle, androgens via AR exert a multitude of effects, ranging from increased muscle mass and strength, to the regulation of muscle fiber type composition, contraction and metabolic functions. In adipose tissue, androgens influence several processes including proliferation, fat distribution, and metabolism but they display depot-specific and organism-specific effects which differ in certain context. This review further explores the potential mechanisms underlying androgen-AR signaling in skeletal muscle and adipose tissue. Understanding the roles of androgens and their receptor in skeletal muscle and adipose tissue is essential for elucidating their contributions to physiological processes, disease conditions, and potential therapeutic interventions.

Fat biology and metabolic balance: On the significance of sex

Obesity and its related metabolic disorders have become prevalent and fatal, which are faced by the entire human beings since decades. An energy equilibrium is urgently important for human metabolic health, which requires the participation of multiple organs, such as adipose tissues, liver and skeletal muscles. It seems that both sex and age play a role in the above processes. In this review, we focus on the sexual dimorphism in energy metabolism mediated by adipose tissues, including white and thermogenic (brown/beige) adipose tissues. Remarkably, past investigations have focused on targeting brown/beige adipose tissues to combat obesity because of their contributions to non-shivering thermogenesis. However, sex differences in the regulation of adipose tissue metabolism are likely overlooked. Particularly, increasing data show that males display more visceral fat than females, and females show increased visceral fat after menopause. Visceral adiposity is more deleterious and closely related to metabolic disorders, such as cardiovascular diseases. In this review, we discuss current findings on sexual dimorphism in WAT and BAT biology for a better metabolic balance in humans.

Sex Steroid Hormones Regulate Leptin Transcript Accumulation and Protein Secretion in 3T3-L1 Cells

Leptin is an adipokine produced by fat cells that regulates food consumption and metabolic activity. Sexual dimorphism in leptin and fat stores have been observed in humans and rodents with females having more leptin and greater levels of subcutaneous fat than males. One potential mechanism leading to this dimorphism is steroid hormone regulated synthesis of transcripts encoding leptin. Identification of direct regulatory mechanisms is difficult in animals or primary adipocytes due to these intertwined dimorphisms. We used well-characterized 3T3-L1 murine adipocytes to demonstrate that dihydrotestosterone (DHT) reduced Leptin (Lep) transcript abundance and cytosolic and secreted leptin protein. The magnitude of this effect was greatest on secreted leptin, which was decreased by DHT to 30% of the control. In contrast, 17β-estradiol significantly increased the abundance of transcripts encoding leptin and increased secreted leptin to 230% of the control. Treatment with estrogen and androgen receptor antagonists had opposite effects on Lep transcript abundance to steroid treatments, indicating that these transcriptional effects are mediated through the canonical steroid hormone signaling pathways. These results indicate that short-term treatments with steroid hormones are sufficient to alter both Lep transcript accumulation and leptin protein secretion, and may play a role in the sexual dimorphism of this adipokine.

Functional differences between AMPK α1 and α2 subunits in osteogenesis, osteoblast-associated induction of osteoclastogenesis, and adipogenesis

The endocrine role of the skeleton-which is impaired in human diseases including osteoporosis, obesity and diabetes-has been highlighted previously. In these diseases, the role of AMPK, a sensor and regulator of energy metabolism, is of biological and clinical importance. Since AMPK’s main catalytic subunit α has two isoforms, it is unclear whether functional differences between them exist in the skeletal system. The current study overexpressed AMPKα1 and α2 in MC3T3-E1 cells, primary osteoblasts and mouse BMSCs by lentiviral transduction. Cells overexpressing AMPKα2 showed higher osteogenesis potential than AMPKα1, wherein androgen receptor (AR) and osteoactivin played important roles. RANKL and M-CSF were secreted at lower levels from cells overexpressing α2 than α1, resulting in decreased osteoblast-associated osteoclastogenesis. Adipogenesis was inhibited to a greater degree in 3T3-L1 cells overexpressing α2 than α1, which was modulated by AR. An abnormal downregulation of AMPKα2 was observed in human BMSCs exhibiting the fibrous dysplasia (FD) phenotype. Overexpression of AMPKα2 in these cells rescued the defect in osteogenesis, suggesting that AMPKα2 plays a role in FD pathogenesis. These findings highlight functional differences between AMPKα1 and α2, and provide a basis for investigating the molecular mechanisms of diseases associated with impaired functioning of the skeletal system.

Inhibition of adipogenic differentiation of human SGBS preadipocytes by androgen-regulated microRNA miR-375

Late-onset hypogonadism (LOH), defined as a combination of low serum testosterone (T) levels in combination with clinical signs and symptoms of androgen deficiency in ageing men, is nowadays a well-characterized disease. Testosterone therapy in males affected by hypogonadism leads to a significant decrease of fat mass. In humans, the exact molecular mechanism of T effects on inhibition of adipogenesis is still unknown. We hypothesized that specific microRNAs could be regulated by androgens which might cause an inhibition of adipogenic differentiation. To confirm this hypothesis, human mesenchymal stem cells and a preadipocyte cell line were differentiated into mature adipocytes and in parallel treated with testosterone and dihydrotestosterone. The expression level of miR-375 was upregulated during adipogenic differentiation and downregulated after androgen treatment. Furthermore, we could show that after androgen treatment the decreased expression of miR-375 led to increased expression levels of adiponectin receptor 2 (ADIPOR2) compared to untreated adipocytes. Moreover, inhibition of miR-375 also mediated a decreased adipogenic differentiation and increased ADIPOR2 expression levels. In summary, we identified miR-375 as an androgen regulated microRNA, which could play an important role for understanding the mechanism of the increase in visceral fat mass and the associated insulin resistance caused by testosterone deficiency.

Reversine increases the plasticity of lineage-committed preadipocytes to osteogenesis by inhibiting adipogenesis through induction of TGF-β pathway in vitro

Reversine has been reported to reverse differentiation of lineage-committed cells to mesenchymal stem cells (MSCs), which then enables them to be differentiated into other various lineages. Both adipocytes and osteoblasts are known to originate from common MSCs, and the balance between adipogenesis and osteogenesis in MSCs is reported to modulate the progression of various human diseases, such as obesity and osteoporosis. However, the role of reversine in modulating the adipogenic potential of lineage-committed preadipocytes and their plasticity to osteogenesis is unclear. Here we report that reversine has an anti-adipogenic function in 3T3-L1 preadipocytes in vitro and alters cell morphology and viability. The transforming growth factor-β (TGF-β) pathway appears to be required for the anti-adipogenic effect of reversine, due to reversine-induced expression of genes involved in TGF-β pathway and reversal of reversine-inhibited adipogenesis by inhibition of TGF-β pathway. We show that treatment with reversine transformed 3T3-L1 preadipocytes into MSC-like cells, as evidenced by the expression of MSCs marker genes. This, in turn, allowed differentiation of lineage-committed 3T3-L1 preadipocytes to osteoblasts under the osteogenic condition in vitro. Collectively, these findings reveal a new function of reversine in reversing lineage-committed preadipocytes to osteogenesis in vitro, and provide new insights into adipose tissue-based regeneration of osteoblasts.