The concentration of ARs in skeletal muscle depends on the muscle fiber type, sex, training status, and androgen concentrations. AR protein content is a critical variable in RT-induced androgen-mediated skeletal muscle protein accretion in healthy men (31). The androgen/AR complex serves as a transcription factor leading to increased protein synthesis. Upon androgen binding to the ligand binding domain (LBD), dissociation from the heat-shock proteins occurs, hyperphosphorylation, dimerization, and conformational changes occur converting the AR to a transcription factor that interacts with androgen response elements or AREs of DNA (58). Phosphorylation may occur during ligand binding and through other signaling pathways indicating that the AR is cross-regulated by other ligand-receptor interactions (54). The signaling effects of androgens are mediated through the AR which belongs to a family of steroid receptors. It's targeted metabolic remodeling driven by receptor-specific hormone signaling. That directly impair insulin signaling in muscle and liver tissue. It activates GHRH receptors in the pituitary gland, triggering a cascade that elevates IGF-1 (insulin-like growth factor 1) and shifts substrate utilization toward lipolysis in visceral fat stores specifically. Tesamorelin is a synthetic analogue of growth hormone-releasing hormone (GHRH) that stimulates endogenous growth hormone (GH) production from the anterior pituitary. Several growth hormone analogues featuring amino acid substitutions, deletions, and extensions have been developed, resulting in variants with distinct pharmacokinetic and pharmacodynamic profiles.citation needed As of 2005, recombinant growth hormones available in the United States (and their manufacturers) included Nutropin (Genentech), Humatrope (Lilly), Genotropin (Pfizer), Norditropin (Novo), and Saizen (Merck Serono). Expression of GRβ in cells is increased by proinflammatory cytokines interleukins IL-1, -2, -4, -7, -8 and -18; tumor necrosis factor -alpha (TNFα); and interferons α and γ (168, 200). In fast fibers, glucocorticoid exposure in the absence of exercise increases the activity of non-lysosomal proteases (214). In skeletal muscle, glucocorticoid receptor expression is more abundant in fast than slow twitch fibers (211, 212). Via these proteolytic systems, expression of genes involved in atrophy ("atrogenes") are increased, which target proteins for degradation by the proteasome machinery (210). Consequently, slow twitch muscle fibers appear to be resistant to the catabolic action of glucocorticoids (213) whereas, fast twitch muscle fibers are more sensitive to the catabolic action of glucocorticoids (214). Atrogenes include transcription factor FOXO, a major switch for the stimulation of several atrogenes, and two ubiquitin ligases atrogin-1 and MuRF-1, involved in the targeting of protein to be degraded by the proteasome machinery, and LC3 (186, 201, 209, 210). The catabolic actions of cortisol resulting in muscle proteolysis occur largely via the ubiquitin–proteasome and lysosomal systems (186, 209–211). These results indicated GH has local effects that may be independent of increased levels of the circulating IGF-I (Ohlsson et al., 2009). The local production of IGF-1 is controlled primarily by GH and other hormones (e.g., parathyroid and thyroid hormones) (Bikle et al., 2015); suggesting GH's effect on growth may be mediated in part via increased local IGF-1 production and/or action. Bikle et al. also showed muscle atrophy was more pronounced after ablation of muscle IGF-1 production than when hepatic IGF-1 production was suppressed (Bikle et al., 2015); exhibiting circulating levels of IGF-1 (i.e., endocrine factor) do not effect overall growth responses (Ohlsson et al., 2000; Velloso, 2008). However, the main muscle anabolic effects of GH are believed to be indirect—via inducing the hepatic generation of IGF-1 triggering the IGF-1-Akt-mTOR pathway; in turn resulting in MPS augmentation and as a consequence muscle maintenance and growth (Sandri et al., 2013; Schiaffino et al., 2013). What is the relationship between IGF-1 and growth hormone? Causes include aging (somatopause), GH deficiency, protein undernutrition, sleep deprivation, severe insulin resistance, hypothyroidism, liver disease, and chronic inflammation. Tesamorelin reduces it specifically, through a mechanism (GHRH receptor activation → pulsatile GH → IGF-1 → hormone-sensitive lipase in visceral adipocytes) that diet and exercise cannot replicate. Visceral adipose tissue accumulation driven by declining growth hormone secretion in aging populations. Age-related GH decline, insulin resistance, inflammatory signaling from existing VAT. While GR expression does not appear to change following resistance exercise (76), receptor activation occurs at a rate that is independent of both fiber type and delivery of steroid to working muscles during exercise (215). Yet, in response to exercise, both fast and slow fibers experience increases in myofibrillar protease activity followed by anti-catabolic actions (214). Raised expression of 11β-HSD1 (Type 1) in skeletal muscle is believed to play role in mechanisms that contribute to the development of metabolic syndrome (180) insulin resistance (181), and hypertension (182).
