Expression of androgen receptor target genes in skeletal muscle

Abstract

We aimed to determine the mechanisms of the anabolic actions of androgens in skeletal muscle by investigating potential androgen receptor (AR)-regulated genes in in vitro and in vivo models. The expression of the myogenic regulatory factor myogenin was significantly decreased in skeletal muscle from testosterone-treated orchidectomized male mice compared to control orchidectomized males, and was increased in muscle from male AR knockout mice that lacked DNA binding activity (ARΔZF2) versus wildtype mice, demonstrating that myogenin is repressed by the androgen/AR pathway. The ubiquitin ligase Fbxo32 was repressed by 12 h dihydrotestosterone treatment in human skeletal muscle cell myoblasts, and c-Myc expression was decreased in testosterone-treated orchidectomized male muscle compared to control orchidectomized male muscle, and increased in ARΔZF2 muscle. The expression of a group of genes that regulate the transition from myoblast proliferation to differentiation, Tceal7, p57Kip2, Igf2 and calcineurin Aa, was increased in ARΔZF2 muscle, and the expression of all but p57Kip2 was also decreased in testosterone-treated orchidectomized male muscle compared to control orchidectomized male muscle. We conclude that in males, androgens act via the AR in part to promote peak muscle mass by maintaining myoblasts in the proliferative state and delaying the transition to differentiation during muscle growth and development, and by suppressing ubiquitin ligase-mediated atrophy pathways to preserve muscle mass in adult muscle.

Muscle-specific androgen receptor deletion shows limited actions in myoblasts but not in myofibers in different muscles in vivo

“…The highly androgen-dependent LA muscle has a 50% reduction in mass in both mAR ΔZF2 lines, associated with decreased fiber cross-sectional area. Although the LA is used widely as a model muscle to assess the anabolic actions of androgens and SARMs, our results show that it differs in both the androgen dependence of its mass as well as the pattern of AR target gene expression compared with hind-limb muscles, as we have previously shown (Rana et al 2014). These facts raise Amd1,Wnt4,Fzd4,p57,Igf2,Tceal7,Itgb1bp3,Tgfb1,Tgfb2 and calcineurin Aa in (A and B) gastrocnemius (GAST) muscle and (C and D) levator ani (LA) muscle from WT and AR ΔZF2 male littermates of the (A and B) MCK mAR ΔZF2 and (C and D) α-actin mAR ΔZF2 lines (n = 9-13/group).…”

“…However, the fact that this loss of repression occurred in the mAR ΔZF2 GAST muscles, while GAST mass was normal, indicates that this change in expression is not sufficient to impact on muscle mass in the absence of other changes (Rana et al 2014). In contrast, Myf5 was upregulated in the LA muscle of both mAR ΔZF2 lines but not in GAST muscle, suggesting that this regulation is specific to the LA.…”

“…Of the myogenic regulatory factors, myogenin showed increased expression in LA but was also increased in the GAST of both mAR ΔZF2 lines and global AR ΔZF2 . Myogenin drives terminal differentiation of myoblasts into myotubes (Hasty et al 1993, Nabeshima et al 1993, Patapoutian et al 1995, Rawls et al 1995, and we previously showed that myogenin is repressed by androgens in vitro and in vivo (Rana et al 2014). The current data Table 2 Gene expression in muscle in MCK mAR ΔZF2 and α-actin mAR ΔZF2 lines, global AR ΔZF2 (MacLean et al 2008b), and testosterone-treated orchidectomy models (Axell et al 2006, Rana et al 2011.…”

“…In contrast, Myf5 was upregulated in the LA muscle of both mAR ΔZF2 lines but not in GAST muscle, suggesting that this regulation is specific to the LA. Myf5 may be one of the genes mediating the androgen effects on muscle mass in the LA, but the fact that it is not upregulated in global AR ΔZF2 hind-limb muscle suggests that this regulatory pathway in the LA is not widely applicable to or representative of all skeletal muscle (Rana et al 2014). We identified a group of genes that showed a consistent pattern of regulation by androgens and the AR in muscles that had decreased mass (Table 2, Supplementary Fig.…”

“…Maximal hypertrophic response to muscle overloading requires myofiber AR, but also involves other target cells (Ferry et al 2014). The relative contribution of these mechanisms in different muscle cell types, as well as the AR target genes mediating these actions (Rana et al 2014), remains to be determined.…”