Use of artificial androgen receptor coactivators to alter myoblast proliferation

Microarray profiling of gene expression in C2C12 myotubes trained by electric pulse stimulation

Electric pulse-stimulated C2C12 myotubes are gaining interest in the field of muscle physiology and biotechnology because electric pulse stimulation (EPS) enhances sarcomere structure development and active tension generation capability. Recently, we found that termination of EPS results in the rapid loss of active tension generation accompanied by disassembly of the sarcomere structure, which may represent an in vitro muscle atrophy model. To elucidate the molecular mechanism underlying this rapid loss of active tension generation and sarcomere structure disassembly after termination of EPS, we performed transcriptomic analysis using microarray. After termination of EPS, 74 genes were upregulated and 120 genes were downregulated after 30 min; however, atrophy-related genes were not found among these genes. To further assess the effect of EPS on gene expression, we re-applied EPS after its termination for 8 h and searched for genes whose expression was reversed. Four genes were upregulated by termination of EPS and downregulated by the re-application of EPS, whereas two genes were downregulated by termination of EPS and upregulated by the re-application of EPS. Although none of these genes were atrophy- or hypertrophy-related, the results presented in this study will contribute to the understanding of gene expression changes that mediate rapid loss of active tension generation and sarcomere structure disassembly following termination of EPS in C2C12 myotubes.

The role of GH and IGF-I in mediating anabolic effects of testosterone on androgen-responsive muscle

Testosterone (T) supplementation increases skeletal muscle mass, circulating GH, IGF-I, and im IGF-I expression, but the role of GH and IGF-I in mediating T's effects on the skeletal muscle remains poorly understood. Here, we show that T administration increased body weight and the mass of the androgen-dependent levator ani muscle in hypophysectomized as well as castrated plus hypophysectomized adult male rats. T stimulated the proliferation of primary human skeletal muscle cells (hSKMCs) in vitro, an effect blocked by transfecting hSKMCs with small interference RNA targeting human IGF-I receptor (IGF-IR). In differentiation conditions, T promoted the fusion of hSKMCs into larger myotubes, an effect attenuated by small interference RNA targeting human IGF-IR. Notably, MKR mice, which express a dominant negative form of the IGF-IR in skeletal muscle fibers, treated with a GnRH antagonist (acyline) to suppress endogenous T, responded to T administration by an attenuated increase in the levator ani muscle mass. In conclusion, circulating GH and IGF-I are not essential for mediating T's effects on an androgen-responsive skeletal muscle. IGF-I signaling plays an important role in mediating T's effects on skeletal muscle progenitor cell growth and differentiation in vitro. However, IGF-IR signaling in skeletal muscle fibers does not appear to be obligatory for mediating the anabolic effects of T on the mass of androgen-responsive skeletal muscles in mice.

Chicken ovalbumin upstream promoter-transcription factor II regulates nuclear receptor, myogenic, and metabolic gene expression in skeletal muscle cells

We demonstrate that chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) mRNA is more abundantly expressed (than COUP-TFI mRNA) in skeletal muscle C2C12 cells and in (type I and II) skeletal muscle tissue from C57BL/10 mice. Consequently, we have utilized the ABI TaqMan Low Density Array (TLDA) platform to analyze gene expression changes specifically attributable to ectopic COUP-TFII (relative to vector only) expression in muscle cells. Utilizing a TLDA-based platform and 5 internal controls, we analyze the entire NR superfamily, 96 critical metabolic genes, and 48 important myogenic regulatory genes on the TLDA platform utilizing 5 internal controls. The low density arrays were analyzed by rigorous statistical analysis (with Genorm normalization, Bioconductor R, and the Empirical Bayes statistic) using the (integromics) statminer software. In addition, we validated the differentially expressed patho-physiologically relevant gene (identified on the TLDA platform) glucose transporter type 4 (Glut4). We demonstrated that COUP-TFII expression increased the steady state levels of Glut4 mRNA and protein, while ectopic expression of truncated COUP-TFII lacking helix 12 (COUP-TFΔH12) reduced Glut4 mRNA expression in C2C12 cells. Moreover, COUP-TFII expression trans-activated the Glut4 promoter (-997/+3), and ChIP analysis identified selective recruitment of COUP-TFII to a region encompassing a highly conserved SP1 binding site (in mouse, rat, and human) at nt positions -131/-118. Mutation of the SpI site ablated COUP-TFII mediated trans-activation of the Glut4 promoter. In conclusion, this study demonstrates that in skeletal muscle cells, COUP-TFII regulates several nuclear hormone receptors, and critical metabolic and muscle specific genes.

Generation and analysis of an androgen-responsive myoblast cell line indicates that androgens regulate myotube protein accretion

Androgens have anabolic actions in skeletal muscle and could potentially act to: (a) increase proliferation of myoblasts; (b) delay differentiation to myotubes; and (c) induce protein accretion in post-proliferative myofibers. To identify the site of androgens action, we investigated the proliferative response of the C2C12 mouse myoblast cell line to testosterone and dihydrotestosterone (DHT) treatment. Neither androgens affected cell proliferation after up to 7 days treatment, nor was there a synergistic effect of androgens on the proliferative response of C2C12 cells to IGF-I treatment. However, proliferating C2C12 cells expressed 0.1% of the level of androgen receptor (AR) mRNA found in adult mouse gastrocnemius muscle (p<0.01). Therefore, we generated mouse C2C12 myoblast cell lines stably transfected with the mouse AR cDNA driven by the SV40 promoter (C2C12-AR). C2C12-AR cell proliferation, differentiation, and protein content were analyzed in response to androgen treatment. Our data demonstrated that androgen treatment does not alter either proliferation rate or differentiation rate of C2C12-AR cells. However, treatment of differentiated C2C12-AR myotubes with 100 nM DHT for 3 days caused a 20% increase in total protein content vs vehicle treatment (p<0.05). This effect was not observed in control C2C12 cells transfected with empty vector. These data suggest that androgens act via the AR to upregulate myotube protein content. This model cell line will be useful to further investigate the molecular mechanisms via which androgens regulate protein accretion.

The role of hormones, cytokines and heat shock proteins during age-related muscle loss

Ageing is associated with a progressive decline of muscle mass, strength, and quality, a condition known as sarcopenia. Due to the progressive ageing of western populations, age-related sarcopenia is a major public health problem. Several possible mechanisms for age-related muscle atrophy have been described; however the precise contribution of each is unknown. Age-related muscle loss is thought to be a multi-factoral process composed of events such as physical activity, nutritional intake, oxidative stress, inflammatory insults and hormonal changes. There is a need for a greater understanding of the loss of muscle mass with age as this could have a dramatic impact on the elderly and critically ill if this research leads to maintenance or improvement in functional ability. This review aims to outline the process of skeletal muscle degeneration with ageing, normal and aberrant skeletal muscle regeneration, and to address recent research on the effects of gender and sex steroid hormones during the process of age-related muscle loss.