There are two oestrogen receptors (ERs), ERalpha and ERbeta. ERbeta protein is expressed in human skeletal muscle in the nuclei of both myofibres and endothelial cells, whether ERalpha protein is present in this tissue is unknown. We studied the expression of ERalpha protein in human skeletal muscle biopsies taken from vastus lateralis from four men, four women, two children and two postmenopausal women. Immunohistochemistry was used to determine the proportions of nuclei that were positively stained for ERalpha, the proportion of ERalpha-positive nuclei located in the muscle fibres and in capillaries and to test for possible co-expression of ERalpha and ERbeta. Both ERs were expressed in all subjects. Of all nuclei, 63% stained for ERalpha with no sex difference. ERalpha was localised both in myofibres and in endothelial cells of the capillaries, 25% of the ERalpha-positive nuclei were located in the capillaries. ERalpha and ERbeta were generally expressed in the same nuclei. The present study shows for the first time the expression of ERalpha protein in human skeletal muscle independently of age and sex. These results might improve understanding of the physiological role of oestrogen in human skeletal muscle and raise new questions about activation of ERs in skeletal muscle.

The present study shows for the first time ERbeta mRNA and protein expression in human skeletal muscle tissue in both males and females.

Context As many sports are divided in male/female categories, governing bodies have formed regulations on the eligibility for transgender individuals to compete in these categories. Yet, the magnitude of change in muscle mass and strength with gender-affirming treatment remains insufficiently explored. Objective This study explored the effects of gender-affirming treatment on muscle function, size, and composition during 12 months of therapy. Design, settings, participants In this single-center observational cohort study, untrained transgender women (TW, n = 11) and transgender men (TM, n = 12), approved to start gender-affirming medical interventions, underwent assessments at baseline, 4 weeks after gonadal suppression of endogenous hormones but before hormone replacement, and 4 and 12 months after treatment initiation. Main outcome measures Knee extensor and flexor strength were assessed at all examination time points, and muscle size and radiological density (using magnetic resonance imaging and computed tomography) at baseline and 12 months after treatment initiation. Results Thigh muscle volume increased (15%) in TM, which was paralleled by increased quadriceps cross-sectional area (CSA) (15%) and radiological density (6%). In TW, the corresponding parameters decreased by –5% (muscle volume) and –4% (CSA), while density remained unaltered. The TM increased strength over the assessment period, while the TW generally maintained their strength levels. Conclusions One year of gender-affirming treatment resulted in robust increases in muscle mass and strength in TM, but modest changes in TW. These findings add new knowledge on the magnitude of changes in muscle function, size, and composition with cross-hormone therapy, which could be relevant when evaluating the transgender eligibility rules for athletic competitions.

These data suggest that the greater mRNA expression of ERalpha and ERbeta and the oestrogen-associated angiogenic factor VEGF support the hypothesis of an involvement of ERs in the adaptation of skeletal muscle to endurance training.

Oestrogen receptor beta (ERbeta) is expressed in human skeletal muscle tissue. In the present study, we have developed an immunohistochemical method to reveal if ERbeta is located within the muscle fibres as well as within capillaries. Skeletal muscle biopsies were obtained from m. quadriceps femoris vastus lateralis in four healthy young subjects. Immunohistochemical triple staining was applied to transverse sections of paraffin-wax-embedded tissue. The basement membrane of muscle fibres and capillaries was identified by using an antibody to collagen IV, endothelial cells using an antibody to CD34 and ERbeta using a corresponding antibody. The ERbeta-positive (ERbeta+) nuclei were located within the muscle fibre defined by the localisation of collagen IV. ERbeta+ nuclei were also, for the first time, found in endothelial cells of capillaries in skeletal muscle tissue. Quantification was performed on transverse cryostat sections after performing a double staining (collagen IV and ERbeta). It was shown that 24% of the ERbeta+ nuclei were located within capillaries, and 76% were located within muscle fibres. In conclusion, ERbeta in human skeletal muscle tissue is expressed not only in the muscle fibres themselves, but also within the capillary endothelial cells. This observation might improve understanding of the physiological role of oestrogen and its receptor.

Wiik A, Hellsten Y, Berthelson P, Lundholm L, Fischer H, Jansson E. Activation of estrogen response elements is mediated both via estrogen and muscle contractions in rat skeletal muscle myotubes. Am J Physiol Cell Physiol 296: C215-C220, 2009. First published November 19, 2008 doi:10.1152/ajpcell.00148.2008The aim of the present study was to investigate the activation of estrogen response elements (EREs) by estrogen and muscle contractions in rat myotubes in culture and to assess whether the activation is dependent on the estrogen receptors (ERs). In addition, the effect of estrogen and contraction on the mRNA levels of ER␣ and ER␤ was studied to determine the functional consequence of the transactivation. Myoblasts were isolated from rat skeletal muscle and transfected with a vector consisting of sequences of EREs coupled to the gene for luciferase. The transfected myoblasts were then differentiated into myotubes and subjected to either estrogen or electrical stimulation. Activation of the ERE sequence was determined by measurement of luciferase activity. The results show that both ER␣ and ER␤ are expressed in myotubes from rats. Both estrogen stimulation and muscle contraction increased (P Ͻ 0.05) transactivation of the ERE sequence and enhanced ER␤ mRNA, whereas ER␣ was unaffected by estrogen and attenuated (P Ͻ 0.05) by muscle contraction. Use of ER antagonists showed that, whereas the estrogen-induced transactivation is mediated via ERs, the effect of muscle contraction is ER independent. The muscle contraction-induced transactivation of ERE and increase in ER␤ mRNA were instead found to be MAP kinase (MAPK) dependent. This study demonstrates for the first time that muscle contractions have a similar functional effect as estrogen in skeletal muscle myotubes, causing ERE activation and an enhancement in ER␤ mRNA. However, in contrast to estrogen, the effect is independent of ERs and dependent on MAPK, suggesting activation via the estrogen related receptor (ERR). electrostimulation; estrogen-related receptor; ligand-independent activity; luciferase; mRNA ESTROGEN RECEPTORS (ERs) are ligand-activated transcription factors that belong to the nuclear hormone receptor super family. Estrogen, which exerts its effect via ERs, is not only a female reproductive hormone but acts almost ubiquitously in the human body and is involved in physiological and pathological states in both males and females. Estrogen has many important effects on the cardiovascular, reproductive, and central nervous system as well as for bone maintenance (14). In skeletal muscle, it has been reported that estrogen for example is involved in regulating carbohydrate and lipid metabolism (22). During exercise, estrogen modifies the energetic substrate mobilization improving fat oxidation while sparing muscle glycogen (22). Previous reports indicate a role of estrogen in muscle growth and strength development (31) but available data are not consistent (13).The two estrogen receptors ER␣ and ER␤ are expressed at the mRNA level in human skeletal mu...

BackgroundAlthough the divergent male and female differentiation depends on key genes, many biological differences seen in men and women are driven by relative differences in estrogen and testosterone levels. Gender dysphoria denotes the distress that gender incongruence with the assigned sex at birth may cause. Gender-affirming treatment includes medical intervention such as inhibition of endogenous sex hormones and subsequent replacement with cross-sex hormones. The aim of this study is to investigate consequences of an altered sex hormone profile on different tissues and metabolic risk factors. By studying subjects undergoing gender-affirming medical intervention with sex hormones, we have the unique opportunity to distinguish between genetic and hormonal effects.MethodsThe study is a single center observational cohort study conducted in Stockholm, Sweden. The subjects are examined at four time points; before initiation of treatment, after endogenous sex hormone inhibition, and three and eleven months following sex hormone treatment. Examinations include blood samples, skeletal muscle-, adipose- and skin tissue biopsies, arteriography, echocardiography, carotid Doppler examination, whole body MRI, CT of muscle and measurements of muscle strength.ResultsThe primary outcome measure is transcriptomic and epigenomic changes in skeletal muscle. Secondary outcome measures include transcriptomic and epigenomic changes associated with metabolism in adipose and skin, muscle strength, fat cell size and ability to release fatty acids from adipose tissue, cardiovascular function, and body composition.ConclusionsThis study will provide novel information on the role of sex hormone treatment in skeletal muscle, adipose and skin, and its relation to cardiovascular and metabolic disease.

Objectives: This study explored the effects of gender-affirming treatment, which includes inhibition of endogenous sex hormones and replacement with cross-sex hormones, on muscle function, size and composition in 11 transwomen (TW) and 12 transmen (TM). Methods:Isokinetic knee extensor and flexor muscle strength was assessed at baseline (T00), 4 weeks after gonadal suppression of endogenous hormones but before hormone replacement (T0), and 3 (T3) and 11 (T12) months after hormone replacement. In addition, at T00 and T12, we assessed lower-limb muscle volume using MRI, and cross-sectional area (CSA) and radiological density using CT. Results: Thigh muscle volume increased (15%) in TM, which was paralleled by increased quadriceps CSA (15%) and radiological density (6%). In TW, the corresponding parameters decreased by -5% (muscle volume) and -4% (CSA), while density remained unaltered. The TM increased strength over the assessment period, while the TW generally maintained or slightly increased in strength. Baseline muscle volume correlated highly with strength (R>0.75), yet the relative change in muscle volume and strength correlated only moderately (R=0.65 in TW and R=0.32 in TM). The absolute levels of muscle volume and knee extension strength after the intervention still favored the TW. Conclusion: Cross-sex hormone treatment markedly affects muscle strength, size and composition in transgender individuals. Despite the robust increases in muscle mass and strength in TM, the TW were still stronger and had more muscle mass following 12 months of treatment. These findings add new knowledge that could be relevant when evaluating transwomen's eligibility to compete in the women's category of athletic competitions. 14