Estrogen receptors in skeletal muscle : expression and activation

Abstract

There are two estrogen receptors (ERs), ERalpha and ERbeta, which are ligand activated transcription factors. 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 effects in bone maintenance and the cardiovascular and central nervous systems as well as the reproductive system. Estrogen reduces blood lipid levels and blood pressure as well as increases insulin sensitivity and endothelial function. The effects of estrogen on skeletal muscle tissue have not been studied extensively, although a few reports indicate a role in muscle strength development and involvement in carbohydrate and lipid metabolism. Just like estrogen physical activity reduces blood lipid levels and blood pressure as well as insulin sensitivity and endothelial function. Interestingly, physical activity transpritionally activates similar genes as estrogen does, for example vascular endothelial growth factor (VEGF). Thus, considering that physical activity and estrogen have actions in common, the question whether estrogen signalling is induced by physical activity and thus could be involved in down-stream exercise-induced gene expression arises.

The overall aim of this thesis was to study the expression of ERs and their activation in skeletal muscle tissue. The specific aims were to investigate if ERalpha and ERbeta are present in human skeletal muscle. Thereafter, the ER expression was studied in subjects with low endogenous estrogen levels such as men, children and postmenopausal women. Furthermore, the localisation and possible co-expression of the both receptors were investigated. The expression levels of ERs were compared in highly endurance-trained men and moderately active men together with the target gene VEGF. Finally, the activation of ERs by estrogen as well as by muscle contractions was investigated. It was hypothesised that ERs in skeletal muscle are functional and activated by estrogen and by muscle contractions and are involved in the adaptation of skeletal muscle to physical training.

For the first time ERalpha and ERbeta were shown to be expressed in human skeletal muscle representing both sexes and various ages. Approximately 65 % of all nuclei expressed ERalpha and 70 % expressed ERbeta. The ERalpha and ERbeta were located not only to the nuclei of muscle fibres themselves but also to capillaries. Of all ERalpha- or ERbeta-positive nuclei about 25 % were located to capillaries. The two receptors were to a major extent co-expressed in the same nuclei. Endurance-trained men had a higher steady-state mRNA level of both ERalpha and ERbeta compared to normally active men, together with higher expression of VEGF. Muscle contractions of myotubes from rat also increased ERbeta mRNA levels without any effect on ERalpha mRNA. An increase in ERbeta mRNA was also seen with estrogen stimulation of the myotubes. Muscle contractions had a similar functional effect as estrogen in myotubes causing activation of estrogen response elements (ERE). In contrast to estrogen, the effects of muscle contractions were most likely independent of ERs.

That ERs are present in the skeletal muscle fibres suggests that this tissue is a target for estrogen action, which was confirmed in myotubes by ERE activation when stimulated with estrogen. In the muscle tissue, estrogen might also have direct effects on the capillaries, since ERs are located to capillaries too. The finding that contraction of myotubes activates ERE-sequences and increases ERbeta mRNA levels as well as the higher mRNA levels of ERs in endurance trained men suggest an involvement of ERs and ER target genes in the adaptation of skeletal muscle to physical exercise.