(M-484) Investigating the relationship between Myogenic Gene Expression and Estradiol Concentrations throughout the Menstrual Cycle

It has been shown that estradiol (E2) can enhance muscle strength, mass, and regeneration (Tiidus et al.).   Nevertheless, it remains unclear the mechanisms driving these favorable effects. It has been shown in post-menopausal females and oral contraceptive users, that administration of conjugated estrogens and exogenous estradiol can increase myogenic and decrease proteolytic factor expression in female subjects (Dieli-Conwright et al., Oxfeldt et al.). However, there is a lack of data regarding the expression of myogenic and proteolytic factors across eumennorheic females over the menstrual cycle, where E2 fluctuations are extreme. We hypothesized that with increased E2 concentrations, myogenic factors would be upregulated while proteolytic factors would be downregulated.

This study was conducted per the UT Southwestern (SW) Human Research Protection Program. Pre-menopausal women between the ages of 18-39 undergoing hip surgery at UTSW Medical Center were recruited (n=19). Whole blood and a biopsy from the gluteus medius were collected from each subject and analyzed for endogenous hormone levels and gene expression, respectively. The blood sent to a clinical laboratory service for hormone characterization. To determine gene expression via quantitative polymerase chain reaction (qPCR), the frozen muscle tissue was pulverized. Total RNA was extracted using Trizol and was reverse transcribed using standard procedures (High Capacity RT Kit, Applied Biosystems). The myogenic factors of interest were myoblast determination protein 1 (MYOD1), myogenic factor (MYF5), MYF6, MYOG, while the proteolytic factors were Forkhead box O3 (FOXO3) and E3 ubiquitin-protein ligase TRIM63 (MuRF1). These transcripts were amplified from cDNA using Taqman Universal PCR Master Mix (Applied Biosystems) and detected using Taqman primers (Applied Biosystems). Data were normalized to 18s (housekeeping gene). For this study, only subjects in the menses and follicular phase of the menstrual cycle (n=12) were included in the analysis to minimize the influence of progesterone on gene expression. Gene expression was reported as a relative quantity (RQ), determined by the delta Ct method.

Estradiol concentrations ranged between 26-356 pg/mL, while progesterone levels were negligible at a range of 0.5 – 1 ng/mL. Both ranges of hormones are consistent with the physiological levels during menses and the follicular phase of the menstrual cycle. There were no statistically significant relationships between the myogenic and proteolytic genes assayed. However, two genes had a tendency towards a significant relationship with estradiol concentrations. Myogenic factor 6 (MYF6) increased as circulation estradiol concentrations increased (p=0.07, Fig.1A), while MuRF1 had a negative relationship with circulating estradiol concentrations (p=0.09, Fig.1B).

While no statistically significant relationships were observed between myogenic or proteolytic factor genes and circulating estradiol concentrations, two factors showed a notable trend towards significance: MYF6 (positive relationship, p=0.07) and MuRF1 (negative relationship, p=0.09). The MYF6 protein, known for its role in myogenic differentiation, has also been shown to play a significant role in maintaining the satellite cell pool by stimulating the secretion of myokines, particularly epidermal growth factor (EGF), which prevents premature differentiation of satellite cells into myoblasts (Lazure et al. 2020). On the other hand, the MuRF1 protein is a critical component of muscle atrophy. It acts as a protease that specifically breaks down myosin heavy chains, leading to sarcomere breakdown and eventual muscle mass reduction (Clarke et al. 2007). Our results suggest a possible association between increased estradiol levels and increased regenerative capacity and decreased catabolic state. However, it is crucial to interpret these findings with caution due to the limited sample size. Nonetheless, these high E2 conditions may create a favorable environment for muscle recovery following injury and may carry potential implications for the regenerative and muscle maintenance capacity of pre-menopausal women with reduces E2 levels (e.g. pituitary amenorrhea). To further explore these findings, future research should aim to increase the sample size and measure the protein levels and localizations of these factors within the muscle.

Dieli-Conwright CM, Spektor TM, Rice JC, Sattler FR, Schroeder ET. Influence of hormone replacement therapy on eccentric exercise induced myogenic gene expression in postmenopausal women. J Appl Physiol. 2009;107(5).

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