In an unexpected finding, research scientists have discovered that a deficiency of estrogen receptor alpha (ERα) in the heart induces obesity in female mice, providing important insights for postmenopausal women.
Estrogen is known to play an important role in the protection of women’s hearts, but once women are postmenopausal and estrogen levels drop, they are at an increased risk of a number of diseases and conditions, including heart disease, obesity and diabetes.
In a joint collaboration between the Baker Heart and Diabetes Institute, and the Department of Cardiovascular Research, Translation and Implementation at La Trobe University, study co-first authors Dr Yow Keat Tham and Dr Bianca Bernardo, and lead authors Professor Julie McMullen and Associate Professor David Greening found that reduced ERα in the cells responsible for heart contraction (cardiomyocytes) led to moderate heart dysfunction and increased adiposity in female mice, but not in male mice.
The surprise findings were published in Nature Cardiovascular Research today.
“We’ve identified a sex hormone receptor in the heart that can regulate adiposity (obesity) in females,” Prof McMullen said. “We’ve been interested in trying to understand the role of this estrogen receptor in the heart for some time, to see how it provides protection to the heart.
“When we blocked this estrogen receptor, we were expecting to see changes and damage largely to the heart. But rather than seeing a dramatic heart phenotype, what we saw was an adiposity phenotype. So, we observed that the female mice were heavier and had more fat mass, which we weren’t expecting at all.”
Genes that are important for contractility of the heart and metabolic function of the heart were also lower in the female heart when ERα was reduced, explaining why the female study hearts did not pump as well.
Particles, called extracellular vesicles, that were released from the female hearts with reduced ERα also contained proteins that differed from both the control group and male hearts, A/Prof Greening, an expert in extracellular vesicle biology, said.
“We found that reducing ERα in heart muscle cells (cardiomyocytes) of female mice leads to transcriptional, lipidomic and metabolic dysregulation in the heart, together with metabolic dysregulation in skeletal muscle and adipose tissue,” he said.
“Furthermore, the extracellular vesicles that are released from heart cells with reduced ERα had the capacity to reprogram skeletal muscle cells in cell culture.
“These changes to tissues, the extracellular vesicles proteome and reprogrammed skeletal muscle cells altered the cells’ molecular landscape and function. So rather than energy being expended, energy is instead stored, which explains the increased adiposity in female mice in the absence of ERα.”
This important work has implications for preventing and treating heart and metabolic disease in postmenopausal women, but also cardiotoxicity in premenopausal women receiving therapies that may inhibit or reduce ERα in the heart.
“Females who have drugs which can interact with or inhibit this particular receptor, including some chemotherapies, often become obese,” Prof McMullen said. “Now we have a better understanding of ERα, we’ve got a better chance of identifying therapies to prevent the obesity from occurring.”
A/Prof Greening said this study demonstrated that “extracellular vesicles – nanovesicles with their packaged molecular cargo – are systemic signalling regulators that can travel to, and impact other organs in the body, including adipose tissue and skeletal muscle”.
“Extracellular vesicles thereby provide a new paradigm in crosstalk between cells, tissues and organs in health and disease,” he said.
Prof McMullen’s work for this study was supported by the Heart Foundation.