New evidence in the April 21, 2006 Cell challenges the long-standing notion that declining estrogen levels alone lead to osteoporosis after women go through menopause. The researchers rather found that high levels of pituitary-derived follicle-stimulating hormone (FSH) cause bone loss in mice. The pituitary is a master gland found at the base of the brain.
What's more, the researchers reported, mice with symptoms of severe estrogen deficiency, lacking either the FSH hormone or its receptor, became resistant to bone loss.
FSH normally triggers egg development and stimulates estrogen production by the ovaries, the researchers explained. As women approach menopause and estrogen levels decline, the pituitary gland responds by releasing more FSH.
Treatments that prevent bone loss by blocking FSH might therefore offer alternative methods to treat or prevent osteoporosis without the risks associated with other hormone replacement therapies, said Mone Zaidi of Mount Sinai School of Medicine. For example, estrogen replacement therapy has been linked to an increased risk of breast cancer, especially when taken in combination with progesterone, he said.
"For the last three decades, the idea has clearly been that estrogen loss is responsible for the changes experienced in postmenopausal women--including flushing, dryness, and bone loss," Zaidi said. "Although FSH levels rise sharply in parallel to estrogen decline, a direct effect of FSH on the skeleton had never before been explored."
"We've now found that, irrespective of the nature or severity of estrogen deficiency, an intact pituitary and, more specifically, high FSH levels are prerequisites for bone loss in animals with reduced or absent ovarian function."
Osteoporosis affects nearly 45 million women worldwide with fracture rates that far exceed the combined incidence of breast cancer, stroke, and heart attacks, the researchers said. The disease results from a disruption of the fine balance between bone formation and resorption. After menopause, resorption exceeds new bone formation, leading to a net bone loss that can be slowed by estrogen therapy through mechanisms that have remained somewhat murky, Zaidi said.
However, emerging evidence has begun to cast some doubt on whether estrogen deficiency can fully explain bone loss after menopause, he added. For example, mice without an intact pituitary gland become resistant to the effects of ovary loss on bone density. FSH levels also show a closer correlation than estrogen levels to the rate of bone turnover in postmenopausal women.
The researchers now show that mice lacking FSH or its receptor become resistant to bone loss despite severe loss of ovarian function. In mice with normal ovaries and approximately half the normal concentration of FSH, bone mass increased due to a decline in bone resorption by cells known as osteoclasts, which break down bone. Indeed, they report, FSH stimulates receptors found on the surface of bone-degrading osteoclasts and their precursors, leading to the bone cells' formation and function.
The study suggests that FSH plays a role in the normal process by which bone is mobilized by osteoclasts before it can be replaced, Zaidi said.
"As you run or walk throughout life, microcracks develop in bone," he explained. "Therefore, bone remodeling and replacement with new bone is required to maintain skeletal integrity. Osteoclasts essentially dig around those cracks to clear the way for repairs."
In combination with the research group's earlier finding that thyroid-stimulating hormone directly regulates the skeleton, the findings revise the understanding of how pituitary-derived hormones function, Zaidi added. Scientist had thought pituitary hormones acted primarily through their effects on other endocrine glands.
"This should change the textbook picture of pituitary hormone physiology," he said.