ATHENS, Ohio – Growth hormone often is prescribed to counteract such common effects of aging as loss of muscle tone and increase in body fat. But new animal studies of the natural hormone suggest that while it may improve quality of life, it may actually shorten the life span.
Researchers at Ohio University's Edison Biotechnology Institute found that mice engineered without the receptor for growth hormone lived almost one year longer than normal mice. The study, published in a recent issue of the journal Endocrinology, is the first to isolate growth hormone and analyze its impact on longevity.
Scientists say the findings have important implications for physicians who prescribe growth hormone replacement therapy for elderly patients and say doctors should use caution with these remedies until more studies are done.
"You may be improving quality of life, but you may be shortening the quantity of life," said Karen Coschigano, a scientist at the institute (EBI) and lead author of the recent study.
The project is one of many conducted at EBI to determine growth hormone's role in human health and illness, including acromegaly, diabetic kidney and eye diseases and breast cancer. To investigate its impact on aging, researchers studied three groups of mice: mice with normal or nearly normal levels of the growth hormone receptor, as well as mice with no growth hormone receptor. Mice in the last group were less than half the size of the other mice but lived almost one year longer than the other animals in the study. The normal life span for the mice used in the study was about two years, Coschigano said. Studies elsewhere have shown a similar increase in life span for mice with impaired pituitary function – which includes a lack of growth hormone activity.
Though the study suggests that disruption of the signal that activates growth hormone may prolong life, she added, the researchers have yet to determine the mechanism by which the process works. One possibility is that this disruption lowers levels of insulin – another growth stimulus – which helps to increase longevity. The Ohio University research may lend support to that theory, as the mice that lived longer also had significantly lower insulin levels than the other mice under study, she said.
More research is needed to determine how the study findings in mice may be relevant to humans, particularly elderly individuals who undergo therapy to replace growth hormone lost during the natural aging process. As growth hormone plays many important, beneficial roles in the human body – including maintaining bone and tissue health – Coschigano says halting the body's production of growth hormone isn't the best approach. For example, researchers found that while the mice without the growth hormone receptor lived longer, they also were less fertile and had weaker bones.
"Growth hormone needs to be maintained at an intermediate level," said study co-author John Kopchick, Goll-Ohio Eminent Scholar and professor of molecular and cellular biology at EBI and the university's College of Osteopathic Medicine. "Not enough is not good, but too much is definitely not good."
Instead, the researchers are searching for genes regulated by growth hormone that might play a specific role in life span. Then, a therapy could be developed that would interrupt specific gene function, allowing the body to continue to reap the benefits of growth hormone without suffering the substance's sometimes negative effects.
The study was funded in part by the State of Ohio's Eminent Scholar Program and the Sensus Drug Development Corporation. Co-authors of the study are Kopchick, Linda Bellush, a former scientist with EBI and David Clemmons of the University of North Carolina at Chapel Hill.
The Edison Biotechnology Institute is a biomedical genetics institute at Ohio University and part of the Ohio Department of Development's Thomas Edison Program.
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