Human Growth Hormone (Somatotropin)
Human growth hormone is required for normal bodily growth and development—growth of the bone, muscle, and cartilage cells. Also called human somatotropin (and abbreviated hGH or hST), the hormone is a protein produced by the pituitary gland. It functions by promoting RNA and protein synthesis so that cells will multiply and differentiate. Human growth hormone is also necessary for metabolism of fat, water, and minerals.
In the 1930s, the American biologist Herbert Evans (1882-1971) demonstrated how pituitary extract can greatly stimulate animal growth, producing a condition called gigantism. In the 1940s, Evans and his colleague Choh Hao Li isolated the growth hormone from cattle. Other scientists isolated growth hormones in different animal species. In 1966, Li and endocrinologist Harold Papkoff (1925-) determined the sequence of human growth hormone, showing that it has 245 amino acids. In 1970, Li and other scientists independently synthesized human growth hormone using genetic engineering techniques. This led to the first production of genetically engineered human growth hormone in the 1980s by Eli Lilly and Co. and Genentech.
Li and Papkoff also demonstrated how the somatotropin molecule varies from species to species in size and composition, so that it is active only in its own or a very closely related species. There is a forty percent variation among species. Human and monkey growth hormones differ greatly in size and composition from those of other species. The growth hormone in cattle, for instance, consists of 396 amino acids, while that of monkeys has 241.
This distinction has become important for human health reasons in the 1990s. Bovine growth hormone--generally referred to has bST or bovine somatotropin--is being considered for general use by the dairy industry to increase the amount of milk a cow produces. There has been concern that trace amounts of bST in milk will affect human consumers. However, many experts think this is unlikely because bST is active only in cattle and closely-related species, such as sheep.
It is now known that human growth hormone is an unusually designed bundle of helixes. It binds to its receptor molecule in a complex of one hGH molecule and two similar but not identical receptor molecules.
Lack of normal amounts of growth hormone during childhood can result in a type of dwarfism in which the person is small but has normal proportions and intelligence. Injections of human growth hormone can help the person reach normal or near-normal size if the condition is diagnosed while the bones can still grow (before the long bones have closed).
If the body produces too much growth hormone in childhood, the person can have much greater than normal height--what is called gigantism. If the growth hormone overproduction occurs in adulthood, the bones of the feet, hands, and face thicken in a condition called acromegaly.
The pituitary releases human growth hormone when it receives a signal from the brain's hypothalamus in the form of growth hormone releasing factor (GHRF). However, release can also be triggered by sleep, exercise, fasting, or hypoglycemia (low blood sugar). Release can be inhibited by the hormone somatostatin, as well as lack of sleep, high blood sugar (hyperglycemia), obesity, and a high blood level of free fatty acids. The American endocrinologist Roger Guillemin and colleagues isolated somatostatin in 1973 and GHRF in 1984.
In 1989 the pharmaceutical firm of Hoffmann-La Roche announced the development of an artificially produced growth hormone releasing factor, which, when administered, stimulates the body to produce normal amounts of growth hormone. This may have advantages for a patient because a smaller dose is required than that of growth hormone. Also GHRF is a much smaller molecule, with only forty-four amino acids, so it can be administered by a skin patch or inhalant, rather than by injection.
In addition to its benefits for children suffering from dwarfism, hGH injections can help reverse many of the biological signs of aging. In 1985, Dr. Daniel Rudman published his findings in the Journal of the American Geriatrics Society that there is a correlation between people who show the signs of aging quickly and low levels of hGH. In a study conducted on twenty-one men between the ages of sixty-one and eighty, hGH injections increased the lean body mass, decreased body fat composition, thickened and firmed the skin, and increased the bone density of his patients. Other benefits of hGH injections include reduction of truncal obesity, increased energy levels, enhanced sexual performance, improved memory and vision, and strengthened immune function. The FDA approved the use of hGH for pituitary-deficient adults and for those suffering from SDS (somatrotrophin deficiency syndrome) in 1996. HGH injections are not approved for use in healthy adults, and are extremely expensive (from $800 a month to $18,000 a year). Further research is being conducted into the use of natural secretagogues (hormone precursors) to boost the release of hGH in adults to help reverse the aging process.
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