Trade off situation between thymus and growth hormone: age-related decline of growth hormone is a cause of thymic involution but favorable for elongation of lifespan.

Abstract

High level of growth hormone (GH) is necessary for the activation of thymic function to promote T cell differentiation in the early stage of animal life. In the later stage of the life, administration of GH promotes the development of immune system and rejuvenates declined immune function of elderly people. By contraries, GH deficiency is favorable for the longer lifespan, as hypo-pituitary dwarf mice such as Ames and Snell dwarf mice exhibit longer lifespan than control. Furthermore over-expression of heterologous or homologous GH in transgenic mice shortens the lifespan. Ecuadorians carrying mutations of GH receptor gene are short in height, but exhibited low frequency of malignancy and no cases of diabetes. These data indicate that GH is necessary for the development of thymus dependent immune system but GH deficiency is favorable for long life span and decreases occurrence of cancer and DM. This situation is a kind of trade off situation between the immune system and GH. Thus the early decline of high level of GH occurring shortly after the birth is a cause of early decline of thymic functions, but favorable for longer lifespan. This situation could be a kind of trade off situation between thymus and GH.