How Hormones, as Ancient Signalling Molecules, Regulate Diverse Biological Processes Through Evolution

Abstract

Hormones are chemical messengers that regulate and coordinate all major metabolic, growth and developmental activities of different populations of cells of an organism. Since most hormones of higher organisms can be detected in primitive organisms, it follows that hormones arose during evolution well before many of the functions they regulate in higher organisms. Two examples to illustrate this principle are the protein hormone prolactin and the iodothyronine thyroid hormone. The first regulates such diverse activities as lactation in mammals, crop sac development in birds and migration in fish; the second hormone controls metabolic rate in homeotherms and different functions in different tissues of the same developing individual as during amphibian metamorphosis, such as restructuring of the digestive system, gene switching for new blood proteins, new cell development during limb formation and programmed cell death in unwanted tissues in the tail and intestine. The acquisition of hormonal function during evolution is likely to have coincided with the appearance of hormonal receptors, which are the key to understanding the mechanism or specificity of action of a given hormone. The hormone-receptor complex for protein hormones can be considered to have co-evolved as a unit. Most animal hormone receptors can be divided into two classes according to their localization in the hormone's target cell: (a) those located in the cell membrane and (b) in those in the nucleus. Work based on the exploitation of recombinant DNA and cell transfection has established a high degree of homology between oncogenes and both membrane and nuclear receptors. An early consequence of the formation of hormone-membrane receptor complex is at the modulation of processes such as phosphorylation of proteins, as exemplified by the control of levels of cyclic AMP. Nuclear receptors control the chromatin structure of their target genes through interaction with or structural modification of chromatin. The key role of receptors in explaining hormone action has to be considered in the context of evolution of a system of molecular linguistics in intercellular communication.