Paracrine Secretion: Function, Role & Diseases

Paracrine secretion is the medical term for hormone secretion into the interstitium that acts on cells in the immediate environment. Paracrine secretion primarily serves to differentiate tissues. Paracrine disorders can affect bone formation, for example, and show effects on the entire hormonal system.

What is paracrine secretion?

Paracrine secretion is the medical term for hormone secretion into the interstitium that affects cells in the immediate environment. Paracrine secretion is a secretory pathway of glands and gland-like cells. In this process, secretions such as growth factors or hormones are not transported endocrine via the blood to the target tissues, but act on their immediate environment. Autocrine secretion is a special form of this principle. In this secretion pathway, the secreted substances act back on the secreting cells themselves. For this purpose, the secreting cells are themselves equipped with receptors to which their own secretions can bind. Although secretions are basically designed to act outside the glandular cells, they thus have an intracellular effect. Paracrine secretions without autocrine action show an effect only on the immediately adjacent cells. Some glandular cells of the human organism are involved in endocrine and paracrine secretions at the same time. An example of such cells are the intermediate Leydig cells located in the interstitium of the testis. Thus, paracrine and endocrine secretion processes are usually complementary rather than mutually exclusive. However, they also possess mutually polar properties. The main function of paracrine secretory processes is to stimulate growth functions or differentiation processes.

Function and task

Hormones control the growth and function of individual cells, tissues, and organs in the human body over the long term. In contrast, the autonomic nervous system merely takes over the control of processes that are limited in time. The hormone system thus has global tasks of life support, since it influences organs and controls cell metabolism and the differentiation of individual cells. The individual effects of all hormones are optimally coordinated and regulate each other to a certain degree by inhibiting or stimulating each other. In the human body, only tissue hormones and so-called cytokines are secreted paracrine. Cytokines are regulatory proteins, i.e. peptides. They mainly control the immune response and are produced by various entities of the immune system, such as lymphocytes. The immunological proteins are not always of paracrine action, but also exhibit endocrine action. Their paracrine effect corresponds to a large extent to the special form of autocrine secretion. The paracrine, autocrine, and endocrine effects of cytokines are network-like and form homeostasis, that is, a balance for maintaining complex organ functions and other processes in the organism. In addition to cytokines, the secretions of some endocrine cells are also partially secreted by paracrine pathways. For example, the beta cells of the pancreas and some cells in the anterior pituitary gland secrete their hormones in a complementary paracrine form to the endocrine form, and thus, after release into the interstitium, act on cells in the immediate vicinity that are equipped with a receptor for the hormone in question. Depending on its type and concentration, the secretion triggers a specific response after binding to the cells. The combination of paracrine and endocrine forms alters the action of the secreted signaling substances. The regulatory patterns in paracrine secretion consist primarily of environmental inhibition. Such environmental inhibition prevents immediately adjacent cells in tissue patterns, for example, from differentiating into exactly the same form during differentiation. In contrast, in the paracrine special form of autocrine secretion, ultrashort feedback is the best-known regulatory mechanism. Here, after secretion, the secretions bind to the receptor proteins of the secreting cell itself, thereby inhibiting its own production.

Diseases and disorders

If too few or too many paracrine hormones are secreted, this has an effect on the entire hormone balance and thus also on the organs or tissues of the organism because of the close interaction of the secretion pathways.The individual symptoms of a hormone formation or secretion disorder are therefore diverse. The medical field of endocrinology deals with hormonal diseases and thus also disturbed paracrine secretion. As a rule, growth and development are disturbed when an endocrine or paracrine disease is present. For example, paracrine factors may play as great a role in the development of osteoporosis as they do in the misregulation of metabolism. One of the most important discoveries is the relevance of paracrine autocrine processes in the development of cancer. In particular, secreted growth factors play a role here, stimulating tissues to grow when the intracellular signaling cascade is disturbed. The molecular mechanisms of action of paracrine and autocrine substances, the receptors for these substances, and the regulatory circuitry involved in the release of growth factors have become a focus of cancer research in recent years. For example, because of autocrine growth control, tumor growth is independent of external factors. Therefore, autocrine-regulated growth control offers itself as a starting point for modern cancer therapy. The concentration of growth factors can be reduced by the administration of monoclonal antibodies, for example. Blocking the respective receptors and thus interfering with paracrine autocrine body processes may also be considered as a promising therapeutic option in cancer.