Autocrine hormones

The main autocrine hormones in the testes are androgens, the most important of which is testosterone. As mentioned above, testosterone is converted to the more potent DHT in peripheral tissues. Testicular androgens produced by Leydig cells act locally to control spermatogenesis. Leydig cells store cholesterol in lipid droplets in their cytoplasm and use the cholesterol to form androgens. 

Hormones can also be classified by the way they get from the point of their release to their target tissue. Endocrine (from the Greek endon, within, and krinein, to release ) hormones are released into the blood and carried to target cells throughout the body (insulin is an example). Paracrine hormones are released into the extracellular space and diffuse to neighboring target cells (the eicosanoid hormones are of this type). Autocrine hormones are released by and affect the same cell, binding to receptors on the cell surface. 

Y = major route of dissemination to target cells is via blood transport N = functions mainly as paracrine or autocrine hormone, although it may enter the blood stream. 

In autocrine signaling, cells of the same type communicate with one another. The hormone produced by the signaling cell affects a cell of the same type by binding to receptors on these cells and initiating an intracellular signal cascade. If an autocrine hormone is secreted simultaneously by many cells then a strong response occurs in the cells. Autocrine mechanisms are of particular importance in the immune response (see Chapter 11). 

Hormones are classified by the distance within which they act. Autocrine hormones act on the same cell that releases them. An example is interleukin-2. Paracrine hormones act on cells close to the cell that releases them. Examples are prostaglandins and polypeptide growth hormones. Endocrines act on cells at a distance. Examples are the endocrine hormones insulin and epinephrine, which are released into the bloodstream by endocrine glands. (The term endocrine signifies internal secretions directly into the bloodstream, whereas the term exocrine signifies secretions through a duct.) A table of various endocrine hormones is provided in the cited reference. Most hormones consist variously of polypeptides, amino acid derivatives, or steroids. Some examples of sources and functions are as follows. 

Autocrine hormones affect the function of the cell they are secreted from. 

Transport in the blood is no longer a requisite for a hormonal response. Responses can occur after release of hormones into the interstitial fluid with binding to receptors in nearby ceUs, called paracrine control, or binding to receptors on the ceU that released the hormone, called autocrine control. A class of hormones shown to be synthesized by the tissue in which they act or to act in the local ceUular environment are the prostaglandins (qv). These ubiquitous compounds are derived from arachidonic acid [506-32-1] which is stored in the ceU membranes as part of phosphoHpids. Prostaglandins bind to specific ceUular receptors and act as important modulators of ceU activity in many tissues. 

PGs act locally in an autocrine or paracrine fashion in the tissues in which they are synthesized, rather than as circulating hormones, which act at a distant site. For this reason, studies localizing the enzymatic machinery, which synthesize prostaglandins, are informative with respect to the site of PG actions. PG synthesis is... 

Sabharwal, P, Varma, S., and Malarkey, W.B., Human thymocytes secrete luteinizing hormone An autocrine regulator of T-cell proliferation, Biochem. Biophys. Res. Com., 187, 1187, 1992. 

Despite the diversity of hormones - endocrine, paracrine, autocrine or pheromones - they all influence intracellular events by intracellular signalling mechanisms, and the number of such mechanisms is remarkably small, hi this chapter, the signalling mechanisms, the effects and the functions of hormones are described, discussed and illustrated by reference to selected hormones. In addition, the principles underlying the mechanisms by which hormones affect the biochemistry in cells are discussed. 

Hormones transfer signals by migrating from their site of synthesis to their site of action. They are usually transported in the blood. In this case, they are said to have an endocrine effect (1 example insulin). By contrast, tissue hormones, the target cells for which are in the immediate vicinity of the glandular cells that produce them, are said to have a paracrine effect (2 example gastrointestinal tract hormones). When signal substances also pass effects back to the cells that synthesize them, they are said to have an autocrine effect (3 example prostaglandins). Autocrine effects are often found in tumor cells (see p. 400), which stimulate their own proliferation in this way. 

Fig. 3.5. Endocrine, paracrine and autocrine signal transduction, a) endocrine signal transduction the hormone is formed in the specialized endocrine tissue, released into the extracellular medium and transported via the circulatory system to the target cells, b) paracrine signal transduction the hormone reaches the target cell, which is found in close juxtaposition to the hormone producing cell, via diffusion, c) autocrine signal transduction the hormone acts on the same ceU type as the one in which it is produced.

In autocrine signaling, cells of the same type commimicate with one other. The hormone produced by the signaling cell affects a cell of the same type by binding to recep-... 

NO fulfills many criteria required to qualify as an intracellular and intercellular messenger. NO is formed with the help of specific enzyme systems activated by extracellular and intracellular signals. NO is synthesized intraceUularly and reaches its effector molecules, which may be localized in the same cell or in neighboring cells, by diffusion. Thus, it has the character of an autocrine or paracrine hormone, as well as an intracellular messenger. 

Endothelins are widely distributed in the body. ET-1 is the predominant endothelin secreted by the vascular endothelium. It is also produced by neurons and astrocytes in the central nervous system and in endometrial, renal mesangial, Sertoli, breast epithelial, and other cells. ET-2 is produced predominantly in the kidneys and intestine, whereas ET-3 is found in highest concentration in the brain but is also present in the gastrointestinal tract, lungs, and kidneys. Endothelins are present in the blood but in low concentration they apparently act locally in a paracrine or autocrine fashion rather than as circulating hormones. 

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