Third Assignment: The Functions of Adrenocorticotropic Hormone!

General Functions and Localizations:     

Adrenocorticotropic hormone has a plethora of functions throughout the mammalian anatomy. Chief of which, is its role in metabolic activities when the body is under stress and during the early morning. ACTH is the tropic hormone involved with the release of Cortisol from the adrenal cortex (Wang and Majzoub, 2011). As stated in a previous assignment, ACTH is a derivative of the precurser hormone Proopiomelanocortin, and thus is classified as a melanocortin. Melanocortin hormones bind to receptors known as melanocortin receptors. ACTH and its derivatives currently affect three out of five of the different melanocortin receptor derivatives that are known in mammals (Wang and Majzoub 2011). Depending on which receptor is bound, the hormone will have a different effect on the target cell. I will discuss the function, effects, and localizations of each receptor once bound by ACTH or its derivatives in this post.

The first receptor I will be discussing is receptor MC1R. Receptor MC1R is primarily localized to melanocytes, keratinocytes, and dermal fibroblasts in mammals (Wang and Majzoub, 2011). These cells are mostly found in the epidermal and dermal layers of the skin. MC1R receptor recruitment is controlled by calcium concentrations and exposure to UV light. When calcium is high, and UV light contacts the skin, it stimulates the recruitment of more MC1R receptors to the cell membrane which increases the cells sensitivity to the receptor ligand. MC1R is primarily acted on by α-MSH, which is a derivative of ACTH. When α-MSH binds to MC1R receptor, it activates a cAMP secondary messenger pathway (Wang and Majzoub, 2011). High levels of cAMP stimulate phosphorylation of tyrosinase which is the main enzyme involved in the movement of itracellular pigment. When tyrosinase is active, it causes conglomeration of vessicles containing eumelanin (A black-brown pigment), and diffuses vessicles containing pheomelanin (red-yellow pigment). This mobilization causes darkening of the cells and thus leads to darkening of the skin. The darkening process is much more active in non-human animals, due to a higher concentration of α-MSH in the blood (Thody, 1999). The darkening of the skin is often essential for protection from high intensity UV rays. The human MC1R receptor is highly polymorphic, and is often associated with the traits of fair/light skin and blond hair (Wang and Majzoub, 2011). In addition to its role in pigmentation, α-MSH is involved in the human inflammatory response. MC1R receptors can be found on macrophage. When α-MSH levels are high, they bind to these recptors on the surface of the macrophage, causeing stifled antibody release and lowering inflammation (Wang, and Majzoub, 2011). Systemic injection of α-MSH has been found to treat arthritis, cancer inflammation, and even inflamed regions of the brain (Schaible et al. 2013). α-MSH is also involved in an antibody regulating axis that is involved with food intake in the intestines (Coquerel et al. 2011).
 

 

Figure. 1: Flow chart demonstrating the relationship between ACTH and
α-MSH, as well as some of their functions. Taken from https://classconnection.s3.amazonaws.com/779/flashcards/1382779/jpg/pomc1334541252805.jpg

 

The next receptor I will be discussing is the main receptor, on which ACTH acts directly. This receptor is known as MC2R, and is found mainly in the human adrenal cortex. As stated in a previous post, ACTH release is controlled by the release of CRH from the hypothalamus. CRH release is regulated by glucose and cortisol levels in the blood, as well as by circadian rhythms (Your bodies 24-hour clock). In the early morning, glucose levels are low, but your body will need glucose in order to wake and be alert. The hypothalamus releases CRH which in turn releases ACTH. ACTH travels through the blood to the adrenals where it binds to MC2R receptors. The binding of the MC2R receptor causes a cascade of effects activated by a secondary messenger pathway (Wang and Majzoub, 2011). Binding causes the cell to increase uptake of lipoproteins into lipid droplets in the cell and begins the hydrolysis of cholesterol within the lipid droplets. Cortisol is a steroid hormone, therefore, it is a cholesterol derivative. The binding of ACTH also causes an increase of cholesterol transport by the steroidogenic acute regulatory protein (StaR) into the mitochondria. Once inside the mitochondria, the cholesterol is converted to pregnenolone which is the precurser to all steroid hormones. The pregnenolone is evetually converted to cortisol and is released into the blood stream where it is bound to a transport protein and sent to other cells (Wang and Majzoub, 2011). Cortisol is responsible for increasing gluconeogenesis in the liver, and inhibiting glucose uptake by adipose tissue and muscle, increasing the concentration of glucose in the blood. Cortisol levels peak in the early morning when the body is waking, and slowly decrease during the day. Thus, ACTH is directly responsible for regulating cortisol levels, and thus regulating blood glucose levels (Ben-Shlomo et al. 2011).

The final receptor that ACTH and α-MSH have an effect on is the MC5R receptor. Studies on this receptor are still limited and so not much is known yet. However, current studies have shown that this receptor is highly concentrated in the aldosterone producing cells of the adrenal cortex, in the zona glomerulosa. It has been proven that high levels of ACTH and α-MSH have caused high levels of aldosterone excretion from these cells (Wang and Majzoub, 2011). Aldosterone is responsible for increasing blood volume and sodium concentration when the blood is at a low blood pressure.

Pathologies:

There are many pathologies associated with ACTH and cortisol levels. One of the most prominently known is Cushing's Syndrome. Cushing's Syndrome, or hypercortisolism, is caused by high levels of ACTH and/or cortisol in the blood (Ben-Shlomo et al. 2010). The raised levels of these hormones are usually caused by one of two things. The first, is cortisol like drugs such as prednisone. These drugs often euther mimic the effects of cortisol, or cause increased cortisol release. The second, is a tumor known as a pituitary adenoma. These tumors occur in the anterior pituitary gland where ACTH is produced. The growth of the tumor causes increased secretion of ACTH, which in turn causes the increased secretion of cortisol (Ben-Schlomo et al. 2010). High levels of cortisol cause increased blood glucose anf thus, cause many problems in the mammalian anatomy. Symptoms of Cushing Syndrome include high blood pressure, abdominal obesity, stretch marks, rounded face, fat lumps on the shoulders, weak muscles, weak bones, acne, and in women, an irregular menstrual cycle (Ben-Shlomo et al 2010).

 

 

 

Another common condition related to ACTH/Cortisol release is known as Addison's Disease.  Addison's Disease, or hypocortisolism, is caused by insufficient cortisol release by the adrenal cortex (Lovas and Husebye, 2005). The adrenals do not release enough cortisol typically because of damage caused by the immune system of the affected. Immune cells break down the cells of the adrenal cortex and thus stymy the production and release of cortisol. Addison's Disease can also be caused by infection in the adrenal cortex (Lovas and Huesebye, 2005). Symptoms typically include fatigue, muscle weakness, lightheadedness, joint pain, sweating, and especially hyperpigmentation. Addison's disease often causes high levels of ACTH in the blood, which cause MC1R receptors to be activated and thus causing high melanin levels in the skin (Lovas and Husebye, 2005).

References:

1) Oulu Wang & Joseph A. Majzoub. (2011). Adrenocorticotropin. In The Pituitary (Third ed., pp. 47-81). San Diego, California: Blackwell Science
2) Anthony J. Thody. (1999). a-MSH and the Regulation of the Melanocyte Function. Annals of the New York Academy of Sciences. Vol 885. pp.217-229
3) Eva-Verena Schaible, Arne Steinsträßer, Antje Jahn-Eimermacher, Clara Luh, Anne Sebastiani, Frida Kornes, Dana Pieter, Michael K. Schäfer, Kristin Engelhard, and Serge C. Thal. (2013). Single Administration of Tripeptide α-MSH(11–13) Attenuates Brain Damage by Reduced Inflammation and Apoptosis after Experimental Traumatic Brain Injury in Mice. PLoS one. 8(8). pp. 1-10
4) Quentin Coquerela, Maria Hamze Sinnoa, Nabile Boukhettalaa, Moïse Coëffiera, Mutsumi Terashia, Christine, Bole-Feysota, Denis Breuilléb, Pierre Déchelottea, Sergueï O. Fetissov. (2011). Intestinal inflammation influences α-MSH reactive autoantibodies: Relevance to food intake and body weight. Psychoneuralendocrinology. 37(1). pp. 94-106
5) Anat Ben-Shlomo, Ning-Ai Liu, and Shlomo Melmed. (2010). Pathogenesis of Corticotropic Tumors. Cushing's Syndrome pp. 1-40). New York City. Humana Press. 
6) Dr Kristian Løvås and  Prof Eystein S Husebye. (2005). Addison's Disease. The Lancet. 365(9476). pp. 2058-2061
7) https://classconnection.s3.amazonaws.com/779/flashcards/1382779/jpg/pomc1334541252805.jpg
8) https://blog.dlvrit.com/wp-content/uploads/Cortisol-Peak-Hours.png
9) https://s-media-cache-ak0.pinimg.com/236x/5b/82/4f/5b824f69dc82065ed723015c12410979.jpg