Downregulation and upregulation
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Downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation.
An example of downregulation is the cellular decrease in the number of receptors to a molecule, such as a hormone or neurotransmitter, which reduces the cell's sensitivity to the molecule. This phenomenon is an example of a locally acting negative feedback mechanism.
An example of upregulation is the increased number of cytochrome P450 enzymes in liver cells when xenobiotic molecules such as dioxin are administered (resulting in greater degradation of these molecules).
Some receptor agonists may cause downregulation of their respective receptor(s), while most receptor antagonists may temporally upregulate their respective receptor(s). The disequilibrium caused by these changes often causes withdrawal when the long-term use of a medication or drug is discontinued. However, the use of certain receptor antagonists may also damage receptors faster than they upregulate (internalization of receptors due to antagonism).
Upregulation and downregulation can also happen as a response to toxins or hormones. An example of upregulation in pregnancy is hormones that cause cells in the uterus to become more sensitive to oxytocin.
Insulin receptor mechanism
The process of downregulation occurs when there are elevated levels of the hormone insulin in the blood. When insulin binds to its receptors on the surface of a cell, the hormone receptor complex undergoes endocytosis and is subsequently attacked by intracellular lysosomal enzymes. The internalization of the insulin molecules provides a pathway for degradation of the hormone as well as for regulation of the number of sites that are available for binding on the cell surface. At high plasma concentrations, the number of surface receptors for insulin is gradually reduced by the accelerated rate of receptor internalization and degradation brought about by increased hormonal binding. The rate of synthesis of new receptors within the endoplasmic reticulum and their insertion in the plasma membrane do not keep pace with their rate of destruction. Over time, this self-induced loss of target cell receptors for insulin reduces the target cell’s sensitivity to the elevated hormone concentration. The process of decreasing the number of receptor sites is virtually the same for all hormones; it varies only in the receptor hormone complex.
This process is illustrated by the insulin receptor sites on target cells in a person with type 2 diabetes. Due to the elevated levels of blood glucose in an overweight individual, the β-cells (islets of Langerhans) in the pancreas must release more insulin than normal to meet the demand and return the blood to homeostatic levels. The near-constant increase in blood insulin levels results from an effort to match the increase in blood glucose, which will cause receptor sites on the liver cells to downregulate and decrease the number of receptors for insulin, increasing the subject’s resistance by decreasing sensitivity to this hormone. There is also a hepatic decrease in sensitivity to insulin. This can be seen in the continuing gluconeogenesis in the liver even when blood glucose levels are elevated. This is the more common process of insulin resistance, which leads to adult-onset diabetes.
The process of downregulation can be counteracted in the above example. A person with type 2 diabetes can increase their sensitivity to insulin through proper diet and regular exercise, resulting in weight loss; some individuals may even return to their pre-diabetic state by following such a regimen.
Sherwood, L. (2004). “Human Physiology From Cells to Systems, 5th Ed” (p. 680). Belmont, CA: Brooks/Cole-Thomson Learning
Wilmore, J., Costill, D. (2004). Physiology of Sport and Exercise, 3rd Ed (p. 164). Champaign, IL: Human Kinetics