Downregulation and upregulation

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 detected, which results in degradation of the molecules.

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’s 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.

Cases
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 &beta;-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 person’s 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.

Another example can be seen in diabetes insipidus, in which the kidneys become insensitive to arginine vasopressin.

Reversal
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.