Amphiregulin
Amphiregulin
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Amphiregulin

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Amphiregulin

Amphiregulin, also known as AREG, is a protein synthesized as a transmembrane glycoprotein with 252 aminoacids and it is encoded by the AREG gene. in humans.

The protein encoded by this gene is a member of the epidermal growth factor (EGF) family.

It is a critical autocrine growth factor as well as a mitogen for astrocytes, Schwann cells, and fibroblasts. It is a ligand for epidermal growth factor (EGF) and it is related to transforming growth factor alpha (TGF-alpha). This protein interacts with the Epidermal growth factor receptor (EGFR) to promote the growth of normal epithelial cells.

AREG is a critical factor in estrogen action and ductal development of the mammary glands. Amphiregulin has been found to be essential for mammary ductal development, as evidenced by absence of ductal growth in amphiregulin knockout mice. This is similar to the phenotypes of EGFR and ERα knockout mice, which also show absence of ductal growth. Amphiregulin is expressed in many parts of body such as ovaries, placenta, pancreas, breasts, lungs and spleen. Expression of amphiregulin can be induced by TGF-α, TNF-α, interleukin 1, and prostaglandins.

Generally, amphiregulin is considered to be a part of type 2 mediated resistance and tolerance, the latter of which occurs by promoting the reestablishment of tissue integrity after damage that is due to acute or chronic inflammation. Its involvement in tissue repair can be explained by its dual role, as amphiregulin can induce mitogenic signals, but it can also lead to cell differentiation of epithelial cells.

While epithelial-derived amphiregulin can promote tissue repair, several immune cells are found to express it in cases of tissue damage, so amphiregulin is part of the crosstalk between immune and epithelial cells.

A population of immune cells that is found to increase its amphiregulin expression after tissue damage, is the innate lymphoid cell 2 (ILC2) population. This has been observed in several organs, such as the lung, the intestine, and the skin. The expression of amphiregulin by ILC2s can be induced by interleukin 33 (IL-33). Also, in skin derived ILC2s, amphiregulin expression was regulated by the interaction of killer-cell lectin-like receptor G1 (KLRG1) with E-cadherin. After intestinal damage, activated intestinal ILC2s produce amphiregulin which enhances the production of mucin by epithelial cells, increases the expression of Claudin-1 and promotes the activity of goblet cells. These functions of amphiregulin lead to increased junction strength, as well as the strengthening of the mucus layer.

Tissue resident regulatory T cells (Tregs) can also express amphiregulin to promote tissue repair. In the skeletal muscle, the IL-33 receptor (ST2) expressing Tregs have a distinct T-cell receptor (TCR) repertoire, and TCR signals don't seem to be required for amphiregulin production, but this process can be dependent on the IL-33/ST2 (or IL-33 receptor) pathway and the expression of interleukin 18 receptor (IL-18R) on tissue resident Tregs. Also, amphiregulin that is expressed from these Tregs can further enhance their function, forming an autocrine positive feedback loop. Amphiregulin-expressing tissue resident Tregs have been observed in the lung, where most of them are CD44hiCD62Llo and they express higher levels of CD103, programmed cell death protein 1 (PD-1), glucocorticoid-induced TNFR-related protein (GITR), cytotoxic T-lymphocyte antigen 4 (CTLA-4) and KLRG1. They have been found in injured muscles, where this population has been associated with eosinophil influx, and the production of amphiregulin could enhance the colony-forming efficiency and myogenic differentiation of skeletal muscle satellite cells in vitro, increasing muscle healing. In the inflamed colon, Gata3+Helios+ Tregs express high levels of amphiregulin too. Moreover, Tregs that express amphiregulin, along with keratinocyte growth factor (KGF), CD39 and CD73, act on parenchymal cells to promote tissue repair and regeneration.

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