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.

Function
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 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.

Biological role
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.

Role in tissue repair
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.

Some unconventional T cells have been associated with the promotion of tissue repair by amphiregulin in a direct or in an indirect manner. After intestinal damage, mucosal-associated invariant T (MAIT) cells produce amphiregulin that leads to epithelial cell turnover and enhances the activity of goblet cells. Also, a pro-repair response by amphiregulin produced by ILC2s and Tregs, that is dependent on IL-33 signaling, is induced by gamma delta (γδ) T cells that produce interleukin 17A (IL-17A). This correlation between IL-17A-producing γδ T cells and amphiregulin has been observed in the lungs and in the oral mucosa.

Psoriasis
Mutations in this encoded protein are associated with a psoriasis-like skin phenotype. Higher circulating levels of amphiregulin are associated with AGVHD progression.

Cancer
Overexpression of amphiregulin is connected with cancer of the breast, prostate, colon, pancreas, lung, spleen, and bladder.

Rheumatoid arthritis
It seems that expression of AREG is connected with proliferation of fibroblasts and production of proinflammatory cytokines interleukin 8 and vascular endothelial growth factor (VEGF).

Fibrosis
Chronic elevation of amphiregulin levels has been associated with fibrosis in several organs. ILC2s are drivers of liver, skin, and pulmonary fibrosis, and their expression of interleukin 13 (IL-13) and amphiregulin is implicated in this process. Pathogenic memory Th2 cells that express amphiregulin are also involved in pulmonary fibrosis. Exposure to house dust mite leads to the increase of amphiregulin-expressing pathogenic memory Th2 cells. This increase might be related to the IL-33/ST2 signaling, as blocking this pathway causes less production of amphiregulin. The function of amphiregulin in airway fibrosis is related to eosinophils that express EGFR, in which amphiregulin binds to, resulting in the upregulation of inflammatory genes, including Spp1 that encodes osteopontin. The expression of osteopontin by eosinophils shapes the pathogenesis of pulmonary fibrosis. Moreover, macrophage-derived amphiregulin is involved in the transforming growth factor beta (TGF-β)-induced fibrosis too, as it has been found to activate latent TGF-β through the activation of integrin-αV complex. In the liver, ongoing necrosis leads to the activation of hepatic ILC2s which release amphiregulin along with IL-13. The release of them activates the hepatic stellate cells that transform into myofibroblasts, and ultimately promotes liver fibrosis.

Inflammation
Amphiregulin is part of cellular response type 2. It was found that the cell source of amphiregulin is innate lymphoid cells 2 (ILC2) which are dependent on interleukin 33. ILC2 expressed amphiregulin after tissue damage of the intestines and activation by IL-33. Moreover, endogenous AREG with IL-33 decreased the intestinal inflammation in mice with normal count of T-lymphocytes and in deficient mice.