Lymphotoxin alpha

Gene

The human gene encoding for LT-α was cloned in 1985. The gene of LT-α is located on chromosome 6 and is in close proximity of the gene encoding major histocompatibility complex.

Structure

LT-α is translated as a 25 kDa glycosylated polypeptide with 171 amino acid residues. Furthermore, human LT-α is 72% identical to mouse LT-α at the protein's primary sequence.

LTα expression is highly inducible and when secreted, forms a soluble homotrimeric molecule. LT-α can also form heterotrimers with lymphotoxin-beta, which anchors lymphotoxin-alpha to the cell surface. The interaction between LT-α and LT-β results in the formation of a membrane bound complex (LT-α1-β2).

Function

Lymphotoxin alpha, a member of the tumor necrosis factor superfamily, is a cytokine produced by lymphocytes. It occurs as soluble homotrimer (LT-α3) and as membrane-bound heterotrimer (LT-α1-β2) with LT-β. Soluble LT-α3 binds TNFR1 and TNFR2 and can induce apoptosis or growth arrest in sensitive target cells, contribute to anti-tumor and antiviral cytotoxicity, and promote inflammatory cytokine production via NF-κB signaling. Membrane LT-α1-β2 signals through LT-β receptors and is crucial for lymphoid organogenesis, maintenance of secondary lymphoid tissues (lymph nodes, Peyer’s patches), and formation of tertiary lymphoid structures in chronic inflammation. Absence of LT-β on the lymphocyte cell surface will diminish the ability of LT-α to form LT-α1-β2, thus decreasing its effective ability as a cytokine.

LT-α mediates a large variety of inflammatory, immunostimulatory, and antiviral responses in target cells. LT-α is also involved in the formation of secondary lymphoid organs during development and plays a role in apoptosis. In LT-α knockout mice, Peyer's patches and lymph nodes will fail to develop, thus illustrating the cytokine's essential role in immunological development.

As a cytotoxic protein, LT-α causes the destruction of cancerous cell lines, activates signaling pathways, and effectively kills transformed tumor cells. However, mice with overexpression of LT-α or LT-β showed increased tumor growth and metastasis in several models of cancer. In other studies, mice with gene knockout of LT-α showed enhanced tumor growth, implicating possible protective role of LT-α in cancer. However, these studies utilized mice with complete LT-α deficiency that did not allow to distinguish effects of soluble versus membrane-associated LT.

LT-α mediated signaling pathway

As a member of the TNF family, LT-α binds to various receptors and activates the NF-κB pathway, thus promoting immune regulation through the innate immune response. In order for activation via LT-β receptor to occur, LT-α must form a complex with LT-β to form the LT-α1-β2 complex in the plasma membrane of the lymphocyte. Formation of LT-α1-β2 complex enables binding to LT-β receptors on target cells and subsequent activation of signaling pathways. Activation of signaling pathways such as NF-κB ultimately leads to various cellular fates, including cell proliferation and cell death. After LT-β receptor activation, IKK-α, β, and γ are produced, which increases degradation of I-κB, an inhibitor of NF-kB, and produce NF-kB1 (p50) and ReIA (p60). The production of NF-kB1 and ReIA increases rates of gene transcription of cytokines and inflammatory-inducing molecules.

Anti-carcinogenic properties

Activation of LT-β receptors is capable of inducing cell death of cancerous cells and suppressing tumor growth. The process of cell death is mediated by the presence of IFN-γ and can involve apoptotic or necrotic pathways. It is seen that LT-β receptors facilitate the upregulation of adhesion molecules and recruit lymphocytes to tumor cells to combat tumor growth. In other words, LT-α interactions with LT-β receptors can increase anti-tumor effects through direct destruction of tumor cells.

Pro-carcinogenic properties

However, recent studies have shown the contribution of LT-α mediated signaling to the development of cancer. As mentioned previously, LT-α signaling can promote inflammatory responses, but prolonged inflammation can cause serious cellular damage and increase the risk of certain diseases including cancer. Thus, mutations in regulatory factors in LT-α signaling pathways can promote cell signaling disruptions and encourage the creation of cancerous cell lines. One of these mutations includes constant binding of LT-α1-β2 complex to LT-β receptors, which results in the constant activation of the NF-κB alternative pathway. Presence of a constitutively active NF-κB pathway manifests in multiple myeloma and other cancer-related diseases. Removal of LT-β receptors has shown to inhibit tumor growth and decrease angiogenesis. Thus, lymphotoxin and its downstream signaling via the NF-κB pathway illustrate the cytokine's influence on tumor development and metastasis.

A fully humanized anti-LT-α antibody (Pateclizumab or MLTA3698A) has been shown to react with both LT-α and LT-β. Clinical trials involving this antibody have yet to be employed, but the creation of this antibody offers alternative inhibitory methods for the NF-κB pathway.

Effects on gastrointestinal system

During embryonic development, LT-α signaling plays an active part in the formation of the gastrointestinal immune system. In particular, LT-α mediated signaling is responsible for the development of intestinal lymphoid structures such as Peyer’s patches. This intestinal lymphoid follicle plays an important role in the immune system of the digestive tract.

Peyer’s patches are highly specialized lymphoid nodules located in the intestine. They are surrounded by follicle-associated epithelium and are able to interact with other immune cells through the transcytosis of foreign antigens. In addition to this function, Peyer’s patches facilitate the production Ig-A producing immunocytes, thus increasing the efficacy of the adaptive immune system.

The development of Peyer’s patches requires the binding and activation of LT-β receptor with LT-α1-β2 complex. Experiments involving transgenic mice have shown that the absence of LT-α resulted in the lack of Peyer’s patches and other lymph nodes. The lack of Peyer’s patches and other lymph nodes have also been shown to reduce levels of Ig-A. Being the most produced immunoglobulin, Ig-A protects against mucosal pathogens by regulating bacterial growth and inhibiting antigen adhesion to the intestine under normal conditions. Reduced levels of Ig-A greatly diminishes gut immune regulation and deregulate protection against microbes, thereby emphasizing the importance of LT-mediated response for the expression of Ig-A.

Nomenclature

Discovered by Granger and his research group in 1968, LT-alpha was known as lymphotoxin. As years progressed, its name was changed to tumor necrosis factor-beta (TNF-β). Later discovery of LT-β and LT-α1-β2 complex prompted the disposal of TNF-β and the subdivision of LT into two classes: LT-α and LT-β.

Interactions

Lymphotoxin alpha has been shown to interact with LTB.

References

References

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