Lymphotoxin alpha

LTA
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 1TNR, 4MXV, 4MXW
Identifiers
Aliases	LTA, LT, TNFB, TNFSF1, Lymphotoxin alpha, TNLG1E
External IDs	OMIM: 153440; MGI: 104797; HomoloGene: 497; GeneCards: LTA; OMA:LTA - orthologs
Gene location (Human) Chr. Chromosome 6 (human)[1] Band 6p21.33 Start 31,572,054 bp[1] End 31,574,324 bp[1]
Gene location (Mouse) Chr. Chromosome 17 (mouse)[2] Band 17 B1|17 18.59 cM Start 35,422,141 bp[2] End 35,424,327 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in testicle granulocyte lymph node blood appendix spleen right testis left testis tonsil bone marrow Top expressed in spleen white pulp embryo embryo thymus cortex of thymus granulocyte medulla of thymus colon muscle of thigh More reference expression data BioGPS More reference expression data
Gene ontology Molecular function cytokine activity tumor necrosis factor receptor binding signaling receptor binding protein binding Cellular component membrane plasma membrane extracellular region extracellular space Biological process positive regulation of chronic inflammatory response to antigenic stimulus response to hypoxia response to nutrient cell-cell signaling positive regulation of humoral immune response mediated by circulating immunoglobulin positive regulation of interferon-gamma production tumor necrosis factor-mediated signaling pathway response to lipopolysaccharide humoral immune response immune response positive regulation of apoptotic process lymph node development positive regulation of glial cell proliferation negative regulation of fibroblast proliferation signal transduction defense response to Gram-positive bacterium apoptotic process regulation of signaling receptor activity Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 4049 16992 Ensembl ENSG00000231408 ENSG00000223919 ENSG00000173503 ENSG00000226275 ENSG00000230279 ENSG00000238130 ENSG00000226979 ENSMUSG00000024402 UniProt P01374 P09225 RefSeq (mRNA) NM_000595 NM_001159740 NM_010735 RefSeq (protein) NP_000586 NP_001153212 NP_034865 Location (UCSC) Chr 6: 31.57 – 31.57 Mb Chr 17: 35.42 – 35.42 Mb PubMed search [3] [4]
Wikidata
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Lymphotoxin-alpha (LT-α) formerly known as tumor necrosis factor-beta (TNF-β)^[5][6] is a protein that in humans is encoded by the LTA gene.^[7][8] Belonging to the hematopoietic cell line, LT-α exhibits anti-proliferative activity and causes the cellular destruction of tumor cell lines.^[7] As a cytotoxic protein, LT-α performs a variety of important roles in immune regulation depending on the form that it is secreted as. Unlike other members of the TNF superfamily, LT-α is only found as a soluble homotrimer, when found at the cell surface it is found only as a heterotrimer with LTβ.^[6]

LT-α has a significant impact on the maintenance of the immune system including the development of secondary lymphoid organs.^[9][10] Absence of LT-α leads to the disruption of gastrointestinal development, prevents Peyer's patch development, and results in a disorganized spleen.^[11]

As a signaling molecule, LT-α is involved in the regulation of cell survival, proliferation, differentiation, and apoptosis.^[12] LT-α plays an important role in innate immune regulation and its presence has been shown to prevent tumor growth and destroy cancerous cell lines.^[13] In contrast, unregulated expression of LT-α can result in a constantly active signaling pathway, thus leading to uncontrolled cellular growth and creation of tumors.^[12] Hence depending on the context, LT-α may function to prevent growth of cancer cells or facilitate the development of tumors. Furthermore, LT-α effects depend on the type of organ it acts upon, type of cancer cells, cellular environment, gender, and time of effect during an immune response.^[14][13]

The human gene encoding for LT-α was cloned in 1985.^[7][15] The gene of LT-α is located on chromosome 6 and is in close proximity of the gene encoding major histocompatibility complex.^[16]

LT-α is translated as a 25 kDa glycosylated polypeptide with 171 amino acid residues.^[8] Furthermore, human LT-α is 72% identical to mouse LT-α at the protein's primary sequence.^[17]

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).^[10]

Lymphotoxin alpha, a member of the tumor necrosis factor superfamily, is a cytokine produced by lymphocytes. LT-α₁-β₂ can interact with receptors such as LT-β receptors.^[12] Absence of LT-β on cell surfaces will diminish the ability of LT-α to form LT-α₁-β₂, thus decreasing its effective ability as a cytokine.^[9][10] LT-α mediates a large variety of inflammatory, immunostimulatory, and antiviral responses. LT-α is also involved in the formation of secondary lymphoid organs during development and plays a role in apoptosis.^[18]

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.^[19]

As a cytotoxic protein, LT-α causes the destruction of cancerous cell lines, activates signaling pathways, and effectively kills transformed tumor cells.^[9][12] 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.^[20]

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.^[12] In order for activation to occur, LT-α must form a complex with LT-β to form the LT-α₁-β₂ complex. Formation of LT-α₁-β₂ complex enables binding to LT-β receptors and subsequent activation of signaling pathways.^[21] 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).^[21] The production of NF-kB1 and ReIA increases rates of gene transcription of cytokines and inflammatory-inducing molecules.^[21][22]

Activation of LT-β receptors is capable of inducing cell death of cancerous cells and suppressing tumor growth.^[23][24] The process of cell death is mediated by the presence of IFN-γ and can involve apoptotic or necrotic pathways.^[23] It is seen that LT-β receptors facilitate the upregulation of adhesion molecules and recruit lymphocytes to tumor cells to combat tumor growth.^[7][12] In other words, LT-α interactions with LT-β receptors can increase anti-tumor effects through direct destruction of tumor cells.

However, recent studies have shown the contribution of LT-α mediated signaling to the development of cancer.^{[9][12][13][14]} 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.^[13] 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.^[12][13] Presence of a constitutively active NF-κB pathway manifests in multiple myeloma and other cancer-related diseases.^[12] Removal of LT-β receptors has shown to inhibit tumor growth and decrease angiogenesis.^[13] 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.^[9] 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.

During embryonic development, LT-α signaling plays an active part in the formation of the gastrointestinal immune system.^[11] In particular, LT-α mediated signaling is responsible for the development of intestinal lymphoid structures such as Peyer’s patches.^[25][26] 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.^[27] In addition to this function, Peyer’s patches facilitate the production Ig-A producing immunocytes, thus increasing the efficacy of the adaptive immune system.^[28]

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.^[11] The lack of Peyer’s patches and other lymph nodes have also been shown to reduce levels of Ig-A.^[11] 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.^[29] 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.

Discovered by Granger and his research group in 1968, LT-alpha was known as lymphotoxin.^[30] As years progressed, its name was changed to tumor necrosis factor-beta (TNF-β).^[31] 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.^[32][33]

Lymphotoxin alpha has been shown to interact with LTB.^[34][35][36]

• Lymphotoxin