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]
Cytokines are a broad and loose category of small proteins important in cell signaling. Due to their size, cytokines cannot cross the lipid bilayer of cells to enter the cytoplasm and therefore typically exert their functions by interacting with specific cytokine receptors on the target cell surface. Cytokines have been shown to be involved in autocrine, paracrine and endocrine signaling as immunomodulating agents.
Tumor necrosis factor ('TNF), also known as cachexin or cachectin, is a cytokine and a member of the TNF superfamily, which comprises various transmembrane proteins with a homologous TNF domain. Initially referred to as TNFα, this cytokine was the first to be identified as an adipokine, secreted by adipose tissue. TNF signaling occurs through two receptors: TNFR1 and TNFR2. TNFR1 is widely expressed across most cell types and is typically associated with pro-inflammatory and apoptotic signaling. In contrast, TNFR2 is mainly found on endothelial, epithelial, and immune cells, where it mediates anti-inflammatory responses and promotes cell proliferation.
Interleukin 12 (IL-12) is an interleukin that is naturally produced by dendritic cells, macrophages, neutrophils, helper T cells and human B-lymphoblastoid cells (NC-37) in response to antigenic stimulation. IL-12 belongs to the family of interleukin-12. IL-12 family is unique in comprising the only heterodimeric cytokines, which includes IL-12, IL-23, IL-27 and IL-35. Despite sharing many structural features and molecular partners, they mediate surprisingly diverse functional effects.
Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily that includes three different mammalian isoforms and many other signaling proteins. TGFB proteins are produced by all white blood cell lineages.
The tumor necrosis factor receptor superfamily (TNFRSF) is a protein superfamily of cytokine receptors characterized by the ability to bind tumor necrosis factors (TNFs) via an extracellular cysteine-rich domain. With the exception of nerve growth factor (NGF), all TNFs are homologous to the archetypal TNF-alpha. In their active form, the majority of TNF receptors form trimeric complexes in the plasma membrane. Accordingly, most TNF receptors contain transmembrane domains (TMDs), although some can be cleaved into soluble forms, and some lack a TMD entirely. In addition, most TNF receptors require specific adaptor protein such as TRADD, TRAF, RIP and FADD for downstream signalling. TNF receptors are primarily involved in apoptosis and inflammation, but they can also take part in other signal transduction pathways, such as proliferation, survival, and differentiation. TNF receptors are expressed in a wide variety of tissues in mammals, especially in leukocytes.
Lymphotoxin is a member of the tumor necrosis factor (TNF) superfamily of cytokines, whose members are responsible for regulating the growth and function of lymphocytes and are expressed by a wide variety of cells in the body.
Tumor necrosis factor receptor superfamily, member 4 (TNFRSF4), also known as CD134 and OX40 receptor, is a member of the TNFR-superfamily of receptors which is not constitutively expressed on resting naïve T cells, unlike CD28. OX40 is a secondary co-stimulatory immune checkpoint molecule, expressed after 24 to 72 hours following activation; its ligand, OX40L, is also not expressed on resting antigen presenting cells, but is following their activation. Expression of OX40 is dependent on full activation of the T cell; without CD28, expression of OX40 is delayed and of fourfold lower levels.
TNF receptor-associated factor 2 is a protein that in humans is encoded by the TRAF2 gene.
Lymphotoxin beta receptor (LTBR), also known as tumor necrosis factor receptor superfamily member 3 (TNFRSF3), is a cell surface receptor for lymphotoxin involved in apoptosis and cytokine release. It is a member of the tumor necrosis factor receptor superfamily.
IKK-β also known as inhibitor of nuclear factor kappa-B kinase subunit beta is a protein that in humans is encoded by the IKBKB gene.
TNF receptor-associated factor 1 is a protein that in humans is encoded by the TRAF1 gene.
TNF receptor-associated factor (TRAF3) is a protein that in humans is encoded by the TRAF3 gene.
Lymphotoxin-beta (LT-beta) also known as tumor necrosis factor C (TNF-C) is a protein that in humans is encoded by the LTB gene.
Interferon alpha-1 is a protein that in humans is encoded by the IFNA1 gene.
Mitogen-activated protein kinase kinase kinase 14 (MAP3K14), also known as NF-kappa-B-inducing kinase (NIK), is a MAP kinase kinase kinase enzyme that in humans is encoded by the MAP3K14 gene.
LIGHT, also known as tumor necrosis factor superfamily member 14 (TNFSF14), is a secreted protein of the TNF superfamily. It is recognized by herpesvirus entry mediator (HVEM), as well as decoy receptor 3.
Tumor necrosis factor receptor superfamily member 18 (TNFRSF18), also known as glucocorticoid-induced TNFR-related protein (GITR) or CD357. GITR is encoded and tnfrsf18 gene at chromosome 4 in mice. GITR is type I transmembrane protein and is described in 4 different isoforms. GITR human orthologue, also called activation-inducible TNFR family receptor (AITR), is encoded by the TNFRSF18 gene at chromosome 1.
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