|Interleukin 1, alpha|
PDB rendering based on 2ila.
|RNA expression pattern|
Interleukin-1 alpha (IL-1α) is a protein of the interleukin-1 family that in humans is encoded by the IL1A gene. In general, Interleukin 1 is responsible for the production of inflammation, as well as the promotion of fever and sepsis. IL-1α inhibitors are being developed to interrupt those processes and treat diseases.
IL-1α is produced mainly by activated macrophages, as well as neutrophils, epithelial cells, and endothelial cells. It possesses metabolic, physiological, haematopoietic activities, and plays one of the central roles in the regulation of the immune responses. It binds to the interleukin-1 receptor. It is on the pathway that activates tumor necrosis factor-alpha.
IL-1α is a cytokine of the interleukin-1 family.
Interleukin 1 was discovered by Gery in 1972. He named it lymphocyte-activating factor (LAF) because it was a lymphocyte mitogen. It was not until 1985 that interleukin 1 was discovered to consist of two distinct proteins, now called interleukin-1 alpha and interleukin-1 beta.
IL-1α is also known as fibroblast-activating factor (FAF), lymphocyte-activating factor (LAF), B-cell-activating factor (BAF), leukocyte endogenous mediator (LEM), epidermal cell-derived thymocyte-activating factor (ETAF), serum amyloid A inducer or hepatocyte-stimulating factor (HSP), catabolin, hemopoetin-1 (H-1), endogenous pyrogen (EP), osteoclast-activating factor (OAF), and proteolysis-inducing factor (PIF).
Synthesis and structure
IL-1α is a unique member in the cytokine family in the sense that the structure of its initially synthesized precursor does not contain a signal peptide fragment (same is known for IL-1β and IL-18). After processing by the removal of N-terminal amino acids by specific proteases, the resulting peptide is called "mature" form. Calpain, a calcium-activated cysteine protease, associated with the plasma membrane, is primarily responsible for the cleavage of the IL-1α precursor into a mature molecule. Both the 31kDa precursor form of IL-1α and its 18kDa mature form are biologically active.
The 31 kDa IL-1α precursor is synthesized in association with cytoskeletal structures (micro-tubules), unlike most proteins, which are translated in the endoplasmic reticulum.
The three-dimensional structure of the IL-1α contains an open-ended barrel composed entirely of beta-pleated strands. Crystal structure analysis of the mature form of IL-1α shows that it has two sites of binding to IL-1 receptor. There is a primary binding site located at the open top of its barrel, which is similar but not identical to that of IL-1β.
Production and cellular sources
IL-1α is constitutively produced by epithelial cells. It is found in substantial amounts in normal human epidermis and is distributed in a 1:1 ratio between living epidermal cells and stratum corneum. The constitutive production of large amounts of IL-1α precursor by healthy epidermal keratinocytes interfere with the important role of IL-1α in immune responses, assuming skin as a barrier, which prevents the entry of pathogenic microorganisms into the body.
The essential role of IL-1α in maintenance of skin barrier function, especially with increasing age, is an additional explanation of IL-1α constitutive production in epidermis.
With the exception of skin keratinocytes, some epithelial cells and certain cells in central nervous system, the mRNA coding for IL-1α (and, thus, IL-1α itself) is not observed in health in most of cell types, tissues, and blood, in spite of wide physiological, metabolic, haematopoietic, and immunological IL-1α activities.
A wide variety of other cells only upon stimulation can be induced to transcribe the IL-1α genes and produce the precursor form of IL-1α, Among them are fibroblasts, macrophages, granulocytes, eosinophils, mast cells and basophils, endothelial cells, platelets, monocytes and myeloid cell lines, blood T-lymphocytes and B-lymphocytes, astrocytes, kidney mesangial cells, Langerhans cells, dermal dendritic cells, natural killer cells, large granular lymphocytes, microglia, blood neutrophils, lymph node cells, maternal placental cells and several other cell types.
These data allow to assume IL-1α as an epidermal cytokine.
Although there are many interactions of IL-1α with other cytokines, the most consistent and most clinically relevant is its synergism with TNF. IL-1α and TNF are both acute-phase cytokines that act to promote fever and inflammation. There are, in fact, few examples in which the synergism between IL-1α and TNFα has not been demonstrated. These include radioprotection, the Shwartzman reaction, PGE2 synthesis, sickness behavior, nitric oxide production, nerve growth factor synthesis, insulin resistance, loss of mean body mass, and IL-8 and chemokine synthesis.
The most important regulatory molecule for IL-1α activity is IL-1Ra, which is usually produced in a 10- to 100-fold molar excess. In addition, the soluble form of the IL-1R type I has a high affinity for IL-1α and is produced in a 5-10 molar excess. IL-10 also inhibits IL-1α synthesis.
IL-1α possesses biological effect on cells in the picomolar to femtomolar range. In particular, IL-1α:
- stimulates keratinocytes and macrophages for induced IL-1α secretion
- induces pro-collagen type I and III synthesis
- causes proliferation of fibroblasts, induces collagenase secretion, induces cytoskeletal rearrangements, induces IL-6 and GCSF secretion
- induces cycloxygenase synthesis and prostaglandin PGE2 release
- causes phosphorylation of heat shock protein
- causes proliferation of smooth muscle cells, keratinocytes and stimulates release of other cytokines by keratinocytes
- induces TNFα release by endothelial cells and Ca2+ release from osteoclasts.
- stimulates hepatocytes for secretion of acute-phase proteins
- induces proliferation of CD4+ cells, IL-2 production, co-stimulates CD8+/IL-1R+ cells, induces proliferation of mature B-cells and immunoglobulin secretion
- kills a limited number of tumor cells types
- stimulates fibroblasts proliferation
- induces synthesis of proteases, subsequent muscle proteolysis, release of all types of amino acids in blood and stimulates acute-phase proteins synthesis
- changes the metallic ion content of blood plasma by increasing copper and decreasing zinc and iron concentration in blood
- increases blood neutrophils
- activates lymphocyte proliferation and induces fever
Topically administered IL-1α also stimulates expression of FGF and EGF, and subsequent fibroblasts and keratinocytes proliferation. This, plus the presence of large depot of IL-1α precursor in keratinocytes, suggests that locally released IL-1α may play an important role and accelerate wound healing.
Clinical trials on IL-1α have been carried out that are specifically designed to mimic the protective studies in animals. IL-1α has been administered to patients during receiving autologous bone marrow transplantation. The treatment with 50 ng/kg IL-1α from day zero of autologous bone marrow or stem cells transfer resulted in an earlier recovery of thrombocytopenia compared with historical controls. There are to date no clinical trials on IL-1α as an adjuvant for tumors.
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