Death receptors are cell surface receptors that transmit apoptotic signals initiated by specific ligands and play a central role in instructive apoptosis. Death receptors belong to the tumor necrosis factor receptor (TNFR) gene superfamily. Eight members of the death receptor family have been characterized so far: TNFR1 (also known as DR1, CD120a, p55 and p60), CD95 (also known as DR2, APO-1 and Fas), DR3 (also known as APO-3, LARD, TRAMP and WSL1), TRAILR1 (also known as DR4 and APO-2), TRAILR2 (also known as DR5, KILLER and TRICK2), DR6, ectodysplasin A receptor (EDAR) and nerve growth factor receptor (NGFR). These death receptors are distinguished by a cytoplasmic region of ~80 residues termed the death domain (DD). When these receptors are triggered by corresponding ligands, a number of molecules are recruited to the death domain and subsequently a signaling cascade is activated. Two types of death receptor signaling complex can be distinguished. The first group comprises the death-inducing signaling complexes (DISCs) that result in the activation of caspase-8, which plays the central role in transduction of the apoptotic signal. DISC is formed at the CD95 receptor, TRAILR1 or TRAILR2. The second group comprises the TNFR1, DR3, DR6 and EDAR. These recruit a different set of molecules, which transduce both apoptotic and survival signals.
|Cytokine & Receptor Information|
Cytokines are a large group of proteins, peptides or glycoproteins that are secreted by specific cells of immune system. Cytokines are a category of signaling molecules that mediate and regulate immunity, inflammation and hematopoiesis. Cytokines are produced throughout the body by cells of diverse embryological origin. Cytokine is a general name; other names are defined based on their presumed function, cell of secretion, or target of action. For example, cytokines made by lymphocytes can also be referred to as lymphokines, while interleukins are made by one leukocyte and act on other leukocytes. And chemokines are cytokines with chemotactic activities.
Cytokines may act on the cells that secrete them (autocrine action), on nearby cells (paracrine action), or in some instances on distant cells (endocrine action).
Several main groups of cytokines include: interleukins, chemokines, interferons, tumor necrosis factors (TNFs), colony-stimulating factors (CSFs), and TGF-beta superfamily members. Interleukins are a group of cytokines that were first seen to be expressed by leukocytes. They modulate inflammation and immunity by regulating growth, mobility and differentiation of lymphoid and other cells. Chemokines are chemotactic cytokines with the ability to induce directed chemotaxis in nearby responsive cells. Stimulated by pro-inflammatory cytokines infected tissues release chemokines, and chemokine gradients induce leukocytes to move between endothelial cells and pass the basement membrane into the infected tissues. Interferons are cytokines which are made and released by the cells of most vertebrates in response to the presence of pathogens (such as viruses, bacteria, or parasites, or tumor cells). Interferons play critical role in host defense mechanisms. The tumor necrosis factor (TNF) superfamily of cytokines act through ligand-mediated trimerization, causing recruitment of several intracellular adaptors to activate multiple signal transduction pathways for cell survival, death, and differentiation. Colony-stimulating factors (CSFs), also called haematopoietic growth factors, are secreted glycoproteins which regulate bone marrow production of circulating red and white cells, and platelets.
An important part of Cytokines’ action on the immune system is to stimulate immune cell proliferation and differentiation. Cytokines involved in this function include interleukin 1 (IL-1), which activates T cells; IL-2, which stimulates proliferation of antigen-activated T and B cells; IL-4, IL-5, and IL-6, which stimulate proliferation and differentiation of B cells; and other cytokines such as, interferon gamma, IL-3, IL-7 and colony-stimulating factor (GM-CSF).
Cytokines act on their target cells by binding specific membrane receptors. The receptors and their corresponding cytokines have been divided into several families based on their structure and activities. Type I cytokine receptors have certain conserved motifs in their extracellular amino-acid domain, and lack an intrinsic protein tyrosine kinase activity. Type II cytokine receptors are multimeric receptors composed of heterologous subunits, and are receptors mainly for interferons. The extracellular domains of type II cytokine receptors share structural similarities in their ligand-binding domain. Some cytokine receptors belong to the immunoglobulin superfamily, such as IL-1R alpha, IL-1R beta, IL-6R alpha, SCFR, c-kit, etc. Other cytokine receptors include TNF receptor family, chemokine receptors, and TGF-beta receptors.
Cytokines have been proved useful in immune-based therapies. For example, interferon-alpha, a cytokine with broad antiviral properties, has been proven to be useful in treating cancers, such as malignant melanoma. Cytokine therapy is not merely a tool of the future. In fact, several cytokine therapies are now routinely used by many people living with HIV.