Recombinant Rat LIF: A Multi-functional Cytokine？ Background [1] Recombinant Rat LIF, a member of the interleukin-6 family, is derived from a precursor of 202 amino acids by removing 22 amino acids at the N-terminus. LIF is a substance cloned from the myeloid leukemia cell line (M1) that induces differentiation and inhibits proliferation. LIF has the following functions: inhibiting the differentiation of embryonic stem cells, stimulating the proliferation of bone marrow-like DA1 cells, inducing liver cell response, facilitating neurotransmitter transmission between neurons, and inhibiting the activation of lipoprotein lipase. The three-dimensional structure of Recombinant Rat LIF has been described using X-ray crystallography and nuclear magnetic resonance (NMR) techniques. The structure of LIF is a compact four-helix bundle. The ILF structure is similar to other cytokines of the interleukin-6 family, consisting of four helices tightly connected in an up-up-down-down arrangement. The first helix (helix A) starts from leucine 44, and the N-terminal portion of helix A and the C-terminal portion of helix 3 are covalently linked together by two disulfide bonds (cysteine 34-cysteine 156 and cysteine 40-cysteine 153). The N-terminal portion is important for receptor binding. What is the Receptor of Recombinant Rat LIF? [1] Recombinant Rat LIF is a multi-functional cytokine, and its functional receptor has been found in various organs, including the liver, skeleton, uterus, kidney, and central nervous system. Quantitative studies have shown that the functional receptor of LIF is present on the surface of ES cells, liver, and monocyte-macrophage cells in the hematopoietic system. Recombinant Rat LIF shares the cytokine receptor gp130 with IL-6. However, these two cytokines have significant differences in chemical stoichiometry, composition, structure, and thermodynamics. The receptor complex formed by IL-6 and its receptor is a hexamer, while the receptor complex formed by LIF and its receptor is a trimer, consisting of Recombinant Rat LIF binding to one molecule of gp130 and one molecule of the secondary receptor LIFR. LIFR is structurally similar to gp130, and LIFRβ binds to JAK through its intracellular domain. Therefore, signal transduction occurs through the transactivation of JAK bound to gp130 and JAK bound to LIFβ. It is worth noting that the α-chain receptor is not required in the LIF signaling pathway. The binding of Recombinant Rat LIF to gp130 and LIFRβ is highly affinity-driven. The binding between LIF and LIFRβ is 80 times tighter than the binding between LIF and gp130. The high-affinity binding of LIF to its receptors in vivo is consistent with the high-affinity interaction between LIF and the two membrane-bound receptor chains. How Does Recombinant Rat LIF Activate Signaling Pathways? [1] Unlike most Janus kinases (JAK) receptors, the intracellular domain of LIFR does not possess any tyrosine kinase activity. Gp130 and LIFRβ together form members of the JAK family of kinases, which exist in an inactive state and are rapidly activated by Recombinant Rat LIF and its receptor. However, the molecular mechanism underlying this activation is still unclear. The JAK family consists of four members (JAK1, JAK2, JAK3, TYK2). Overexpression experiments have shown that LIF can bind to three of the four members (JAK1, JAK2, TYK2). Blocking the LIF signaling pathway occurs when JAK1 is knocked out, but knocking out JAK2 or TYK2 does not affect the LIF signaling pathway. This suggests that JAK1 is the dominant member of the JAK family. Activation of JAK1 occurs through phosphorylation, where the receptor chain of JAK1 activates the other chain by phosphorylating specific tyrosine residues. This tyrosine residue is located on the "activation loop" of the kinase domain, which induces conformational changes, allowing the enzyme to dock with the substrate and MG-ATP at the active site. Therefore, Recombinant Rat LIF, along with its receptor, enables JAK1 to have catalytic activity. JAK1 initiates a series of tyrosine phosphorylations, stimulating three different signaling pathways: JAK/STAT signaling pathway, MAP kinase signaling pathway, and PI(3) kinase signaling pathway 2 (Figure 1-3). These signaling pathways contribute to cell differentiation, survival, and self-renewal. After stimulation by LIF, the JAK/STAT signaling pathway, MAP kinase signaling pathway, and PI(3) kinase signaling pathway make major contributions to the quantity and quality of cell type specificity. References [1] Deng, X. (Year). Expression, regulation, and knockout studies of the LIF gene in the implantation period of rats.查看更多
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