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Interleukin 24 (IL-24) is a protein that in humans is encoded by the IL24 gene.

IL-24 is a cytokine belonging to the IL-10 family of cytokines that signals through two heterodimeric receptors: IL-20R1/IL-20R2 and IL-22R1/IL-20R2. This interleukin is also known as melanoma differentiation-associated 7 (mda-7) due to its discovery as a tumour suppressing protein. IL-24 appears to control in cell survival and proliferation by inducing rapid activation of particular transcription factors called STAT1 and STAT3. This cytokine is predominantly released by activated monocytes, macrophages and T helper 2 (Th2) cells and acts on non-haematopoietic tissues such as skin, lung and reproductive tissues. IL-24 performs important roles in wound healing, arthritis, psoriasis and cancer. Several studies have shown that cell death occurs in cancer cells/cell lines following exposure to IL-24. The gene for IL-24 is located on chromosome 1 in humans.

Structure
IL-24 is a secreted protein that is highly conserved throughout evolution with sequence homology between species including yeast, dog, cat, monkey and cow. It is located on chromosome 1q32-33 in humans along with several other IL-10 cytokine family gene members. IL-24 encompasses seven exons and six introns. The cDNA of IL-24 is 1,718 base pairs in length and encodes a protein of 206 amino acid with a predicted molecular size of ˜24 kDa. IL-24 also contains an IL-10 signature motif at amino acids 101–121 shared by other IL-10 family member cytokines. IL-24 possibly can form functionally active dimers due to the presence of potential disulfide bonds. Researchers identified a number of splice variants of IL-24 lacking one or more exons.

Function
At low concentrations, IL-24 predominantly functions as a cytokine. IL-24 is normally expressed in humans in tissues of the immune system such as the thymus, spleen, peripheral blood leukocytes (PBL) and normal melanocytes. IL-24 plays a role in wound healing, in autoimmune diseases such as psoriasis, rheumatoid arthritis and spondyloarthropathy, and protection against infectious diseases caused by bacteria such as Pseudomonas aeruginosa, Salmonella typhimurium and Mycobacterium tuberculosis. In normal skin cells, it suppress keratinocyte proliferation during wound healing.

Besides the normal physiological role that IL-24 plays in the immune system, IL-24 has been studied in great detail for its role in cancer.

Cancer
IL-24 is an immunomodulatory cytokine which can also display broad cancer-specific suppressor effects. The tumor suppressor activities of IL-24 include inhibition of angiogenesis, sensitization to chemotherapy, and induction of cancer-specific apoptosis. Given its ubiquitous apoptotic effect on malignant cells, lack of an effect on normal cells, and absence of significant side effects, IL-24 is an important candidate for cancer therapy.

IL-24 is able to induce apoptosis via both intracellular and extracellular signaling mechanisms. Secreted IL-24 protein induces a robust expression of endogenous IL-24 and subsequent induction of tumor-specific killing through an ER stress-mediated pathway as well as by ROS production. The ER stress is the initial pathway in IL-24-induced apoptosis.

An important question, which remained unresolved, is why IL-24 has the abilities to selectively induce apoptosis in a large spectrum of human cancer-derived cell lines without harming normal cells. One possible reason for this differential killing effect involves inherent biochemical differences between normal and cancer cells (ER stress, ROS production and ceramide), another possibility is that IL-24 is able to target a molecule that only triggers apoptosis in cancer cells. The third option for this differential killing effect is that both of the above hypotheses are correct.