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Insulin is a peptide hormone synthesized by pancreatic beta cells located in the islets of Langerhans. Secreted in response to elevated levels of nutrients in the blood, it controls the metabolism of glucose, fatty acids as well as proteins.
Insulin controls the uptake of glucose by cells by stimulating translocation of the glucose transporter GLUT-4 to the cell membrane. Once glucose is internalized, it promotes its storage in the form of glucogen. In both cases, this is achieved through the action of PI3K/Akt who are activated by the insulin receptor substrate protein IRS-1.
Insulin also promotes protein synthesis by inhibiting the action of GSK3, which normally phosphorylates and inactivates eukaryotic initiation factors eIF2B and eIF4B.