Regulation and Physiological Roles of Serum- and Glucocorticoid-Induced Protein Kinase Isoforms

Florian Lang¹ and Philip Cohen²

¹Department of Physiology, University of Tubingen, Germany.
²Medical Research Council Protein Phosphorylation Unit, School of Life Sciences, University of Dundee, Dundee, Scotland, DD1 4HN, United Kingdom.

Gloss: The covalent attachment of phosphate to proteins (phosphorylation), catalyzed by enzymes known as protein kinases, and the removal of phosphate from proteins (dephosphorylation), catalyzed by protein phosphatases, regulate most aspects of cell life. Phosphorylation or dephosphorylation alters the conformation of a protein and can change its ability to function in almost every conceivable way. For example, it cannot only switch activity on or off, but alter the rate at which a protein is degraded or its ability to move from one subcellular compartment to another. There are about 500 protein kinases and 150 protein phosphatases encoded by the human genome, and discovering their biological roles is one of the major tasks of the postgenomic era. This article reviews our current knowledge about one of the protein kinase subfamilies, termed serum- and glucocorticoid-induced protein kinases because the first member (SGK1) was identified as a gene that is rapidly transcribed into mRNA when cells are stimulated by serum or glucocorticoid hormones. However, we now know that the SGK1 gene is transcribed in response to a great variety of extracellular signals. Moreover, the SGK1 enzyme is itself activated by phosphorylation in response to different extracellular signals that act via the formation of a lipid mediator called phosphatidylinositol-3,4,5-trisphosphate (PIP3). Evidence is accumulating that SGK1 plays important roles in activating certain potassium, sodium, and chloride channels, suggesting an involvement in the regulation of processes such as cell survival, the functioning of the brain, and the excretion of sodium by the kidney. For the last mentioned reason, sustained high levels of SGK1 protein and activity may contribute to diseases and conditions, such as hypertension and long-term damage to the kidney in type II diabetes. This raises the possibility that drugs that inhibit SGK1 specifically may have therapeutic potential for the treatment of several diseases.

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