Hormone-Sensitive Lipase and Protein Kinase B; Molecular characterization in testis, adipose tissue and pancreatic beta cells
Sammanfattning: Hormone-sensitive lipase (HSL) and protein kinase B (PKB) are enzymes that in different ways are associated with lipid metabolism. While HSL is the well-known lipolytic enzyme responsible for hydrolysis of stored triacylglycerols in adipose tissue, PKB is a recently identified serine/threonine protein kinase, ubiquitously expressed and involved in signal transduction pathways induced by insulin and growth factors. Here, with focus on lipid-containing tissues from which information on their presence was previously scarce or lacking, expression of HSL in testis and expression/regulation of PKB in adipocytes and pancreatic beta cells are reported. A testicular isoform of HSL (HSLtes) is formed through addition of a unique N-terminal domain, comprising 300 amino acids, to the 768/775 amino acids, respectively, of rat and human HSLadi, i.e., the adipocyte form. The testis-specific part is more hydrophilic than HSLadi. In man, this addition occurs through alternative splicing of a testis-specific exon located 16 kb upstream of the first exon encoding HSLadi. HSLtes expression occurs primarily in spermatozoa and maturing sperm and not in the endocrine Leydig cells. HSLtes appears to exert a lipase/cholesterol esterase activity similar to that of HSLadi but its exact role is yet unknown. PKB isoforms (alfa, beta and gamma) are expressed in rat adipocytes and beta cells of pancreatic islets. Insulin stimulation of rat adipocytes leads to rapid and reversible phosphorylation and activation of PKB. Activation of PKB by the insulin-mimetic compound peroxovanadate is associated with translocation of PKB from the cytosol to the membrane fraction of rat adipocytes. In clonal beta cells, PKB is phosphorylated and activated upon stimulation with IGF-I. In both cells, the activation of PKB is abolished through inhibition of phosphatidylinositol-3 kinase (PI3K), suggesting a position for PKB down-stream of PI3K in signalling by insulin/IGF-I in these cells.
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