Lena.Stenson-Holst, Institutionen för Cell - och Molekylärbiologi, Lund den 13 :e januari, 1999 kl 09:09

Hormone-Sensitive Lipase and Protein Kinase B - Molecular characterization in testis, adipose tissue and pancreatic b cells

Lena Stenson Holst

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 b 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 (a, b and g) are expressed in rat adipocytes and b 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 b 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.

Section for Molecular Signalling Department of Cell and Molecular Biology Lund University, S-221 00 Lund, Sweden List of papers

I. Stenson Holst, L., Hoffmann, A.M., Mulder, H., Sundler, F., Holm, C., Bergh, A. and Fredrikson, G. (1994): Localization of hormone-sensitive lipase to rat Sertoli cells and its expression in developing and degenerating testes. FEBS Lett 355, 125-30.

II. Stenson Holst, L., Langin, D., Mulder, H., Laurell, H., Grober, J., Bergh, A., Mohrenweiser, H.W., Edgren, G. and Holm, C. (1996): Molecular cloning, genomic organization, and expression of a testicular isoform of hormone-sensitive lipase. Genomics 35, 441-7.

III. Wijkander, J., Stenson Holst, L., Rahn, T., Resjö, S., Castan, I., Manganiello, V., Belfrage, P. and Degerman, E. (1997): Regulation of protein kinase B in rat adipocytes by insulin, vanadate, and peroxovanadate. Membrane translocation in response to peroxovanadate. J Biol Chem 272, 21520-6.

IV. Stenson Holst, L., Mulder, H., Manganiello, V., Sundler, F., Ahrén, B., Holm, C. and Degerman, E. (1998): Protein kinase B is expressed in pancreatic beta cells and activated upon stimulation with insulin-like growth factor I. Biochem Biophys Res Commun 250, 181-6.

Från Avdelningen för Molekylär Signalering Institutionen för Cell - och Molekylärbiologi Akademisk avhandling som för vinnande av doktorsexamen i medicinsk vetenskap vid Medicinska fakulteten i Lund kommer att offentligen försvaras i Hörsal A, Kemicentrum, Lund fredagen den 11 december 1998, kl. 915 Fakultetsopponent: Docent Lars Rönnstrand Ludwig Institute for Cancer Research BMC, Uppsala