som med vederbörligt tillstånd från Rektorsämbetet vid Umeå Universitet för avläggande av doktorsexamen i medicinsk vetenskap kommer att offentligen försvaras i Sal E 04, by 6E, Norrlands Universitetssjukhus, fredagen den 6 februari 1998, klockan 09.00 av Zhi-Wen Yu
Zhi-Wen YuInsulin-like action of vanadium compounds in fat cells
ABSTRACTUMEÅ UNIVERSITY MEDICAL DISSERTATIONS New series No 535 ISSN 0346-6612 ISBN 91-7191-402-1
From the Department of Medicine, Umeå University and the Institute of Internal Medicine, University of Göteborg, Sweden
Fakultetsopponent: Docent Hans Arnqvist, Institutionen för Medicin, Universitetet i Linköping Umeå 1998
Insulin-like action of vanadium compounds in fat cells by Zhi-Wen Yu, Department of Medicine, University of Umeå, Umeå University Hospital, S-901 85 Umeå, Sweden ABSTRACT
Vanadium compounds are potent tyrosine phosphatase inhibitors interacting with insulin signalling and they are potentially useful in the treatment of diabetes due to their insulin-like effect and enhancing effect on insulin action. The aim of this study was to elucidate the mechanisms of vanadate action particularly with respect to effects on insulin sensitivity.
In isolated rat adipocytes short-term pretreatment with insulin or vanadate increased cell surface insulin binding capacity. The insulin effect was ATP- and glucose-dependent. The effect of vanadate was more pronounced and less glucose-dependent than that of insulin but was markedly inhibited by the tyrosine kinase inhibitor genistein, suggesting the importance of tyrosine phosphorylation-related mechanism(s) for the effect. Furthermore, the stable peroxide of vanadate, bpV(pic) also markedly increased insulin binding in rat adipocytes. In addition, both vanadate and bpV(pic) inhibited cellular insulin degradation. Insulin sensitivity with respect to glucose uptake was also enhanced by the peroxovanadate (pV) compound as well as by vanadate.
Cleavage of phospholipids in the plasma membrane of rat adipocytes by phospholipase C increased insulin-like growth factor II (IGF-II) binding, suggesting cryptic receptors for IGF-II in the plasma membrane. The ability of insulin to increase IGF-II binding was impaired in cells that were insulin-resistant following cAMP treatment. Phospholipase C also enhanced the effect of low concentrations of insulin on IGF-II binding in these insulin-resistant cells indicating that an increased proportion of cryptic IGF-II receptors may be associated with insulin resistance. The serine phosphatase inhibitor, okadaic acid, further impaired and vanadate, in contrast, improved insulin sensitivity in cAMP-treated cells, suggesting opposing effects of tyrosine vs serine phosphorylation events on the availability of IGF-II receptors at the cell surface.
Short-term treatment with bpV(pic) but not vanadate exerted a stimulating effect on glucose uptake and an antilipolytic effect in isolated human adipocytes from both non-insulin-dependent diabetes mellitus (NIDDM) patients and control subjects. In addition, it markedly enhanced tyrosine phosphorylation of several cellular peptides including the insulin receptor b-subunit and insulin receptor substrate 1 which are part of the insulin signalling machinery. However, in NIDDM adipocytes both peroxovanadate and insulin action on glucose uptake was impaired to a similar degree. Moreover, bpV(pic) was unable to improve insulin sensitivity, indicating that impaired tyrosine phosphorylation of insulin signalling proteins is probably not a single critical mechanism for cellular insulin resistance in NIDDM. In contrast, the antilipolytic effect of bpV(pic) as well as insulin was similar in NIDDM and control cells.
Glucose transporter 4 (GLUT4) gene expression in cultured 3T3-F442A adipocytes was promoted by long-term insulin or vanadate treatment but this was found only when a low glucose concentration (5.6 mM) in the culture medium was used. Cyclic AMP inhibited the expression of this gene irrespective of glucose concentrations. It also inhibited the enhancing effect of insulin but this inhibition was prevented by concomitant treatment with vanadate. These agents produced similar alterations in GLUT4 protein content. Moreover, the insulin response with respect to glucose uptake was enhanced by long-term vanadate treatment in 3T3-F442A cells, whereas cAMP impaired the effect of insulin.
In conclusion, the observed effects of vanadium compounds on insulin binding, degradation and functional exposure of insulin-regulated plasma membrane proteins in adipocytes are possible mechanisms for their ability to improve insulin sensitivity. In addition, the promoting effect on GLUT4 gene expression may be associated with a long-term insulin-enhancing effect. The present data also indicate the involvement of tyrosine phosphorylation-related mechanisms in these regulatory processes and in the insulin-like action of pV in human adipocytes. However, the enhanced tyrosine phosphorylation of insulin signalling proteins produced by pV does not restore insulin sensitivity in NIDDM cells, indicating a limited role of tyrosine phosphorylation as a mechanism for cellular insulin resistance in NIDDM.
Key words: adipocyte, insulin, vanadate, peroxovanadate, cAMP, glucose, insulin receptor, tyrosine phosphorylation, GLUT4, glucose uptake, lipolysis, insulin sensitivity, insulin resistance, diabetes.
New series No 535 ISSN 0346-6612 ISBN 91-7191-402-1
This thesis is based on the following papers, which will be referred to by their roman numerals I - V.
I. Yu Z-W, Posner, BI, Smith, U and Eriksson, JW. Effects of peroxovanadate and vanadate on insulin binding, degradation and sensitivity in rat adipocytes. Biochim Biophys Acta, 1996, 1310:103-109.
II. Yu Z-W, Wickman A and Eriksson JW. Cryptic receptors for insulin-like growth factor II in the plasma membrane in rat adipocytes - a possible link to cellular insulin resistance. Biochim Biophys Acta, 1996, 1282:57-62.
III. Yu Z-W and Eriksson, JW. The role of glucose for the upregulating effect of insulin and vanadate on cell-surface insulin receptors in rat adipocytes. Submitted.
IV. Yu Z-W, Jansson P-A, Posner BI, Smith U and Eriksson JW. Peroxovanadate and insulin action in adipocytes from NIDDM patients. Evidence against a primary defect in tyrosine phosphorylation. Diabetologia, 1997, 40:1197-1203.
V. Yu Z-W, Enerbäck S, Nilsson E, Samuelsson L and Eriksson JW. Insulin and vanadate enhance but cAMP impairs GLUT4 gene expression in 3T3-F442A cells. Possible involvement of tyrosine phosphorylation-related mechanisms. Manuscript.