The pancreatic peptide glucagon, is secreted by the islets of Langerhans to maintain plasma glucose concentrations during fasting. The biological action of glucagon is exerted upon target tissue following receptor binding and adenylyl cyclase activation. Diabetes mellitus, a disease of deranged carbohydrate metabolism, is manifested by marked elevation in the plasma glucose levels. Paradoxically, high glucagon levels have been reported in the plasma of diabetic patients.
The Iwanij laboratory have been interested in the biochemical and molecular characterization of the glucagon receptor. In order to identify the glucagon receptor, a minor component of the plasma membrane, we have developed an affinity labeling procedure that involves direct UV irradiation as a mean of cross-linking radiolabeled glucagon to its binding sites. Using this approach the lab has identified and characterized the liver glucagon receptor, the major target tissue for this hormone. As the cross-linking technique proved to be highly sensitive, the lab was able to identify and characterize the glucagon receptor in other tissues, such as adipose tissue and kidney, where receptors are present at low densities.
The identification of the glucagon receptor has lead to a partial purification of the denatured form of the glucagon receptor and the development of monoclonal antibodies against the receptor. The lab is currently studying two specific aspects of the receptor physiology: receptor phosphorylation, and function of the N-terminal domain of the receptor.
Cloning and molecular characterization of the liver plasma membrane proteins.
In the course of preparing monoclonal antibodies for the glucagon receptor, we have generated other antibodies that recognize additional components of the liver plasma membrane. Using these antibodies as a screening probes we have cloned two new genes that encode membrane proteins; a novel liver-specific transporter, and a putative adhesion protein. The transporter gene represents a single copy gene and its expression is highly regulated in tissue-specific and developmental stage-specific manner. The human homologue of the liver transporter gene and cDNA has been also cloned and sequenced. Presently, the lab has prepared expression constructs for the transient and stable expression of these proteins in cell lines for use in functional studies.
Selected Publications (Pubmed Search)
Iwanij, V. (1996) The glucagon receptor gene: Organization and tissue distribution. (Invited review) IN: Handbook of Experiemental Pharmacology; Vol. 123, Glucagon, III, Chapter 4 (P.J. Lefebvre, ed). Springer-Verlag, Heidelberg. pp. 53-74.
Iwanij, V. (1995) Canine kidney glucagon receptor: Evidence for a structurally-different, tissue-specific variant of the glucagon receptor. Mol. Cell Endocrinol. 115:21-28.
Iwanij, V., and H. Stukenbrok (1995) Uptake and transport of glucagon by rat liver: Evidence for transcytotic pathway. Protoplasma 188:202-212.
Simonson, G.C., and V. Iwanij (1995) Genomic organization and promoter sequence of a gene encoding a rate liver-specific type-I transport protein. Gene 154:243-247.