Protein tyrosine kinases in insulin producing cells : Expression and putative importance for beta-cell function

Sammanfattning: Protein tyrosine kinases are of importance for cell replication and differentiation in manysystems. The mechanisms controlling the replication and differentiation of insulin producingcells are unknown. The present study was therefore conducted in order to characterize theexpression of various tyrosine kinases in insulin producing cells, and to assess the possibleimportance of these for beta-cell function by addition of ligands to the tyrosine kinasereceptors expressed to in vitro cultures of fetal islet-like structures.Several tyrosine kinases were found to be expressed in different preparations of insulinproducing cells, among them the receptors Flk-1, c-Kit, the FGF receptor 4 and the IGF-Ireceptor. Flk-1 and c-Kit were expressed mainly in ductal epithelium, whereas the FGFR4was widely expressed throughout the pancreatic parenchyma. The ligand to Flk-1, VEGF,stimulated after culture both insulin release to the culture medium and insulin contents/DNAin fetal rat islet-like structures. In addition, it caused an increased replication of isolatedductal epithelial cells. The possibility that VEGF is a duct cell mitogen may be ofimportance, since the increase in beta-cell mass during fetal development is largely due toneoformation of islet cells from stem cells located in the ductal epithelium. SCF, the ligand toc-Kit increased the insulin/DNA content in fetal rat islet-like structures, but decreased theDNA content in the same culture system, whereas both acidic FGF and growth hormonestimulated the insulin accumulation to the culture medium. Taken together, the data indicatethat ligands to tyrasine kinase receptors or receptors signalling through cytoplasmic tyrosinekinases may play a role for islet development and/or function. Specifically, VEGF may playa role in this process by stimulating proliferation of the ductal epithelium, which may be aprerequisite for neoformtation of islet cells from stem cells in the epithellum.Furthermore, a novel tyrosine kinase, Bsk, was cloned from insulin producing cells, and itsstructure and function has been investigated. Bsk is a member of the Src-family of cytoplasmictyrosine kinases, highly homologous to the human gene Frk. The Bsk gene is expressed inpancreas, liver, lung and kidney. Transfection of NIH3T3 fibroblasts with Bsk cDNAcontaining mutations of carboxyterminal regulatory tyrosine residues 497 and 504 results ininhibited cell growth and a decrease in thymidine incorporation rate. Cell cycle analysisrevealed that this is a consequence of Gl-prolongation. This suggests a role for Bsk in theregulation of cell proliferation, although the exact function of the gene product remains to beelucidated.