Rat insulin II gene expression by extraplacental membranes. A non-pancreatic source for fetal insulin

Using cDNA cloning, ribonuclease protection, and Northern hybridization analysis, we showed that insulin gene expression occurs in yolk sac-derived fetal extraplacental membranes throughout the last half of rat fetal development. The mRNA product of the ancestral rat insulin II but not the duplicated rat insulin I gene was present in high copy number, and its abundance was regulated during development. Insulin mRNA was present in extraplacental membranes before pancreatic differentiation; membrane insulin mRNA content greatly exceeded that in pancreas until the last 2 days of gestation when content in each tissue became similar. Polyadenylation and intron splicing occurred at the same sites used in pancreas, but initiation of transcription occurred at multiple sites in membranes. Minces of membranes maintained in culture produced approximately 10 ng of radioimmunoassayable insulin/mg membrane protein/day. Over a 4-day period, approximately 50 times more insulin accumulated in medium than that present in membranes at the time of isolation. These studies indicate that yolk sac is a source for insulin during fetal development and that the mechanisms regulating insulin gene expression in this tissue differ from those in pancreatic beta cells.