Kinetics and regulation of site-specific endonucleolytic cleavage of human IGF-II mRNAs

Human insulin-like growth factor II (IGF-II) mRNA can be cleaved at a specific site in its 4 kb long 3 ' -UTR. This yields a stable 3 ' cleavage product of 1.8 kb consisting of a 3 ' -UTR and a poly(A) tail and an unstable 5 ' cleavage product containing the IGF-II coding region. After cleavage, the 5 ' cleavage product is targeted to rapid degradation and consequently is no longer involved in IGF-II protein synthesis. Cleavage is therefore thought to provide an additional way to control IGF-II gene expression. In this paper the kinetics and the efficiency of cleavage of IGF-II mRNAs are examined. The cleavage efficiency of IGF-II mRNAs carrying four different leaders (L1-L4) is enhanced in the highly structured leaders Ll and L3. Additionally, under standard cell culture conditions cleavage is a slow process that only plays a limited role in destabilisation and translation of the IGF-II mRNAs. However, in human Hep3B cells and CaCo2 cells which express IGF-II endogenously, cleavage is upregulated 3-5-fold at high cell densities. Regulated endonucleolytic cleavage of IGF-II mRNAs is restricted to cells in which IGF-II expression is related to specific cell processes.