Expression and stability of insulin-like growth factor-I (IGF-I) mRNA splicing variants in the GH₃ rat pituitary cell line

We have employed Northern blot analyses and solution hybridization/RNase protection assays to evaluate the presence and stability of IGF-I mRNA splicing variants in the GH₃ rat pituitary cell line. All of the IGF-I mRNA size classes and IGF-I mRNAs with alternately-spliced 5′-untranslated regions and E-peptide coding regions seen in adult rat liver also were present in GH₃ cells, although the proportions of the 5′ splicing variants were significantly altered. In actinomycin D-treated cells, all IGF-I mRNA splicing variants were equally stable; thus, changes in the levels of some splicing variants were not due to differential mRNA stability. Additionally, all IGF-I mRNA size classes seen on Northern blots were equally stable; this data suggests that the large IGF-I mRNA species is not a precursor of the smaller species.