Expression of insulin-like growth factor I (IGF-I) mRNA variants in rat bone

The rat insulin-like growth factor I (IGF-I) gene is characterized by the presence of multiple mRNA transcripts, which differ in the 5'- and/or 3'-untranslated regions (UTRs). Transcript initiation occurs in either exon 1 or exon 2, giving rise to mRNA species which differ in the length and sequence of the 5'-UTR, while further variation is due to multiple transcription start sites and differential splicing within exon 1. This heterogeneity is indicative of multifaceted regulation of gene expression, and it is likely that differences in the nature of transcript expression reflect cell-specific regulation. As IGF-I is an important factor in skeletal growth and development, the aim of this study was to determine the pattern of transcript expression in rat whole bone and osteoblast-enriched cultures isolated from long bones. The relative proportions of transcripts differing in the 5'-UTR were determined by RNase protection assays and compared to expression in rat liver. These studies revealed a significantly lower expression of exon 2-derived transcripts in bone cells compared to liver (approximately 10% compared to 40% of total transcripts). There were also important differences in start site usage in exon 1 in bone cells. In osteoblastic cells, transcripts initiated at start site 3 were the predominant species (50% +/- 12% of total exon 1-derived mRNAs; Mean +/- SD) whereas the alternately spliced transcripts represented only 20% +/- 3%. This was in contrast to the profile in liver in which 47% +/- 9% of total exon 1-derived transcripts were the alternately spliced mRNAs, but start site 3-initiated transcripts represented only 11% +/- 3%. In addition, the proportion of transcripts initiated at start site 4 was about twofold greater in liver than in bone cells (32% +/- 7% compared to 16% +/- 8%). However, expression of full-length transcripts was similar in both tissues. The distribution in osteoblastic cells reflected that in whole bone. These results demonstrate that the IGF-I transcript profile in bone cells differs to that in liver cells. Since the mRNA variants exhibit different properties, including half-life and translatability, such cell-specific variation in their relative expression is likely to reflect differential regulation of IGF-I in these tissues.