EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) MESSENGER-RNA 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 ei ther 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 ex on 1, This heterogeneity is indicative of multifaceted regulation of g ene expression, and it is likely that differences in the nature of tra nscript expression reflect cell-specific regulation, As IGF-I is an im portant factor in skeletal growth and development, the aim of this stu dy was to determine the pattern of transcript expression in rat whole bone and osteoblast-enriched cultures isolated from long bones, The re lative proportions of transcripts differing in the 5'-UTR were determi ned by RNase protection assays and compared to expression in rat liver . These studies revealed a significantly lower expression of exon 2-de rived transcripts in bone cells compared to liver (approximately 10% c ompared to 40% of total transcripts), There were also important differ ences in start site usage in exon 1 in bone cells, In osteoblastic cel ls, 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 si te 3-initiated transcripts represented only 11% +/- 3%, In addition, t he proportion of transcripts initiated at start site 4 was about twofo ld greater in liver than in bone cells (32% +/- 7% compared to 16% +/- 8%), However, expression of full-length transcripts was similar in bo th tissues, The distribution in osteoblastic cells reflected that in w hole 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.