DEVELOPMENTAL REGULATION OF BOVINE INSULIN-LIKE GROWTH FACTOR-II (IGF-II) GENE-EXPRESSION - HOMOLOGY BETWEEN BOVINE TRANSCRIPTS AND HUMAN IGF-II EXONS

Initial observations have indicated similarities between bovine and hu man IGF-II production during development. The aim of the present study was to investigate whether cattle could provide an experimental model that would mimic the complex pattern of human IGF-II gene expression. Expression of bovine IGF-II gene during development was studied by RN A hybridization using various human IGF-II probes. In fetal tissues an d in adult muscle, the bovine IGF-II gene was expressed as a family of eight transcripts ranging in size from 5.2 to 1.1 kb. In adult bovine liver, a major IGF-II transcript of 4.4 kb was expressed that could n ot be detected in any fetal or adult extra-hepatic tissue. During feta l life, quantitative IGF-II mRNA expression differed in liver and musc le, and the relative amounts of the different transcripts varied with the tissue of origin. These observations suggest that the regulation o f bovine IGF-II gene expression is specific to the stage of developmen t and the tissue concerned. Moreover its pattern is very similar to th at in its human counterpart. In order to identify a putative homology between human and bovine gene structures, bovine mRNAs were examined f or cross-hybridization with various non-coding exons of the human gene . Cross-hybridization was detected with human untranslated exons 5 and 6, suggesting the presence of two distinct promoters similar to the h uman promoters P3 and P4. The 4.4 kb mRNA species expressed in adult b ovine liver failed to hybridize to a probe for human exons 1 and 2, su ggesting that the leader sequences of this transcript were different f rom those present in the human gene. Finally, results obtained with a probe containing the 3' untranslated end of exon 9 suggested the prese nce of at least two polyadenylation sites in the bovine gene. Although differences in IGF-II gene structures were found between cattle and m an, the similarities in the pattern of gene expression between the two species suggest that cattle may be a useful model to investigate some developmental aspects of the expression of the human IGF-II gene.