The porcine insulin-like growth factor-I gene: characterization and expression of alternate transcription sites.
Genomic DNA encoding the 5' region of the porcine IGF-I gene was cloned and sequenced and shown to be highly homologous to that of man, rats and sheep. Two leader exons (exons 1 and 2), which are alternately spliced to exon 3 (encoding part of the mature IGF-I molecule), were identified by RNase protection analysis. In both cases, transcription initiates upstream from exons 1 and 2 at multiple dispersed start sites to yield two distinct IGF-I mRNA transcript classes (1 and 2) which differ in the precursor peptides predicted from their individual leader sequences. The expression of class 1 and 2 transcripts was measured in liver and muscle RNA from two groups of 2-month-old pigs whose energy status had been manipulated within physiological limits to produce marked differences in plasma IGF-I levels and growth rates. For this purpose, RNase protection probes were developed that contained the individual leader exons 1 and 2 linked separately to the common exon 3, so that class-specific and total IGF-I gene expression could be determined in a single assay. At normal plasma IGF-I concentrations (200 ng/ml), class 1 and 2 transcripts comprised 81 and 19% respectively of total liver IGF-I mRNA, while at a lower plasma concentration (90 ng/ml) the corresponding values were 95 and 5% respectively. Although both classes of transcript declined with the decrease in plasma IGF-I, the relative drop in levels of class 2 transcripts (84%) was substantially greater than that of class 1 (54%). In longissimus dorsi, cardiac and soleus muscles IGF-I mRNA was predominantly of class 1 and did not change in response to decreased plasma IGF-I. This suggests that liver-derived endocrine IGF-I has an important function in the regulation of muscle growth and that class 2 IGF-I transcripts are more sensitive to conditions that promote optimal growth.
Weller
P A
PA
Department of Cellular Physiology, AFRC Babraham Institute, Cambridge, UK.
Dickson
M C
MC
Huskisson
N S
NS
Dauncey
M J
MJ
Buttery
P J
PJ
Gilmour
R S
RS
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005786:Gene Expression Regulation; D005796:Genes; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis / Q000235:genetics*; D008969:Molecular Sequence Data; D051381:Rats / Q000235:genetics; D012756:Sheep / Q000235:genetics; D013045:Species Specificity; D013552:Swine / Q000235:genetics*; D014158:Transcription, Genetic
D007334:Insulin-Like Growth Factor I