Regulation of hepatic insulin-like growth factor I leader exonusage in lambs: effect of immunization against growth hormone-releasing factor and subsequent growth hormone treatment.
The establishment of a GH-responsive endocrine IGF-I network is essential for the regulation of postnatal growth. Transcripts of exons 1 and 2 of the mammalian IGF-I gene are alternately spliced onto exon 3, generating class 1 and class 2 mRNA, respectively, each encoding individual signal peptides. The liver is largely responsible for the synthesis of circulating IGF-I and is the main site of expression for class 2 mRNA. The aim of this study was to examine the regulation of hepatic class 1 and 2 mRNA levels in response to changed GH status. Lambs were actively immunized against GRF to suppress GH secretion; hepatic IGF-I mRNA leader exon usage was examined in the presence and absence of GH replacement and in control-immunized lambs. Lambs immunized against GRF exhibited a 17% (P < 0.001) decrease in growth rate as assessed by whole body weight gain, accompanied by decreased circulating IGF-I concentrations (P < 0.001), which were increased by subsequent GH treatment (P < 0.001). Hepatic class 1 and 2 IGF-I mRNA levels decreased in GRF-immunized lambs, although only class 2 transcripts decreased significantly (P < 0.001). Subsequent GH treatment induced increases in class 1 and 2 mRNA levels (P < 0.001) but the increase in class 2 message exceeded that for class 1 (P < 0.001). Thus, the percentage of total IGF-I mRNA accounted for by class 2 mRNA was 45% in control lambs, decreased to less than 20% in GRF-immunized lambs, but increased to 72% in the GRF-immunized lambs treated with GH and correlated with circulating IGF-I concentrations. These data suggest physiological significance for class 1 and 2 IGF-I mRNA species in GH action. Possible functions for such alternative splicing mechanisms are discussed.
O'Sullivan
D C
DC
The Babraham Institute, Cambridge, UK.
Szestak
T A M
TA
Pell
J M
JM
D000367:Age Factors; D017398:Alternative Splicing; D000818:Animals; D000906:Antibodies / Q000097:blood; D001823:Body Composition; D005091:Exons; D015870:Gene Expression; D013006:Growth Hormone / Q000378:metabolism / Q000502:physiology*; D013007:Growth Hormone-Releasing Hormone / Q000276:immunology*; D007114:Immunization / Q000662:veterinary; D018969:Insulin-Like Growth Factor Binding Proteins / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000378:metabolism*; D008099:Liver / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D012756:Sheep / Q000254:growth & development* / Q000276:immunology; D015430:Weight Gain / Q000276:immunology
D000906:Antibodies; D018969:Insulin-Like Growth Factor Binding Proteins; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone