Somatomedin-C/Insulinlike Growth Factor I (Sm-C/IGF-I) and Insulinlike Growth Factor II (IGF-II) mRNAs during Lung Development in the Rat

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Abstract
Somatomedins/insulinlike growth factors (Sm/IGFs) are peptide growth factors that are synthesized in multiple tissues and have been implicated in organ growth. Poly(A +)RNAsfrom lungs of fetal and postnatal rats were examined by dot and Northern blot analyses to investigate the local synthesis of IGFs during lung organogenesis. Blots were successively hybridized with 32P-labeled human IGF-II and mouse Sm-C/IGF-I cDNAs followed by human ubiquitin cDNAs (a control). Multiple IGF-II mRNA species of 4.7, 3.9, 3.1, 2.2, 1.75, and 1.2 kb were observed. Prenatally, the 3.9 kb species was predominant. In adult lung, the 4.7 kb species was the major species, and the 1.2 kb species was not detectable. Multiple Sm-C/IGF-I mRNA species of estimated sizes 7.5, 4.7, 1.7, and 1.0 kb were observed and their relative abundance did not change discernibly throughout development. Densitometric quantification revealed that IGF-II mRNAs were 7-20 fold more abundant in fetal lung than adult lung. In addition, they were 5-8 times more abundant than Sm-C/IGF-I mRNAs in embryonic lung, and were slightly less abundant than Sm-C/IGF-I mRNAs in adult lung. These data suggest synthesis of both Sm-C/IGF-I and IGF-II throughout lung organogenesis. Developmental changes in IGF-II mRNA abundance during lung organogenesis imply regulation at the level of gene transcription and/or mRNA stability.