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SOMATOMEDIN-C/INSULIN-LIKE GROWTH FACTORS I (Sm-C/IGF-I) AND IGF-II mRNAs DURING LUNG DEVELOPMENT IN THE RAT

Sm-C/IGF-I and IGF-II are peptide mitogens that are synthesized in multiple organs and are implicated in regulation of organ growth. To investigate the ontogeny of their synthesis during lung development, Northern and dot blot hybridizations were performed on poly(A+)RNAs from lungs of fetal rats at 16, 17, 19, and 21 days gestation and postnatally at 0, 1, 2, 3-4, 6-7, 10, 14, 21, 28 and >50 days. A 32P-labelled rat Sm-C/IGF-I genomic fragment, a mouse Sm-C/IGF-I cDNA, and a human IGF-II cDNA were used as probes. These analyses revealed that the abundance of total Sm-C/IGF-I mRNA is 1.5 to 2.5-fold higher in lungs of fetal than postnatal animals and that multiple Sm-C/IGF-I mRNA species of estimated sizes 7.5, 4.7, 1.7 and 1.2 kb are observed at every age examined. For IGF-II, a high abundance of total hybridizing mRNA is found in fetal lung; it rapidly declines in the first week after birth to a plateau that is 20-fold less than in the fetus. Multiple lung IGF-II mRNAs of 4.7, 3.9, 2.2, 1.75 and 1.2 kb are observed. In the fetal and immediate postnatal periods, the 4.7 and 3.9 kb mRNAs are predominant. By one week postnatally, the 2.2kb mRNA is increased in relative abundance and the 1.2 kb mRNA is not detected. These data suggest synthesis of Sm-C/IGF-I and IGF-II throughout lung organogenesis. The abundance of mRNA for each peptide is developmentally regulated, and for IGF-II the developmental stage may determine the mRNA species expressed.