Cellular distribution of insulin-like growth factor binding protein mRNAs and peptides during rat lung development

A role for IGF binding proteins (IGFBPs) in lung development is suggested by the identification of IGFBPs in lung tissue and production of IGFBPs by fetal lung cells in culture. To characterize the expression of IGFBPs during lung development in the rat in vivo (16 days gestation through adulthood), the expression of IGFBP mRNAs (IGFBP-1 to IGFBP-6) was examined by Northern analysis and in situ hybridization, and IGFBP peptides (IGFBP-2, IGFBP-3, and IGFBP-5) were localized by immunohistochemistry. IGFBP-1 mRNA was not detectable. IGFBP-2 mRNA (1.8 kb) was expressed in both fetal and postnatal life with peak expression during the fetal pseudoglandular stage. IGFBP-2 mRNA was localized mainly to airway epithelium. IGFBP-3 mRNA (2.4 kb) was maximally expressed postnatally in the saccular stage of lung development; it was identified in airway epithelium and interstitium in the fetal lung, but predominantly in airway epithelium after birth. IGFBP-4 (2.6 kb) and IGFBP-5 (6.0 kb) mRNA levels were maximal after birth, from 3 to 21 days postnatal (saccular and alveolar stage). IGFBP-4 mRNA was localized primarily to the interstitium and blood vessels early in development, but was abundant in airway epithelium in the adult. IGFBP-5 mRNA was most abundant in the airway epithelium. IGFBP-3, IGFBP-4, IGFBP-5, and to a lesser extent IGFBP-6 were localized to the large cartilaginous airways in the adult. IGFBP-2, IGFBP-3, and IGFBP-5 peptides were distributed more widely than their respective mRNAs, with a temporal pattern of immunoreactivity following that of their mRNAs. Maximal staining was noted in airway epithelium for IGFBP-2 in the newborn, for IGFBP-3 in the saccular stage (newborn to 3 days postnatal), and for IGFBP-5 in the alveolar stage (5 to 21 days postnatal). Our studies demonstrate that IGFBP-2, IGFBP-3, IGFBP-4, and IGFBP-5 are synthesized and distributed in spatially and temporally different patterns in the developing lung. The widespread distribution of IGFBP immunoreactivity compared with their respective mRNAs suggests that IGFBPs are important paracrine factors in the regulation of IGF action in the developing lung.