Cellular localization of messenger RNAs for insulin-like growth factors (IGFs), their receptors and binding proteins during fetal rat lung development

To gain insight into the role of the insulin-like growth factors (IGFs) in regulating lung development, we have used in situ hybridization histochemistry (ISHH) to examine the ontogeny and sites of expression of IGF-I and IGF-II, IGF binding proteins (IGFBP-1 to IGFBP-6), and IGF cell surface receptors in fetal rat lung from 15 to 21 days of gestation. Both IGF-I and IGF-II mRNAs were expressed throughout the developmental period studied with little change in apparent abundance, IGF-I mRNA localized to mesenchymal cells, especially those surrounding airway epithelium, while IGF-II mRNA, which was somewhat more abundant, localized predominantly to epithelia. The type 1 IGF receptor, the receptor that likely mediates the actions of both IGFs, was expressed widely in virtually all cells, whereas the expression of the type 2 IGF receptor, thought to be involved in IGF internalization and degradation, was confined to the mesenchyme and medial layers of intrapulmonary vessels. As with the IGFs, there was little apparent change in the abundance of IGF receptor mRNAs through fetal development, and the type 2 IGF receptor mRNA was more abundant. The expression of IGFBPs changed significantly during lung development. IGFBP-2, -3, -4, and -5 were expressed from day 15 of gestation, but their sites of expression and ontogeny differed. IGFBP-2 mRNA expression was abundant and constant throughout gestation and was confined to proximal and distal airway epithelia. IGFBP-3 and IGFBP-5 also were expressed by proximal airway epithelia, but also exhibited significant expression in interstitial mesenchyme and in mesenchyme surrounding vessels. The abundance of both increased as gestation progressed (IGFBP-5 greater than IGFBP-3). IGFBP-4 mRNA was confined to interstitial mesenchyme and its abundance peaked at days 16 to 19 of gestation. We found no evidence for expression of either IGFBP-1 or IGFBP-6 We conclude that the expression of IGF-I, IGF-II, and the type 1 IGF receptor throughout gestation in the lung supports a role for the IGFs in lung growth and development. The complex pattern of IGFBP expression (differing sites and ontogeny of expression) suggests that the IGFBPs modulate IGF actions at specific target sites. Furthermore, because there is little change in the expression of IGFs or IGF receptor mRNAs during fetal lung development, regulation of IGFBP expression may be essential to the control of IGF actions during lung development.