Immunohistochemical localization of insulin-like growth factor binding protein-1, -3 and -4 in human fetal tissues and their analysis in media from fetal tissue explants.

The actions of insulin-like growth factor (IGF) II that are important in the regulation of fetal growth and development, are modulated by IGF binding proteins (IGFBPs). We have determined the cellular distribution of IGF-II and IGFBP-1, -3 and -4 in 12-week gestation human fetal tissues using immunocytochemistry. IGF-II immunostaining was found in all organs examined, with strongest immunoreactivity in spinal ganglia, tubular cells of the mesonephros and peri- and epidermal layers of the skin. The immunoreactivity distribution of all IGFBPs was similar to that of IGF-II except lung, hepatic parenchyma, fibrocytes of connective tissue and cells of the growth plate in the cartilage. When conditioned media from skin, liver, lung and kidney explants were analyzed, phosphorylated IGFBP-1 was only detected in liver samples whereas IGFBP-3 was found in all media. Weak immunoreactivity of IGFBP-4 was seen in media from lung tissue. To determine whether proteolytic degradation of IGFBPs were responsible for the different IGFBP levels, cell-free conditioned media were incubated with recombinant human IGFBPs. At neutral pH only proteolysis of IGFBP-4 was observed in media from skin and lung tissue. Upon acidification of the medium samples, IGFBP-1 fragments were formed in skin-derived medium and IGFBP-3 was cleaved by medium from lung and kidney tissue. Acid-activated proteolytic activity against IGFBP-4 was found in the media from lung and liver. These findings suggest that IGFBP proteases may be important in locally defining the concentrations of IGFBPs and contribute to tissue-specific growth response to IGFs.