IMMUNOHISTOCHEMICAL LOCALIZATION OF INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN-1, BINDING-PROTEIN-3 AND BINDING-PROTEIN-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 IG F binding proteins (IGFBPs). We have determined the cellular distribut ion 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 t he skin. The immunoreactivity distribution of all IGFBPs was similar t o that of IGF-II except lung, hepatic parenchyma, fibrocytes of connec tive tissue and cells of the growth plate in the cartilage. When condi tioned media from skin, liver, lung and kidney explants were analyzed, phosphorylated IGFBP-1 was only detected in liver samples whereas IGF BP-3 was found in all media. Weak immunoreactivity of IGFBP-4 was seen in media from lung tissue. To determine whether proteolytic degradati on of IGFBPs were responsible for the different IGFBP levels, cell-fre e conditioned media were incubated with recombinant human IGFBPs. At n eutral pH only proteolysis of IGFBP-4 was observed in media from skin and lung tissue. Upon acidification of the medium samples, IGFBP-1 fra gments were formed in skin-derived medium and IGFBP-3 was cleaved by m edium from lung and kidney tissue. Acid-activated proteolytic activity against IGFBP-4 was found in the media from lung and liver. These fin dings suggest that IGFBP proteases may be important in locally definin g the concentrations of IGFBPs and contribute to tissue-specific growt h response to IGFs.