Distribution of IGFBP-rP1 in normal human tissues

IGFBP-rP1/mac25 is a recently described member of the insulin-like growth f actor binding protein (IGFBP) family. It has structural homology to the oth er members of the IGFBP family but has a lower affinity for insulin-like gr owth factors (IGFs). In previous studies using RNA blot hybridization, it w as shown that the expression of IGFBP-rP1/mac25 was ubiquitous in normal hu man tissues. In this report we show by immunohistochemistry that the expres sion of ICFBP-rP1/mac25 is actually restricted to certain organs and specif ic cell types. We used an antibody raised against a decapeptide of the C-te rminal part of the protein that recognizes a approximate to 37-kD protein u nder reduced conditions. The immunohistochemistry performed on normal human tissues showed a ubiquitous intense staining of peripheral nerves and a va riable degree of positive staining in smooth muscle cells, including those from blood vessel walls, gut, bladder, and prostate. Cilia from the respira tory system, epididymis, and fallopian tube showed intense immunoreactivity . Most endothelial cells showed some positivity, whereas fat cells, plasma cells, and lymphocytes were negative. There was specific expression limited to certain cell types in the kidney, adrenal gland, and skeletal muscle, i ndicating a possible specialized function of IGFBP-rP1/mac25 in these organ s. We further noted an opposite pattern of staining in the lining epitheliu m of breast (typically positive) and prostate glands (largely negative). Th e specific localization of IGFBP-rP1/mac25 as described implies a function of the protein. However, its regulation within the IGF axis or a possible d irect action of IGFBP-rP1/mac25 remains to be demonstrated.