Interstitial retinol-binding protein (IRBP) in retinoblastoma

Interstitial retinol-binding protein (IRBP) is an extracellular glycoprotein that appears to be synthesized by the photoreceptors in the normal retina and may be involved in shuttling retinoids through the interphotoreceptor matrix between the retina and pigment epithelium. The present work demonstrated immunochemically that IRBP of the same molecular weight as normal human IRBP (135,000) was present in three retinoblastomas, irrespective of their degree of differentiation. Tumor 1 was classified as differentiated; Tumors 2 and 3 were classified as poorly differentiated. The level of IRBP in the soluble proteins of Tumor 2 was about 8 times that in Tumor 1 and about one-half that in the soluble proteins (including adhering interphotoreceptor matrix) from a pair of normal retinas from a 31-year-old donor. IRBP occurred in the interstitial space of Tumor 3. Most of the IRBP in this tumor was recovered from the medium in which the undifferentiated cells were dispersed. Incubation of the isolated cells from Tumor 3 in medium containing [(3)H]leucine demonstrated that [(3)H]IRBP was secreted into the medium. The [(3)H]IRBP was immunoreactive with rabbit antibovine IRBP antibodies. It was inferred that the [(3)H]IRBP was glycosylated because it was bound by immobilized concanavalin A and could be displaced with ?-methylmannopyranoside. Since IRBP is not normally found in retinas until the time of photoreceptor differentiation, regulation of its gene may be defective in this malignant neuroectodermal neoplasm. The present findings are relevant to the possible role of retinoids and their binding proteins in neoplastic cells, because they demonstrate for the first time the presence of an extracellular retinoid-binding protein in tumor tissue.