Butyrate enhances the synthesis of interphotoreceptor retinoid‐binding protein (IRBP) by Y‐79 human retinoblastoma cells

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Abstract
The synthesis and secretion of interphotoreceptor retinoid‐binding protein (IRBP) from Y‐79 human retinoblastoma cells was investigated using immunocytochemistry and SDS‐polyacrylamide gel electrophoresis. Indirect immunofluorescence of cells growing in monolayer culture for 11 and 13 days showed no significant IRBP staining although by SDS‐polyacrylamide gel electrophoresis, a small amount of IRBP was detected in the culture medium, suggesting synthesis and extracellular secretion. Butyrate (2mM) treatment of cells starting on the eighth day of culture resulted in a dramatic increase of IRBP fluorescence 3–5 days after treatment. Treatment of cells in all conditions with 1 μM monensin for 3 h showed concentration of IRBP in the Golgi apparatus of about 10–20% of cells as proved by a double immunofluorescent technique, employing anti‐IRBP antibody and wheat‐germ agglutinin. Incubation of cells with either radiolabeled amino acids or glucosamine followed by analysis of cell cytosol and culture medium by SDS‐polyacrylamide gel electrophoresis also confirmed that (1) IRBP is synthesized by the Y‐79 cells and secreted into the medium and (2) its production is markedly increased by butyrate treatment. The enhancement of IRBP synthesis by butyrate suggests biochemical differentiation of Y‐79 cells possibly into photoreceptor‐like cells and offers a new system for studying the properties of this unique retinoid‐binding protein and of factors that control its synthesis and secretion.