A reassessment of insulin-like growth factor binding protein gene expression in the human retinal pigment epithelium

The role of insulin-like growth factors (IGF) in regulating cell differentiation and proliferation is in part modulated by the IGF binding protein (IGFBP) family of genes. Previous studies of the human retinal pigment epithelium (RPE) have detected expression of IGFBP-2, -3, and -6. However, recent experiments in our lab have suggested a broader pattern of IGFBP gene family expression in the RPE cell than has previously been recognized. We have examined the gene expression profile of IGFBP-1 to -6 and the related protein, IGFBP-rP1, in RPE cell lines derived from ten donors eyes using RT-PCR, ELISA, and Western methods. Transcripts of IGFBP-1 to -6 and -rP1 were consistently detected in human RPE cells. IGFBP-3, -5, -6, and -rP-1, appear to be constitutively expressed in the RPE, whereas IGFBP-1, -2, and -4, were expressed at variable levels in the cell lines examined. IGFBP secretion by the RPE in vitro was confirmed by ELISA (IGFBP-1, -2, -3, -4, and -6) and Western blot analysis (IGFBP-5 and -rP1). There was, in general, a strong correlation between gene-specific transcription levels and protein secretion by the RPE. Our studies demonstrate that the major IGFBP family genes are ubiquitously expressed in explanted human RPE cells in vitro. This broad expression profile and the recent evidence that IGFBPs have IGF-independent biological activity suggest that the IGFBP family genes may constitute a previously unrecognized and complex regulatory system in the human retina and RPE.