Regulation of insulin‐like growth factor binding protein synthesis and secretion in human retinal pigment epithelial cells

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Abstract
Cultured human retinal pigment epithelial cells (RPE) secrete insulin‐like growth factor binding proteins (IGFBPs), a family of polypeptides which modulate the actions of the insulin‐like growth factors. RPE cells secrete two IGFBPs with Mr estimates of 34,000 and 46,000, respectively. Treatment of RPE cells with IGF‐I markedly stimulated the secretion of the 46,000 Mr form. This stimulation occurred via an IGF‐I receptor independent mechanism because both [QAYL]IGF‐I (an IGF‐I analogue with decreased affinity for the IGFBPs but normal affinity for the IGF‐I receptor) and α‐IR3 (a blocking monoclonal antibody against the IGF‐I receptor) had no effect on IGF‐I stimulated increases in IGFBPs. Additionally, [QAYL]IGF‐I enhanced RPE cell proliferation to the same magnitude as IGF‐I. Treatment with IGF‐I, [QAYL]IGF‐I, or α‐IR3 had no effect on steady‐state levels of the 2.5 kb IGFBP‐3 or the 1.3 kb IGFBP‐6 mRNA transcripts as measured by Northern blotting and quantitative autoradiography. Forskolin and a group of candidate growth factors, including platelet‐derived growth factor, epidermal growth factor, and acidic and basic fibroblast growth factor, modestly increased IGFBP secretion when compared to untreated cells, but these effects were small when compared to IGF‐I treatment. Fetal calf serum enhanced the presence of the 2.5 kb IGFBP‐3 mRNA transcript in a dose‐dependent fashion but had no effect on the 1.3 kb IGFBP‐6 mRNA transcript. IGF‐I, forskolin, and the candidate growth factors had no effect on either IGFBP‐3 or IGFBP‐6 mRNA. These data suggest that the production of IGFBPs in human RPE cells is regulated by distinct mechanisms which include (1) an IGF‐I receptor independent interaction of IGF‐I with secreted IGFBPs and (2) de novo synthesis of IGFBPs by serum‐containing factors.

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