INSULIN-LIKE-GROWTH-FACTOR BINDING-PROTEIN PRODUCTION IN BOVINE RETINAL ENDOTHELIAL-CELLS

Retinopathy is the most frequent microangiopathic complication in diab etes. Many circulating hormones and locally produced mitogenic factors have been involved. Bovine retinal endothelial cells (BRECs) were cul tured to investigate if insulin, insulin-like growth factors (IGFs), I GF binding proteins (IGFBPs), and a chronic high-glucose condition cou ld control endothelial cell growth. Specific IGF-I receptors with two binding sites with high (K-d 0.03 nmol/L) and low (K-d 1.3 nmol/L) aff inity were found when analyzing families of displacement curves betwee n IGF-I versus IGF-I and IGF-I versus insulin. However, IGFs failed to be mitogenic factors in these cells. This could be explained by an in hibitory effect due to the presence of specific IGFBPs with a molecula r weight between 24 and 43 kd. Using Western blot and immunoblot analy sis, Northern blot study, and specific radioimmunoassay (RIA), these I GFBPs have been identified as IGFBP-3, -2, -5, and -4. Insulin, which does not bind to IGFBPs, was a potent mitogenic factor in these cells at a high concentration (10 nmol/L), suggesting a cross-reaction to IG F-I receptor. These IGFBPs, except the 24-kd form (IGFBP-4), were modu lated by both IGF-I and IGF-II, with a maximum effect at 100 and 10 nm ol/L, respectively. This regulation on IGFBPs was IGF-I receptor-indep endent. In fact, (1) IGFBP mRNA levels were not modified after stimula tion with 100 nmol/L IGF-I, (2) 100 nmol/L IGF plus an equimolar conce ntration of alpha IR3 did not affect IGFBP production, (3) Des(1-3)IGF -I had no effect on IGFBP modulation, whereas at 10 nmol/L it enhanced BREC thymidine cell incorporation, and (4) 100 nmol/L insulin, which at this concentration can cross-react with the IGF-I receptor, did not modify the IGFBP pattern. Chronic exposure (4 weeks) of BRECs to 25 m mol/L glucose had no effect on cell growth. However, after 3 weeks, we observed a decreased IGFBP detection, and addition of 100 nmol/L IGF- I did not change IGFBP levels and did not modify cell growth. Converse ly, BRECs grown in regular medium for 4 weeks showed increased IGFBP p roduction. In conclusion, we showed that conditions mimicking hyperins ulinemia, rather than high levels of IGFs, could regulate BREC growth and that the IGF-I analog, Des(1-3), even with reduced affinity for IG FBPs but in Part capable of binding to IGFBP-3, significantly stimulat ed BRECs growth only at 10 nmol/L. IGF actions are modulated by locall y produced endothelial IGFBPs, and in turn, these endothelial IGFBPs a re regulated, via an IGF-I receptor-independent mechanism, by the pres ence of IGFs. The autoregulatory IGF system together with the direct g lucose modulation of IGFBPs could contribute in diabetic subjects to t he retinal endothelial cell growth and metabolism through local change s in IGF bioavailability. Copyright (C) 1997 by W.B. Saunders Company.