Distribution of IGF-I and -II, IGF binding proteins (IGFBPs) and IGFBP mRNA in ocular fluids and tissues: potential sites of synthesis of IGFBPs in aqueous and vitreous

Abstract

Levels of insulin-like growth factor-I and II (IGF-I and IGF-II) in bovine aqueous humor are twice those found in the vitreous (aqueal IGF-I = 0.62 nM, vitreal IGF-I = 0.30 nM; aqueal IGF-II = 0.028 nM, vitreal IGF-II = 0.017 nM). IGF-I and II binding assays and IGF-II Western ligand blots indicate that aqueous and vitreous humor have equal overall levels of binding (binding assays, mean +/- S.E.M. bound/free per 50 microliters of fluid: vitreal IGF-II = 7.28 +/- 1.6, IGF-I = 0.3 +/- 0.078; aqueal IGF-II = 7.21 +/- 0.072; IGF-I = 0.3 +/- 0.078). In addition, the ligand blots reveal that aqueous and vitreous have markedly different complements of specific IGF binding proteins (IGFBPs). Aqueal levels of a 34 kDa IGFBP, immunologically identified as IGFBP-2, exceed those in the vitreous by two-fold. In contrast, the vitreous exhibits a two- to three-fold higher level of smaller (28-24 kDa), yet unidentified, IGFBPs. Aqueal and vitreal IGFBP patterns are also different from those found in serum. IGFBP-2 found in the aqueous and vitreous may be synthesized by ciliary body and/or cornea since these structures contain high levels of IGFBP-2 mRNA. Lens epithelial cells may also contribute IGFBP-2 to the aqueous since they also contain IGFBP-2 mRNA, albeit at substantially lower levels than the cornea and ciliary body. The retina has the lowest level of IGFBP-2 mRNA. IGF-II binding assays of cornea, ciliary body, retina and retinal pigment epithelium (RPE) indicate that the cornea has the highest level of binding (mean +/- S.E.M. IGF-II B/F per 50 micrograms protein: cornea = 84.52 +/- 28.8; iris/ciliary body complex = 0.61 +/- 0.078; retina = 0.47 +/- 0.096; RPE = 0.069 +/- 0.019). IGF-II ligand blots confirm these tissue-specific differences in binding and show that each ocular tissue contains IGFBP-2. In addition, ligand blots indicate that each ocular tissue contains a complex and distinctive population of IGFBPs. For example, the cornea and retina (but not the ciliary body, aqueous or vitreous) contain a 46 kDa IGFBP that may be IGFBP-3. The finding that cornea and retina also contain IGFBP-3 mRNA suggests that these structures may synthesize IGFBP-3 for local use within the eye.