Local synthesis and developmental regulation of avian vitreal insulin-like growth factor-binding proteins: a model for independent regulation in extravascular and vascular compartments.

Abstract

The expression and regulation of insulin-like growth factor-binding proteins (IGFBPs) in developing avian vitreous humor and serum were compared. Vitreal IGF-I-binding activity was highest on embryonic day 6 [E-6; bound/free ratio (B/F), 0.22 +/- 0.019/50 microliters), decreased 10-fold between E-6 and E-19, and then remained stable through the remainder of embryonic development. In contrast, serum IGF-I binding increased 2-fold over this period, from a B/F of 0.380 +/- 0.056 (E-6) to a B/F of 0.89 +/- 0.18 (E-19). After hatching, serum IGF-I-binding activity continued to increase through posthatching week 12, while vitreal IGF-I binding increased only slightly and then remained constant. Although IGF-II binding in the vitreous humor and serum is 2- to 3-fold higher than that of IGF-I, the same pattern of developmental regulation was observed as with IGF-I. Western ligand blots revealed a vitreal 24-kilodalton (kDa) IGFBP that was absent from both embryonic and adult sera. Likewise, posthatching serum was found to contain a 70-kDa IGFBP absent in vitreous humor. Deglycosylation of vitreal and serum IGFBPs followed by Western ligand blotting revealed unique glycosylation patterns for vitreal and serum IGFBPs. One of the IGFBPs that is differentially glycosylated in vitreous and serum is a 33-kDa IGFBP that is precipitated with human IGFBP-2 antiserum. Northern blot analysis revealed the presence of IGFBP-2 mRNA in several embryonic ocular tissues as well as liver. The observations that vitreal and serum IGFBP levels are independently regulated during development and that IGFBPs from these two compartments have different molecular weights and glycosylation patterns suggest that the vitreal IGFBPs are not derived from serum. The presence of IGFBP-2 mRNA in ocular tissue surrounding the vitreal chamber supports the view that certain vitreal IGFBPs may be synthesized locally.