Localization studies of IGFBP-2 and IGFBP-5 in the anterior compartment of the eye

Abstract

Purpose. Insulin-like growth factor binding proteins (IGFBPs) may modulate insulin-like growth factor-I (IGF-I) action and are important regulating factors in the IGF system. Our aim was to determine the presence of IGFBP-2 and -5 in the anterior compartment of the eye and to compare the histological sites of these IGFBP proteins with the respective IGFBP mRNAs. Methods. To investigate this, immunohistochemistry was used to detect the presence of IGFBP-2 and -5 proteins, and in situ hybridization was used to determine the presence of IGFBP-2 and -5 mRNAs. The studies were performed in normal adult male Sprague-Dawley rats. Immunohistochemistry was performed by the immunoperoxidase method with polyclonal anti-bovine-IGFBP-2 and antihuman-IGFBP-5 antibodies. In situ hybridization was performed using 35 S-radiolabelled riboprobes. Results. IGFBP-2 mRNA and protein were demonstrated in the outer non-pigmented ciliary epithelium, the corneal germinal epithelium and the corneal endothelium. IGFBP-2 mRNA was detected in these same histological layers of the ciliary processes and the cornea. IGFBP-5 mRNA localized to the stroma and also to the inner pigmented ciliary epithelium and IGFBP-5 protein was demonstrated in the adjacent outer non-pigmented ciliary epithelium. IGFBP-5 mRNA and protein were not demonstrated in the cornea. IGFBPs-2 and -5 were not demonstrated by immunohistochemistry in other structures of the eye. Conclusion. We have shown co-localization of IGFBP-2 mRNA and protein and adjacent cellular localization of IGFBP-5 mRNA and proteins in the anterior compartment of the eye. The presence of IGFBP-2 and -5 in the outer ciliary epithelium suggests secretion into the aqueous humour, where they may enhance trapping of IGF-I which may be important for lens and corneal cell survival. Our studies outlining the site of locally synthesized IGFBPs suggests specific roles in regulation of IGFs and highlights the potential importance of the IGF system in the eye.