Kallistatin in human ocular tissues: reduced levels in vitreous fluids from patients with diabetic retinopathy.

PURPOSE: Kallistatin is a serine proteinase inhibitor, which binds to tissue kallikrein and inhibits its proteolytic activity. This study is to determine the expression, cellular localization and the potential function of kallistatin in the eye.
METHODS: Tissue kallikrein-kallistatin complex formation was performed to detect the kallikrein-binding activity in ocular tissues. Immunoreactive kallistatin was detected and quantified by an enzyme-linked immunosorbent assay using polyclonal antibody specific to human kallistatin. In situ hybridization histochemistry was employed to localize the kallistatin mRNA in human eyes using an antisense riboprobe of kallistatin.
RESULTS: We have identified active kallistatin in the cornea, ciliary body, sclera, choroid, optic nerve, retina, vitreous and aqeous fluids. Kallistatin binds to tissue kallikrein and forms an SDS-stable complex. Immunoreactive kallistatin was identified in these tissues. Linear dose-dependent curves of the tissue extracts of the retina and choroid are parallel to that of purified human kallistatin, suggesting their immunological identity. The kallistatin mRNA was identified in the ciliary muscle, lens epithelial cells, all the layers of retina cells, optic nerve, choroid and vascular endothelial cells. These cells were not stained by the sense riboprobe under the same conditions, indicating the specificity of the hybridization. We also compared immunoreactive kallistatin levels in vitreous fluids from 18 patients with diabetic retinopathy and 17 non-diabetic subjects. The results show that diabetic subjects have significantly lower kallistatin levels (233.0 +/- 14.6 ng/mg protein) compared to non-diabetic subjects (334.1 +/- 26.9 ng/mg protein).
CONCLUSIONS: Kallistatin is produced endogenously in the eye and the decrease in the vitreous kallistatin levels may be involved in diabetic retinopathy.