Transferrin secretion by lens epithelial cells in culture

Transferrin (Tf), the plasma iron transport protein which supports cell proliferation and differentiation and has bacteriostatic, antioxidant and anti-inflammatory activity, has been found in relatively high concentrations in the intraocular fluids. Intraocular synthesis of Tf has recently been demonstrated, although the intraocular tissue(s) responsible have not been identified. We designed this study to determine whether certain ocular tissues can make and secrete transferrin. Transferrin content of aqueous and vitreous humors and whole lenses was determined by ELISA. Transferrin secretion by cultured epithelia from lens and ciliary body was also measured. In addition, Northern blots of RNA from cultured lens epithelial cells, ciliary body pigmented and non-pigmented epithelial cells, and from whole iris, ciliary body and retina were probed with riboprobes for Tf mRNA and 18S rRNA. Transferrin made up 23% and 16% of total canine aqueous and vitreous protein. All ocular tissues and cultured cells tested contained mRNA for Tf, however Tf was secreted into the bathing medium from lens epithelial cell cultures, but not from either the pigmented or non-pigmented epithelial cells of the ciliary body cultures, but not from either the pigmented or non-pigmented epithelial cells of the ciliary body Cycloheximide inhibited secretion of Tf from the lens epithelial cells. Lenses from inflamed eyes contained higher levels of Tf than their contralateral controls. This is the first experimental demonstration that an intraocular tissue can make and secrete Tf. Transferrin secretion by the lens may contribute significantly to the IOF content of this important intraocular protein.