Purpose: Limbal basal cells and corneal endothelial cells appear to be inhibited in the G1 phase of the cell cycle. As a preliminary to determining whether transforming growth factor-beta (TGF-beta) might mediate this inhibition, investigation was made to determine whether human corneal and limbal cells express TGF-beta receptor types I (RI), II (RII), and III (RIII).
Methods: Corneas from eight human donors, aged stillborn to 85 years, were fresh frozen, cryostat sectioned, and prepared for indirect immunofluorescence localization of RI, RII, and RIII, using an established protocol. Corneas from donors 50 years of age or older were used to prepare RNA from the epithelium and endothelium. Reverse transcription-polymerase chain reaction was conducted using primers specific for each TGF-beta receptor type.
Results: Immunolocalization patterns for RI, RII, and RIII were similar, regardless of donor age. Binding of RI and RII antibodies was barely detectable in central corneal epithelium; however, most limbal basal cells stained positively for RI and RII. All layers of central corneal epithelium and the suprabasal layers of the limbus stained positively for RIII, whereas staining for this receptor was markedly decreased in limbal basal cells. Corneal endothelium bound the antibody for all three TGF-beta receptor types. In the same tissue sections, antibody staining for the RIII protein was more intense in corneal endothelial cells than in limbal basal cells. Polymerase chain reaction product for RI, RII, and RIII was detected in the epithelium and in the endothelium.
Conclusions: Limbal basal cells and corneal endothelial cells expressed mRNA and protein for TGF-beta receptor types I, II, and III, suggesting that both cell types can transmit a TGF-beta-induced signal. These two cell types may differ in their relative response to those TGF-beta isoforms that require binding to RIII for signal transduction, in that staining intensity for RIII was relatively low in limbal basal cells compared with that in the endothelium. That limbal basal and corneal endothelial cells express receptors for TGF-beta suggests that this cytokine could mediate G1 phase arrest in these two cell types.