Joyce N C, Zieske J D
Schepens Eye Research Institute and the Department of Ophthalmology, Harvard Medical School, Boston, Massachusetts 02114, USA.
Invest Ophthalmol Vis Sci. 1997 Sep;38(10):1922-8.
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).
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.
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.
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.
角膜缘基底细胞和角膜内皮细胞似乎在细胞周期的G1期受到抑制。作为确定转化生长因子-β（TGF-β）是否可能介导这种抑制作用的初步研究，进行了调查以确定人角膜和角膜缘细胞是否表达I型（RI）、II型（RII）和III型（RIII）TGF-β受体。
取自8名年龄从死产儿到85岁的人类供体的角膜进行新鲜冷冻，用低温恒温器切片，并按照既定方案制备用于RI、RII和RIII间接免疫荧光定位的样本。使用来自50岁及以上供体的角膜制备上皮和内皮的RNA。使用针对每种TGF-β受体类型的特异性引物进行逆转录-聚合酶链反应。
无论供体年龄如何，RI、RII和RIII的免疫定位模式相似。在中央角膜上皮中几乎检测不到RI和RII抗体的结合；然而，大多数角膜缘基底细胞RI和RII染色呈阳性。中央角膜上皮的所有层和角膜缘的基底上层RIII染色呈阳性，而角膜缘基底细胞中该受体的染色明显减少。角膜内皮与所有三种TGF-β受体类型的抗体结合。在同一组织切片中，角膜内皮细胞中RIII蛋白的抗体染色比角膜缘基底细胞中更强烈。在角膜上皮和内皮中检测到RI、RII和RIII的聚合酶链反应产物。
角膜缘基底细胞和角膜内皮细胞表达I型、II型和III型TGF-β受体的mRNA和蛋白质，表明这两种细胞类型都可以传递TGF-β诱导的信号。这两种细胞类型对那些需要与RIII结合进行信号转导的TGF-β亚型的相对反应可能不同，因为与内皮相比，角膜缘基底细胞中RIII的染色强度相对较低。角膜缘基底细胞和角膜内皮细胞表达TGF-β受体表明这种细胞因子可能介导这两种细胞类型的G1期停滞。
