Up-regulation of fibroblast growth factor receptor-1 in lens epithelial cells paralleled by growth stimulation

A study of the proliferating activity in lens epithelium and the identification of tissue-type stem cells

Using a newly established fixation method and immunohistochemical methods, we precisely described the regions of cells stained with various antibodies relating to cell proliferation; this method enabled us to make cellular-level diagrams of the epithelium in which the position of every lens epithelial cell (LEC) was determined in reference to the cell located at the top of the bow area. The proliferating activity of LECs of 4-week-old (4W) mice was examined either by labeling with 5-bromodeoxyuridine (BrdU) in vivo or by measuring the amount of mRNA prepared from LECs, which had been separated into the posterior part, containing the germinative zone, and the anterior part and then cultured. The epithelial region stained with antibody for proliferating cell nuclear antigen (PCNA) and cyclin D1 remained relatively constant during the study period, although the positive region was reduced a little from embryonic day 18 (E18) to 12W. This region at 4W overlapped well with the DNA synthesizing region. Therefore, we reasoned that this region would correspond to the germinative zone of the adult mouse. Considering together with results of the reactivation pattern of genes, we considered that the location of tissue-type stem cells in lens epithelium (LE) as immediately anterior to the germinative zone.

Growth factor receptor gene and protein expressions in the human lens

In this study the mRNAs encoding epidermal growth factor receptor (EGFR), basic fibroblast growth factor receptor (FGFR-2) and insulin-like growth factor receptor (IGFR-1) genes of the human normal lenses at ages varying from 0.5 to 72 years, were identified by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Regulation of EGFR gene expression in the lens did not change with aging, and of FGFR-2 and IGFR-1 genes also remained unaltered up to age 53 years. However, expressions of FGFR-2 and IGFR-1 genes were decreased at ages above 60 years. EGFR, FGFR-2 and IGFR-1 proteins were detected by immunoblot analysis in the epithelial cell membranes of lens at age varying from 40 to 72 years. There was no detectable amount of EGFR protein in fiber cell membranes of the lens, and the levels of FGFR-2 and IGFR-1 proteins were much lower than those in the epithelial cell membranes. The low levels of these receptor proteins in the fiber cell membranes of lens, suggest their possible role in keeping the differentiated function of these unique transparent cells. The findings of the increased protein levels with age of EGFR with the appearance of some degradation products at age 48 years and higher, and the increased FGFR-2 protein at age 60 years and higher in the epithelial cell membranes of lens, were of interest. It appears that this could be a compensatory protective response of the lens to aging process for lifelong continuation of normal growth by proliferation and differentiation of its epithelial cells into new fiber cells in the germinative zone at the equatorial region. Thus, these results could provide a basis for further studies on growth factor receptor gene and protein regulations in the mechanism of lens aging and progression of age-related human cataract.