A tumor promoter-resistant subpopulation of progenitor cells is larger in limbal epithelium than in corneal epithelium.

In the epidermis, proliferative basal cells can be divided into two subpopulations according to their response to phorbol ester tumor promoters. The tumor promoter-sensitive subpopulation ceases mitosis and initiates terminal differentiation and, thus, represents more differentiated transient amplifying cells. In contrast, the tumor promoter-resistant subpopulation that continues to proliferate may be the target of neoplastic transformation by chemical carcinogens and may contain stem cells. Based on this concept, we examined the differential response of stem cell-containing limbal epithelium and transient amplifying cell-containing corneal epithelium to phorbol 12-myristate 13-acetate (PMA) treatment. A reported serum-free clonal growth assay was used. The mitogenic response was measured by colony-forming efficiency (CFE), colony size, bromodeoxyuridine (BrdU) labeling index; the differentiation was assessed by colony morphology, AE-5 monoclonal antibody staining. The addition of PMA dose dependently inhibited the clonal proliferation of both limbal and corneal epithelial cultures with respect to CFE, colony size, and BrdU labeling index, suggesting that both cultures contain PMA-sensitive subpopulations. Nevertheless, the magnitudes of a decrease in CFE and colony size in peripheral corneal cultures were significantly greater than those in limbal cultures, indicating that the size of the PMA-resistant subpopulation is larger in the limbal epithelium. The inhibitory effect of PMA on clonal proliferation was partially reversible upon its early withdrawal, indicating that its inhibitory effect is continuous and coupled with progressive differentiation of progenitor cells in this culture system. These results further suggest that the cell cycle length of progenitor cells correlates with the mitogenic pathway mediated via calcium- and phospholipid-dependent protein kinase C, the receptor inhibited by prolonged treatment of phorbol ester tumor promoters.