A TUMOR PROMOTER-RESISTANT SUBPOPULATION OF PROGENITOR CELLS IS LARGER IN LIMBAL EPITHELIUM THAN IN CORNEAL EPITHELIUM

Purpose. In the epidermis, proliferative basal cells can be divided in to two subpopulations according to their response to phorbol ester tum or promoters. The tumor promoter-sensitive subpopulation ceases mitosi s and initiates terminal differentiation and, thus, represents more di fferentiated transient amplifying cells. In contrast, the tumor promot er-resistant subpopulation that continues to proliferate may be the ta rget of neoplastic transformation by chemical carcinogens and may cont ain stem cells. Based on this concept, we examined the differential re sponse of stem cell-containing limbal epithelium and transient amplify ing cell-containing corneal epithelium to phorbol 12-myristate 13-acet ate (PMA) treatment. Methods. A reported serum-free clonal growth assa y was used. The mitogenic response was measured by colony-forming effi ciency (CFE), colony size, bromodeoxyuridine (BrdU) labeling index; th e differentiation was assessed by colony morphology, AE-5 monoclonal a ntibody staining. Results. The addition of PMA dose dependently inhibi ted the clonal proliferation of both limbal and corneal epithelial cul tures with respect to CFE, colony size, and BrdU labeling index, sugge sting that both cultures contain PMA-sensitive subpopulations. Neverth eless, the magnitudes of a decrease in CFE and colony size in peripher al corneal cultures were significantly greater than those in limbal cu ltures, indicating that the size of the PMA-resistant subpopulation is larger in the limbal epithelium. The inhibitory effect of PMA on clon al proliferation was partially reversible upon its early withdrawal, i ndicating that its inhibitory effect is continuous and coupled with pr ogressive differentiation of progenitor cells in this culture system. Conclusion. These results further suggest that the cell cycle length o f progenitor cells correlates with the mitogenic pathway mediated via calcium- and phospholipid-dependent protein kinase C, the receptor inh ibited by prolonged treatment of phorbol ester tumor promoters.