Activity of diverse tumor promoters in a keratinocyte co-culture model of initiated epidermis.

The suppression of focal growth of initiated mouse keratinocytes by co-culture with normal keratinocytes can be overcome by treatment of co-cultures with the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). This keratinocyte co-culture model was developed as an analog of initiated mouse epidermis to facilitate the study of tumor promotion in cell culture. A number of promoters of TPA-type [those with protein kinase C (PKC) as receptor] were compared to non-TPA-type promoters for activity in the keratinocyte co-culture model. The TPA-type promoters teleocidin and aplysiatoxin show comparable activity to that of TPA. Exposure of co-cultures to oleoyl-2-acetylglycerol, a diacylglycerol activator of PKC, also induces focal outgrowth of initiated cells, suggesting that PKC is likely to be involved in the mechanism of action of these compounds. However, the involvement of alternative pathways (not involving PKC directly) for clonal selection are evident since the non-TPA-type promoters okadaic acid, staurosporine and thapsigargin are also active in the assay. Thus, the keratinocyte co-culture model differs from fibroblast models of normal and neoplastic co-cultures in which only TPA-type promoters are active. In further contrast to certain fibroblast co-cultures, TPA does not inhibit cell-cell communication under conditions that suppress focus formation. Taken together with previous data, we conclude that the keratinocyte co-culture model may have broad application for detecting skin tumor promoters, and may be useful for dissecting the mechanism by which normal epidermal cells suppress the growth of initiated cells.