Quantitative correlation between in vitro and in vivo activities of phorbol esters.

been pretreated with a single subthreshold dose of a carcin ogen (2, 6, 9, 44). If either treatment is omitted or if the order of treatments is reversed, no tumors result. On the other hand, delay of up to a year between exposure to the carcinogen and the subsequent exposure to the phorbob derivative do not prevent tumor appearance. The reversibil ity of the action of the phorbol esters and their lack of mutagenicity (33) and carcinogenicity (6) clearly distinguish these compounds from carcinogens. Understanding of their mechanism of action is thus of considerable interest and importance. Because mouse skin is a complex, heterogeneous organ, tissue culture systems provide a means for simplifying the study of phorbol ester action at the cellular level. The phorbol esters show diverse biological effects on many different types of cells in culture (for reviews, see Refs. 6 and 45; also see Ref. 39). Our laboratory has shown that PMA induces in CEF a series of phenotypic changes that strongly resemble changes induced in CEF by oncogenic transformation with RSV (see “Discussion―). We have hy pothesized that the ability to induce partial mimicry of the transformed phenotype may account for the tumor-promot ing effects of the phorbol esters in vivo (4). The CEF system has also been used to examine other aspects of the tumor promotion phenomenon. We found that compounds unre lated to phorbol which are both inflammatory agents and tumor promoters on mouse skin do not mimic the effects of PMA on CEF (12). These results suggested that the phorbol esters may have a unique mechanism of action. A crucial issue regarding the use of CEF or other in vitro systems for studying the mode of action of the phorbol esters is the relevance of these systems to the in vivo phenomena in the mouse. Detailed comparison of struc tune-activity relationships for phorbol derivatives in in vitro systems and in the mouse would thus be desirable. In this paper, we report the effects of various phorbol derivatives on surface LETSP levels and on DG transport in CEF. The activities that we found show excellent quantitative come lation with the inflammatory activities of the same corn pounds determined in the mouse ear assay by Hecker et a!. (20). These results argue for homologous targets for the phorbob esters in the 2 systems.