Cell membrane associated protein kinase C as receptor of diterpene ester co-carcinogens of the tumor promoter type and the phenotypic expression of tumors.

For investigations of biochemical and molecular mechanism(s) involved in carcinogenesis the three stage initiation/promotion/progression approach in mouse skin provides one of the most developed experimental models. As initiators (and as progressors), solitary carcinogenic polycyclic aromatic hydrocarbons (PAH), such as 7,12-dimethylbenz [a]anthracene (DMBA), are used. As promoters in the last about 15 years, cocarcinogenic skin irritant polyfunctional diterpene esters of plant origin have become most powerful tools. Their low activity "cryptic forms" are activated metabolically by hydrolases yielding highly active skin irritants and cocarcinogens of the "ultimate promoter" type such as the phorbol-12,13-diester TPA and congeners of the ingenane- and daphnane type. In their target cells the ultimate promoters are bound to specific binding sites both through their ester moieties as well as through their diterpene moieties as revealed by structure/activity relationships. In such investigations most recently, also fine structural comparison of threedimensional computergraphs of prototype promoters were included. In correlation with their irritant and promoting activities, diterpene ester promoters exhibit agonist type specific (non-covalent) binding to the particulate fraction of epidermis and other organs or cells. The specifically bound portion of certain electron spin and photoaffinity labeled phorbol ester analogs indicates their distinct molecular orientation in membranes and, in their microenvironment, presence of phospholipids essential for activity of the Ca++ dependent proteinkinase C (PKC). Specific bindings to and activation of PKC was shown for TPA and congeners particularly also in mouse epidermis as one of the target organs of initiation/promotion by DMBA/TPA. Diterpene ester promoters are postulated to interact with the second messenger system operated by inositolphospholipid/diacylglycerol which is linked to (i) receptors of certain (endocrine) hormones or growth factors and (ii) to certain oncogene derived (autocrine) molecular signals. In this way, diterpene ester promoters, mostly taylor-made by partial syntheses, have lead to new dimensions the investigations of specific mechanistic problems of processes of carcinogenesis. Interlinkage of parts of these processes with receptor research opens up new and fascinating aspects of mechanisms of carcinogenesis at the cell and/or molecular level. They will provide important leads in gaining a more differentiated system of assessment of environmental risk factors of cancer for cancer prevention and in developing more purposeful and selective antineoplastic drugs for clinical use.