Correlation between tissue growth kinetics and modulation of mouse skin tumorigenesis by phorbol esters.

Previous studies on the influence of phorbol esters on mouse skin tumorigenesis have shown that 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances development of malignant epithelial and mesenchymal skin tumors by a completely carcinogenic dose of 3-methylcholanthrene (MCA), while its congener phorbol-12, 13-diacetate (PDA) exerts an inhibitory effect. Differential effects of these two agents were analysed by histology, morphometry and cell kinetic techniques including autoradiography and estimation of labelled precursor incorporation into DNA by liquid scintillation counting. Epidermal hyperplasia induced on exposure of S/RV Cri mouse skin to a single or multiple TPA application after MCA injection was associated with a significant increase in the thickness of nucleated cell layers, stratum granulosum, number of suprabasal cells and dark basal cells. Enhancing effect of TPA on MCA-induced neoplastic development correlated well with an increase in mitotic activity, number of cells in S-phase and increased rate of DNA synthesis in the epidermis, dermis and subcutis as also mast cell number. In contrast, treatment of MCA-injected preneoplastic mouse skin with PDA resulted in epidermal hypoplasia and cellular damage evident as cytoplasmic vacuolation and nuclear pyknosis. Multiple PDA exposure also reduced the thickness, mitotic index and number of cells in S-phase in epidermis, dermis and subcutis. Thus, cellular toxicity and inability to recruit cells in DNA-synthetic phase may account for inhibition of progression of preneoplastic epithelial and mesenchymal cells into overt tumors by PDA.     

Distinct effects of phorbol esters and exogenous diacylglycerols in the induction of murine thymocyte proliferation.

Murine thymocytes were stimulated with the protein kinase C activating agents Phorbol-12-myristate-13-acetate (PMA) or a more physiological membrane permeant diacylglycerol (dioctanoyl-sn-glycerol, DiC8) in the presence or absence of exogenous lymphokines (rIL-1 beta, rIL-2). Whereas PMA directly induced reactivity to rIL-2, DiC8 did not but had to synergize with the calcium ionophore Ionomycin. Expression of the p55 chain of the IL-2 receptor behaved similarly. In the absence of exogenous rIL-2, thymocytes proliferated in response to a combination of Ionomycin and PMA; however, replacing PMA by a single addition of DiC8 did not result in proliferation. Stimulation with Ionomycin plus repeated addition of DiC8 induced a low level of thymocyte proliferation and further addition of rIL-1 beta resulted in a significant increase. Purified immature (L3T4-Lyt2-) thymocytes behaved similarly, but showed an increased sensitivity to rIL-1 beta. Taken together, the data support the idea that PMA and the more physiological diacylglycerols do not possess totally equivalent activities in lymphocyte stimulation.     

Inhibition of the mixed lymphocyte proliferative response by phorbol esters.

The phorbol diester, 12-O-tetradecanoyl-phorbol-13-acetate, a potent cocarcinogen in mice, blocks the induction of DNA synthesis in lymphocytes undergoing the mixed lymphocyte response. At 10(-7) M diester, the induced DNA synthesis is inhibited almost completely (99%). This action of the diester affects some early step in the response which is necessary for the triggering of cell replication; on-going DNA replication is not significantly affected. Phorbol 12,13-diacetate, a less potent analogue in tumor promotion in vivo, is also a less potent inhibitor of the mixed lymphocyte response (75% inhibition at 10(-6) M). Phorbol, the parent alcohol, is not effective in either system. The use of phorbol diesters in the molecular dissection of mixed lymphocyte responses is discussed.     

Prostaglandin E and F levels in mouse epidermis are increased by tumor-promoting phorbol esters

After topical application of tumor-promoting phorbol esters, immunoreactive prostaglandins E and F in mouse epidermis were increased several-fold over basal levels. The increases were doubled by 3 hours and lasted until 5 days after phorbol ester treatment. The activities of various phorbol esters for increasing epidermal prostaglandin levels paralleled the tumorpromoting activities of these compounds. Topical pretreatment with nonsteroidal anti-inflammatory drugs inhibited the effect of phorbol esters on epidermal prostaglandin levels.     

Inhibition of denuded mouse oocyte meiotic maturation by tumor-promoting phorbol esters and its reversal by retinoids

Two tumor-promoting phorbol esters, phorbol 12,13-dibutyrate (PDBu) and phorbol 12-myristate 13-acetate (PMA), when added to the culture medium of denuded mouse oocytes prevent their spontaneous meiotic maturation, whereas phorbol 13-acetate, which is inactive as a tumor promoter, does not inhibit this process. Retinoids appear to antagonize this inhibitory action of tumor promoters. However, the inhibitory effect of forskolin on meiotic maturation is not prevented, but is potentiated by retinal. These data indirectly suggest a role for calcium and/or phospholipids in the regulation of meiotic maturation. They also suggest that forskolin and phorbol esters mediate their effects through different pathways.     

Stimulation of DNA synthesis in murine fibroblasts by the tumour promoter teleocidin: relationship to phorbol esters and vasopressin

The recently reported tumour promoter teleocidin is a potent mitogen for murine fibroblasts, at nM concentrations. Despite its unrelated structure, teleocidin inhibits binding of phorbol dibutyrate to Swiss 3T3 cells, and is inactive as a mitogen for cells in which the phorbol dibutyrate receptors have been down-regulated. Teleocidin shows synergistic stimulation of DNA synthesis with a wide range of purified growth factors, but fails to synergise with vasopressin, suggesting that it utilises the same mitogenic pathway as this physiological ligand.