Action of phorbol esters in cell culture: Mimicry of transformation, altered differentiation, and effects on cell membranes

Abstract

The carcinogenic process is usually multifactor in its causation and multistep in its evolution. It is likely that entirely different molecular mechanisms underlie the many steps in this process. In contrast to initiating carcinogens, the action of the tumor-promoting phorbol esters does not appear to involve covalent binding to cellular DNA and they are not mutagenic. Recent studies in cell culture have revealed two interesting biologic effects of the phorbol esters and related macrocyclic plant diterpenes. The first is that at nanomolar concentrations they induce several changes that resemble those seen in cells transformed by chemical carcinogens or tumor viruses. These include altered morphology and increased saturation density, altered cell surface fucose-glycopeptides, decrease in the LETS protein, increased transport of deoxyglucose, and increased levels of plasminogen activator and ornithine decarboxylase. In transformed cells exposed to phorbol esters the expression of these features is further accentuated. Phorbol esters do not induce normal cells to grow in agar but they do enhance the growth in agar of certain transformed cells. The second effect of the phorbol esters is inhibition of terminal differentiation. This effect extends to a variety of programs of differentiation and is reversible when the agent is removed. With certain cell culture systems induction of differentiation, rather than inhibition, is observed. Both the transformation mimetic and the differentiation effects are exerted by plant diterpenes that have tumor-promoting activity but not by congeners that lack such activity. The primary target of phorbol esters appears to be the cell membrane. Early membrane-related effects include enhanced uptake of 2-deoxyglucose and other nutrients, altered cell adhesion, induction of arachidonic acid release and prostaglandin synthesis, inhibition of the binding of epidermal growth factor to cell surface receptors, altered lipid metabolism, and modifications in the activities of other cell surface receptors. A model of "two stage" carcinogenesis encompassing the known molecular and cellular effects of initiating carcinogens and tumor promoters is presented. According to this model, initiating carcinogens induce stable alterations in the cellular genome but these are not manifested until tumor promoters modulate programs of gene expression and induce the clonal outgrowth of the initiated cell.

Flavonoids from Citrus unshiu Marc. inhibit cancer cell adhesion to endothelial cells by selective inhibition of VCAM-1

“…In this selective suppression of VCAM-1 expression, Akt and PKC are involved (17), and our result showed that FCM inhibit VCAM-1 expression by TNF through regulation of PKC pathway. PKC pathways is well-known to be involved in cancer cell mobility (11). In the process of cancer invasion, cell migration is also required to pass though the basement membrane.…”

“…Activation of PKC induces rapid changes in cell morphology, cell-cell adhesion, and cell migration (11). Therefore, we also performed Matrigel invasion assays to assess the effects of FCM on cancer cell invasion.…”

The NF-kappa B-binding site mediates phorbol ester-inducible transcription in nonlymphoid cells.

“…Tumor-promoting phorbol esters cause a wide variety of effects (4,32,36), presumably by binding to and activating cellular protein kinase C. Some of these effects are probably due to the differential transcriptional regulation of endogenous genes. For example, the rat prolactin gene (8,34), the human metallothioenin and collagenase genes (16), and the proenkephalin gene (7) are strongly stimulated by the action of phorbol ester.…”

Increased protein kinase Cδ in mammary tumor cells: relationship to transformation and metastatic progression

“…Protein kinase C (PKC) is thought to play a major role in tumor promotion/progression in other tissues due to the fact that PKC is the major cellular receptor for tumor promoting phorbol esters (reviewed in Nishizuka, 1995;Weinstein et al, 1997). In cultured cells, phorbol esters rapidly induce changes in cell morphology, cell ± cell communication and membrane ru ing Weinstein et al, 1979), indicating that PKC has a primary e ect on the organization of cytoskeletal structures. Over a longer time course, phorbol esters also in¯uence cell growth, transformation and gene expression.…”