Alkylformamides as inducers of tumour cell differentiation--a mini-review

Cytoskeleton-dependent surface blebbing induced by the polar solvent N-methylformamide

In vivo and in vitro studies performed on the polar solvent N-methylformamide (NMF), as well as on its association with chemotherapeutic agents or X rays, have clearly demonstrated that this compound is capable of inducing changes in biological characteristics of tumor cells, e.g., cell differentiation. However, the mechanism of action of NMF is far from being elucidated. Hence, in order to better clarify such a mechanism an in vitro study was carried out by using mouse fibroblasts in primary culture (MEF) and human melanoma cultured cells (M14). Results obtained by immunocytochemical and ultrastructural methods with doses of NMF ranging from 0.1 to 7% are reported here. As a general rule, a different sensitivity (in terms of cytopathologic changes induced by NMF) was found between the cell types considered. In fact, melanoma cells appeared to be highly susceptible to the action of the drug, undergoing severe morphological modifications represented mainly by a reversible dose and time-dependent cell rounding and surface blebbing. In contrast, NMF-induced injury in MEF cells was characterized mainly by a simple retraction of the cell body. A cytochemical analysis of the expression of certain membrane antigens (e.g., glycoproteins, epidermal growth factor receptor, B2 microglobulin) in NMF-treated M14 cells undergoing blebbing was also carried out. A randomly distributed labeling of such molecules was observed. Accordingly, freeze-fracturing electron microscopic analysis also displayed a random distribution of intramembrane particles over the plasma membrane. When subcellular changes induced by the drug were investigated, a remarkable modification of cytoskeletal components was detected in both cell types. In particular, cross-linked actin microfilament bundles were easily observed in NMF-exposed MEF cells. Finally, when different experimental conditions which perturb calcium ion homeostasis or restore protein thiol group reduced state were analyzed, a noticeable impairment of the blebbing phenomenon was observed. Thus, a target effect of NMF on the microfilament system, probably leading, in turn, to several subcellular changes and cell surface blebbing, can be hypothesized. Such a cytoskeletal element-dependent cytopathology appears to be related to changes of the oxidized state of such molecules as well as to calcium ion perturbations.

Alteration of lacto-series glycolipid glycosyltransferase activities in human colonic adenocarcinoma DLD-1 cells after culture in N,N-dimethylformamide-containing medium

Human colonic adenocarcinoma DLD-1 cells were grown under conditions which induce characteristics of differentiated cells using medium containing 0.8% N,N-dimethylformamide in order to study alterations in glycosphingolipid glycosyltransferase activities during this process. Analysis of biosynthetic reactions involved in lacto-series antigen synthesis revealed no changes in the specific activities of either beta 1----4galactosyltransferase or alpha 1----3/4fucosyltransferase with N,N-dimethylformamide treatment. However, a dramatic decrease of from 14- to 20-fold in the beta 1----3N-acetylglucosaminyltransferase activity was observed in the treated cells. This enzyme catalyzes the rate-limiting step in lacto-series core chain synthesis. This is consistent with the pattern of regulation of lacto-series antigen expression found to occur during oncogenesis in human colonic mucosa (Holmes EH, Hakomori S, Ostrander GK: J Biol Chem 262:15649, 1987). Total glycolipids from untreated and N,N-dimethylformamide-treated cells were isolated and subjected to TLC immunostain analysis and solid phase radioimmunoassay with a series of monoclonal antibodies specific for lacto-series-based carbohydrate antigens. A decrease of about 2-fold or less in the quantity of lacto-series antigens was observed as a consequence of N,N-dimethylformamide treatment in both neutral glycolipid and ganglioside fractions. The results suggest that only very low levels of beta 1----3N-acetylglucosaminyltransferase activity are required for the steady state expression of significant levels of lacto-series based glycolipids and that modulation of its activity levels by N,N-dimethylformamide treatment in DLD-1 cells represents a convenient in vitro system for studying aspects of regulation of lacto-series antigen expression.

Synergism between 5-fluorouracil and N-methylformamide in HT29 human colon cancer line

In HT29 cells 5-fluorouracil (5FU) cytotoxicity is enhanced by subsequent incubation of cells in medium containing 1% N-methylformamide (NMF). This enhancement does not appear to be related to differences in the repair of 5FU-induced DNA damage. It is proposed that the inhibition of DNA synthesis by NMF (that is reversible and does not result in any detectable toxicity) becomes a lethal event in a cell in which DNA synthesis has already been altered by 5FU exposure. The synergism is sequence dependent (i.e. it does not occur when NMF is given before 5FU) and specific for some cell types as shown by the fact that no synergism was found in L1210 mouse leukaemia cells. In nude mice transplanted s.c. with HT29 cells daily 5FU treatment (for 5 days) followed by daily NMF treatment (for 10 days) caused much greater inhibition of tumour growth than either drug alone or the same combination given in the opposite order (NMF then 5FU). These results, if confirmed on other human colon tumours, could be of clinical interest as a means of increasing the therapeutic efficacy of 5FU in patients with colon cancer.

Cytoskeletal changes induced in vitro by 2,5-hexanedione: an immunocytochemical study

The effects of 2,5-hexanedione, the main metabolite of the solvents hexane and methyl butyl ketone, have been explored in different in vitro epithelial (CG5 and HEp-2) and melanoma (JR8) cells by means of immunochemistry and electron microscopy. The administration of the toxicant to the cell monolayers at noncytolytic concentrations for 24 and 48 hr exerted several effects on the cell lines studied. Most epithelial and melanoma cells detached from the substrate were in the mitotic phase, whereas cells adhering to the substrate showed time-dependent organelle changes. In fact, after treatment with 2,5-hexanedione, mitochondria appeared swollen, with distorted cristae and rarefied matrix; changes in intracytoplasmic vesicles were also detected. Cytoskeletal components were also investigated. A remarkable rearrangement of microfilaments and intermediate filaments (keratin and vimentin) was detected in a time-dependent manner. In particular, actin ruffles and intermediate filament aggregates were observed. Furthermore, the microtubular apparatus seemed to be less affected. The results here reported seem to indicate cytoskeletal components as probable targets of 2,5-hexanedione cytotoxicity in cultured cells.

Cytotoxicity and metabolism of the hepatotoxin N-methylformamide and related formamides in mouse hepatocytes

Some N-alkylformamides such as N-methylformamide (NMF) possess hepatotoxic properties in vivo. To study the mechanism of this toxicity, suspensions of mouse hepatocytes were tested as an in vitro model system suitable for the study of the relationship between (i) the toxic potential of formamides, (ii) their metabolism to N-alkylcarbamoylating species, and (iii) their ability to deplete hepatic glutathione pools. The effects of NMF were compared with those of its analogs N-ethylformamide (NEF), N,N-dimethylformamide (DMF), formamide (F), N-methylacetamide (NMA), and N-methyldeuteroformamide ([2H]NMF). Only NEF and [2H]NMF share with NMF the ability to cause liver damage in vivo in mice. Hepatocellular toxicity was determined by measuring LDH leakage into the extracellular medium; metabolism to N-alkylcarbamoylating species was measured by GLC after derivatization with propanol to form propyl N-alkylcarbamate; glutathione concentrations were determined spectrophotometrically. Of the formamide analogs studied, only NMF and NEF caused cytotoxicity, being apparently equipotent. NMF, NEF, and [2H]NMF gave rise to the formation of detectable levels of N-alkylcarbamoylating metabolites and depleted glutathione pools. Toxicity, metabolism, and glutathione depletion were dependent on NMF concentration. [2H]NMF was markedly less cytotoxic than NMF, yielding only 35% of the amount of N-methylcarbamoylating metabolite compared to NMF and caused less depletion of glutathione than did NMF. These results parallel closely the in vivo hepatotoxic potential of NMF and its analogs, their metabolism to urinary S-(N-alkylcarbamoyl)mercapturates and their ability to deplete hepatic glutathione in mice. The results provide support for the contention that metabolism is involved with formamide-induced hepatotoxicity and suggest that suspensions of isolated mouse hepatocytes are an appropriate in vitro model for the further study of the mechanism by which formamides cause toxicity.

Investigations of the relationship between cell proliferation and differentiation of HL-60 cells induced to differentiate by N-methylformamide

The relationship between the N-methylformamide (NMF)-induced differentiation of HL-60 human promyelocytic leukemia cells, to myeloid cells, and their proliferative potential was determined in suspension culture. Continuous incubation with 170 mM NMF induced 70% of cells to differentiate after 2-2.5 cell divisions, whether or not cells were replenished with fresh medium and serum. 100 mM NMF induced significant levels of differentiation only when cells were allowed to reach a plateau of growth, and this was prevented by replenishment of the medium and serum. Under these conditions of "re-feeding", the cells could be made to undergo greater than 10 cell divisions, and differentiated only when they were allowed to reach a plateau in their growth. The data suggests that NMF may induce differentiation only when the proliferative potential of the cells is limited, either by the drug itself or by the conditions of growth.

In vitro effects of styrene on cytoskeletal apparatus: an immunocytochemical study

Styrene is known as an organic solvent implicated in neurological disorders occurring in exposed workers. Our studies were focused on the effects of styrene on the cytoskeletal apparatus, involved in several toxic neuropathies. The cultured cells were considered as living systems useful to investigate the mechanisms of cytotoxicity. Preliminary results reported here were obtained on two different epithelial cell lines (CG5 and HEp-2) by immunocytochemical methods. Treatment with styrene at 0.04% and 0.08% for 24 and 48 hours, induced changes in cytoskeletal elements. In particular, styrene seemed to induce a decrease in number of cells adhering to the substrate and some alterations in microtubule assembly. Moreover, a rearrangement of the keratin filaments was observed while styrene did not seem to induce noticeable changes in actin filament network. Data obtained seem to confirm in vitro studies as a useful tool in toxicity assessment of xenobiotic compounds at subcellular levels.