Spontaneous and 5-azacytidine-induced revertants of methionine-dependent tumor-derived and H-ras-1-transformed cells

Methionine Dependence of Cancer

Tumorigenesis is accompanied by the reprogramming of cellular metabolism. The shift from oxidative phosphorylation to predominantly glycolytic pathways to support rapid growth is well known and is often referred to as the Warburg effect. However, other metabolic changes and acquired needs that distinguish cancer cells from normal cells have also been discovered. The dependence of cancer cells on exogenous methionine is one of them and is known as methionine dependence or the Hoffman effect. This phenomenon describes the inability of cancer cells to proliferate when methionine is replaced with its metabolic precursor, homocysteine, while proliferation of non-tumor cells is unaffected by these conditions. Surprisingly, cancer cells can readily synthesize methionine from homocysteine, so their dependency on exogenous methionine reflects a general need for altered metabolic flux through pathways linked to methionine. In this review, an overview of the field will be provided and recent discoveries will be discussed.

Linkage of Methionine Dependence and Other Features of Malignancy

Cancer cells have an elevated methionine (MET) requirement compared to normal cells and are termed MET dependent. Cancer cells were isolated in MET-restricted (MR) medium that reverted from MET dependence to MET independence. Increased MET biosynthesis was not a prerequisite for reversion to MET independence, indicating that MET dependence was not due to reduced endogenous MET synthesis. MET-independent revertants of cancer cells concomitantly reverted for some of the other properties associated with malignancy: Of the 13 MET-independent revertants isolated 5 showed increased anchorage dependence as reflected by reduced cloning efficiencies in methylcellulose; 8 showed an increased serum requirement for optimal growth; 8 showed decreased cell density in medium containing high serum; and 3 altered their cell morphology significantly. Eight of the 13 revertants have increased chromosome numbers. Thus, by selecting for MET independence, it is possible to obtain heterogeneous reduced-malignancy revertants, indicating further a relationship between altered MET metabolism and other fundamental properties of oncogenic transformation.

Regulation of O6-methylguanine-DNA methyltransferase by methionine in human tumour cells

Methionine (MET)-dependent cell lines require MET to proliferate, and homocysteine (HCY) does not act as a substitute for this requirement. From six O6-methylguanine-DNA methyltransferase (MGMT)-efficient (mer+) cell lines tested, two medulloblastomas (Daoy and D-341) and a lung non-small-cell adenocarcinoma with metastatic potential (H-1623) were most sensitive to MET deprivation, while two glioblastomas (U-138, D-263) and a small-cell lung carcinoma H-1944 were moderately to weakly dependent. Regardless of the degree of MET dependence, all of these lines down-regulated their MGMT activity within 48-72 h of transfer from MET+HCY- to MET-HCY+ media, long before the eradication of the culture. Reduction of MGMT activity was due to a decline of both MGMT mRNA and protein levels. However, the reduction was not related to the methylation status of the MGMT promoter at the SmaI site or the HpaII sites in the body of the gene; such sites have been shown to be associated in MGMT regulation and in defining the mer phenotype. MET-dependent, mer+ tumour cells cultured in MET-HCY+ were more sensitive to BCNU (IC50 = 5-10 microM) than those cultured in MET+HCY-(IC50 = 45-90 microM), while MET-independent or mer- cell lines were unaffected. This indicates that reduction of MGMT, imposed by the absence of MET, renders mer+ tumour cells more susceptible to alkylating agents. The relatively selective suppression of MGMT activity in mer+ MET-dependent tumour cells, in combination with the inability of such cells to proliferate in the absence of MET, may lead to the development of more effective treatment strategies for mer+ MET-dependent tumours.

Methionine dependence of virus-infected cells

Mouse embryo cells nonproductively infected with human cytomegalovirus differed from noninfected cells by the impaired ability to grow in the medium containing homocysteine instead of methionine. Virus infection of mouse embryo cells grown in both kinds of media resulted in the increase of protein synthesis. In the infected cells grown on homocysteine this increase was followed by a quick decrease. The effects of homocysteine substitution could be abolished by the addition of low amounts of methionine (0.1 mM). Methionine uptake in the infected cells grown on homocysteine for 48 h was significantly higher than that in the noninfected cells.

Specificity of the suppression of metastatic phenotype by tyrosine and phenylalanine restriction

Amino acid restriction modulates tumor growth, although effects on metastasis are poorly documented. We demonstrate that low levels of tyrosine (Tyr) and phenylalanine (Phe) suppress metastasis of B16-BL6 melanoma and that these effects are specific to these two amino acids. Weight loss and sustained low body weight in mice fed low Tyr and Phe diet do not contribute to the antimetastatic effects. Furthermore, methionine (Met) restriction, which decreased survival of mice inoculated i.p. with B16 melanoma, only slightly inhibited spontaneous metastasis compared to the dramatic inhibition during Tyr and Phe restriction. Tyr and Phe restriction inhibited spontaneous metastasis by impairing the ability of tumor cells to establish metastatic foci and not via differential tumor cell removal from the blood. Spontaneous metastasis is blocked by Tyr and Phe intervention even in mice with established lymph node tumors. Tumors isolated from mice fed low Tyr and Phe diet reinoculated into mice fed normal diet exhibited lower experimental metastatic potential, reflected by decreased formation of lung tumor colonies and increased survival of inoculated mice. This decrease in metastatic potential is not associated with tumor chemosensitivity. These findings indicate that Tyr and Phe restriction could become an important adjuvant to effective melanoma treatment.

Unbalanced transmethylation and the perturbation of the differentiated state leading to cancer

It is proposed that the perturbation of the differentiated state in cancer is related to alterations in DNA methylation as well as to alterations in methylation of other cellular molecules, leading to an imbalance in global cellular methylation. It is hypothesized that the global imbalance in methylation is reflected in the enhanced levels of transmethylation seen in many cancer cell types as well as in a number of undermethylated molecules.

Reversion to a homocysteine-responsive phenotype in a human melanoma cell line is associated with diminished growth potential and increased methionine biosynthesis

Our aim was to determine if the isolation of cells capable of proliferating in methionine-free homocysteine-containing medium, from the human MeWo-LC1 melanoma tumor cell line which is unable to proliferate or survive under such conditions, was associated with altered growth properties. Cells which were able to proliferate in methionine-free homocysteine-containing medium (homocysteine-responsive cells) were obtained from the homocysteine-nonresponsive MeWo-LC1 cell line after 8 months of continuous exposure to methionine-free homocysteine-containing medium. Unlike the parental MeWo-LC1 cell line, these homocysteine-responsive cells were also able to proliferate normally in methionine-free medium containing 5'-deoxy-5'-methylthioadenosine. In vitro growth rate, methionine requirement, and capacity to form colonies on soft agarose of these homocysteine-responsive cells were reduced compared to those of the homocysteine-nonresponsive parental MeWo-LC1 cell line. Unlike MeWo-LC1, these homocysteine-responsive cells were able to synthesize [3H]S-adenosylmethionine from [3H]adenine and homocysteine. The failure of the MeWo-LC1 cell line to proliferate in methionine-free homocysteine-containing medium may be due to a deficiency in the synthesis of methionine from homocysteine and 5-methyltetrahydrofolic acid. These results indicate that acquisition of a homocysteine-responsive phenotype in homocysteine-nonresponsive malignant human tumor cells is associated with a reduction in the autonomous growth potential of such cells.

Methionine dependence of rat hepatocarcinoma cells in culture: relationship with tumourigenicity and oncogene expression

The culture in methionine-deprived medium of a rat hepatoma cell line (LF) significantly decreases: (1) the growth in vitro of these cells; (2) their tumourigenicity; and (3) the expression of a panel of oncogenes. When the LF cells are replaced in a medium containing methionine, they maintain a diminished in vitro growth and tumourigenic capacity, despite an overexpression of the same oncogenes. These data point out the determinant role of a nutritional factor, methionine, in tumour growth.

Cytokeratins and cytokeratin filaments in subpopulations of cultured human and rodent cells of nonepithelial origin: modes and patterns of formation

Using immunofluorescence microscopy, we observed that in several established cell culture lines derived from different nonepithelial tissues and species, cells spontaneously emerge, usually at low frequencies, which contain cytoplasmic structures decorated by antibodies specific for cytokeratins 8 and 18. This phenomenon was further examined at both the protein (gel electrophoreses of cytoskeletal proteins, followed by immunoblotting) and the RNA (Northern blots, "nuclear run-on" analysis, in situ hybridization) level. Positive cell lines included simian virus (SV40)-transformed human fibroblasts (HF-SV80, WI-38 VA13), human astrocytic glioma cells (U333 CG/343MG), rat (RVF-SMC) and hamster (BHK-21/13) cells derived from vascular smooth muscle and murine sarcoma MS-180 cells. In two cell lines (HF-SV80 and BHK-21/13), the frequency of the cytokeratin-containing cells and of the cytokeratin fibril arrays per cell was drastically increased upon treatment with 5-azacytidine. The structural appearance of the cytokeratins was variable in the different cell lines but could also differ among cells of the same culture: While small granular or comma-shaped structures or bizarrely shaped filament arrays prevailed in WI-38, RVF and normally grown BHK-21 cells, most of the other lines revealed extended normal-looking, fibrillar arrays. In one line (MS-180), the appearance of cytokeratins was associated with a morphological change, as it was only found in a subpopulation of cells that had lost their typical elongated and spindle-shaped phenotype and assumed a rounded ("coccoid") shape. Our results show that the expression of the genes encoding cytokeratins 8 and 18 is not necessarily restricted to programs of epithelial differentiation and that factors stochastically effective appear in cultured cell lines that allow the synthesis of these cytoskeletal components. Mechanisms possibly involved in this spontaneous and selective advent of cytokeratins 8 and 18 and implications for tumor diagnosis are discussed.