5-methyltetrahydrofolate: synthesis and utilization in normal and SV40-transformed BHK-21 cells

Targeting methionine auxotrophy in cancer: discovery & exploration

INTRODUCTION

Amino acid auxotrophy or the metabolic defect which renders cancer incapable of surviving under amino acid depleted conditions is being exploited and explored as a therapeutic against cancer. Early clinical data on asparagine- and arginine-depleting drugs have demonstrated low toxicity and efficacy in melanoma, hepatocellular carcinoma and acute lymphoblastic leukemia. Methionine auxotrophy is a novel niche currently under exploration for targeting certain cancers.

AREAS COVERED

In this review we explore the discovery of methionine auxotrophy followed by in vitro, in vivo and patient data on targeting cancer with methionine depletion. We end with a small discussion on bioengineering, pegylation and red blood cell encapsulation as mechanisms for decreasing immunogenicity of methionine-depleting drugs. We hope to provide a platform for future pharmacology, toxicology and cytotoxicity studies with methionine depletion therapy and drugs.

EXPERT OPINION

Although methionine auxotrophy seems as a viable target, extensive research addressing normal versus cancer cell toxicity needs to be conducted. Further research also needs to be conducted into the molecular mechanism associated with methionine depletion therapy. Finally, novel methods need to be developed to decrease the immunogenicity of methionine-depleting drugs, a current issue with protein therapeutics.

Methionine dependency and cancer treatment

Conventional chemotherapies have showed their limits, notably for patients with advanced cancer. New therapeutic strategies must be identified, and the metabolic abnormalities of cancer cells offer such opportunities. Many human cancer cell lines and primary tumors have absolute requirements for methionine, an essential amino acid. In contrast, normal cells are relatively resistant to exogenous methionine restriction. The biochemical mechanism for methionine dependency has been studied extensively, but the fundamental mechanism remains unclear. A number of investigators have attempted to exploit the methionine dependence of tumors for therapeutic effects in vivo. To reduce in vivo methionine in plasma and tumours, dietary and pharmacological treatments have been used. Methionine-free diet or methionine-deprived total parenteral nutrition causes regression of a variety of animal tumours. Alternatively, methionine depletion was achieved by the use of methioninase. This enzyme specifically degrades methionine and inhibits tumour growth in preclinical models. Because of potential toxicity and quality of life problems, prolonged methionine restriction with diet or with methioninase is not suitable for clinical use. Methionine restriction may find greater application in association with various chemotherapeutic agents. Several preclinical studies have demonstrated synergy between methionine restriction and various cytotoxic chemotherapy drugs. The experimental results accumulated during the last three decades suggest that methionine restriction can become an additional cancer therapeutic strategy, notably in association with chemotherapy.

Loss of heterozygosity at the 5,10-methylenetetrahydrofolate reductase locus in human ovarian carcinomas

The high-affinity folate-binding protein (FBP) is primarily involved in the uptake of the 5-methyltetrahydrofolate, and its expression may be physiologically regulated by the intracellular folate content. The overexpression of FBP on the cell surface of ovarian carcinoma cells may be responsible for an increased folate uptake. We tested the hypothesis of the existence of a defect in the 5, 10-methylenetetrahydrofolate reductase (MTHFR) in ovarian tumours that could cause reduced intracellular regeneration of the 5-methyltetrahydrofolate and induce increased FBP expression. No sequence mutations were found in the MTHFR gene, but allelic deletions of this gene were frequently detected in ovarian tumours (59%). Chromosomal losses appeared to be confined to the 1p36.3 region to which the MTHFR gene maps. Although it cannot be stated that MTHFR is the target gene of the chromosomal loss involving the 1p36.3 region, a correlation between loss of heterozygosity at this locus and decrease in MTHFR activity was shown, suggesting a role of these allelic deletions in generating a biochemical defect in folate metabolism. Further studies are needed to assess further the relationship between MTHFR and FBP overexpression, but the demonstration of the alteration of a key metabolic enzyme of the folate cycle in a subset of human ovarian tumours is in accordance with the hypothesis of an altered folate metabolism in these neoplasias and might be exploited for therapeutic purposes.

Role of vitamin B12 and folate deficiencies in carcinogenesis

A significant body of experimental evidence supports the notion that a deficiency of either vitamin B12 or folic acid enhances the activity of various carcinogens. Unifying mechanisms are proposed to explain this cocarcinogenic role.

Formation and utilization of methionine by rat liver cells in culture

The enzyme N5-methyltetrahydrofolate:homocysteine methyltransferase (methionine synthetase) catalyzes the synthesis of methionine from homocysteine. Methylcobalamin is a cofactor for the reaction. The effects of methionine deprivation and methylcobalamin supplementation on the growth of normal and transformed rat liver epithelial cell lines were determined using growth constants to quantitate cell proliferation. No marked specific requirement by the transformed cell lines for methionine relative to leucine was observed. A sigmoidal relationship, however, was found to exist between growth constants and the logarithms of the amino acid concentrations for both normal and transformed cells. Methylcobalamin stimulated the growth rates of the normal and transformed liver cells in methionine-deficient, homocysteine-containing medium. Growth on methionine was not increased by the addition of methylcobalamin. The growth constants for two normal, two spontaneously transformed, one chemically transformed, and one tumor cell line grown in medium in which methionine was replaced by homocysteine were found to be proportional to the level of methionine synthetase. The results demonstrate the utility of growth quantitation to study the methionine dependency of transformed cells.

Constitutive behavior of methionyl-tRNA synthetase compared to repressible behavior of methionine adenosyltransferase in mammalian cells

Methionine adenosyltransferase, one of the two major enzymes utilizing methionine, is regulated by the levels of methionine in the growth medium (Jacobsen, S.J., Hoffman, R.M. and Erbe, R.W. (1980) J. Natl. Cancer Inst. 65, 1237--1244, and Caboche, M. and Mulsant, P. (1978) Somatic Cell Genet. 4, 407--421). We report here that methionyl-tRNA synthetase, unlike methionine adenosyltransferase, behaves in a constitutive manner with respect to the concentration of methionine in the culture medium. This behavior is seen in Chinese hamster ovary cells and in normal diploid and SV40-transformed human fibroblasts. Although the kinetics of regulation of methionine adenosyltransferase and methionyl-tRNA synthetase by exogenous methionine are clearly different, the levels of the two enzymes in the human cell lines are similar.

Reversible growth arrest in simian virus 40-transformed human fibroblasts

A reversible growth arrest of simian virus 40-transformed human fibroblasts has been produced by replacement of methionine in the growth medium by its immediate metabolic precursor, homocysteine, Although these arrested cells exhibit a greatly reduced cloning efficiency when plated in methionine-supplemented medium, they resume rapid proliferation without a lag when subconfluent cells are refed with methionine-supplemented medium. This growth arrest is accompanied by a reduction in the percentage of mitotic cells in the cell population. Furthermore, data obtained using fluorescence-activated cell sorting techniques indicate that the cells are arrested i the S and G2 phases of the cell cycle. This is in contrast to a G1-phase accumulation of cells, which occurs only in methionine-supplemented medium at very high densities and which is similar to the G1 block seen in cultures of normal fibroblasts at high density. The apparent relationship between specific events in the DNA-synthetic and premitotic phase of the cell cycle and methionine dependence in these transformed cultures is discussed.