Changes in cobalamin metabolism are associated with the altered methionine auxotrophy of highly growth autonomous human melanoma cells

Abstract

Our aim was to identify the biochemical defect responsible for the inability of highly growth autonomous human tumor cells to proliferate in culture medium devoid of methionine, but containing homocysteine and 5-methyletrahydrofolic acid. We have adopted the terms "homocysteine-responsive" and "homocysteine-nonresponsive" to describe cells which can or cannot proliferate in methionine-free homocysteine-supplemented medium. Using a panel of genetically related homocysteine-responsive and -nonresponsive human melanoma cell lines, the results from a number of experiments indicate that acquisition of the "homocysteine-nonresponsive phenotype" is associated with the reduced intracellular accumulation of methyl-cobalamin, a critical cofactor of the methionine synthase enzyme. When in vitro methionine synthase assays were performed in the presence of exogenously added methyl-cobalamin, specific methionine synthase activity in extracts obtained from homocysteine-responsive cells was only twofold greater than that observed with extracts prepared from homocysteine-nonresponsive cells. However, when exogenous methyl-cobalamin was omitted from the enzyme assays, methionine synthase activity in extracts derived from homocysteine-nonresponsive cells was dramatically reduced, compared with the small decrease observed with homocysteine-responsive cell extracts. Compared with their homocysteine-responsive counterparts, homocysteine-nonresponsive cells exhibited increased levels of cobalamin efflux and decreased intracellular accumulation of methyl-cobalamin. There was a clear relationship between the abilities of these related melanoma cell lines to proliferate in methionine-free homocysteine-supplemented medium, and the extent of cobalamin loss and capacity of exogenously added methyl-cobalamin to stimulate in vitro methionine synthase activity. These results indicate a link between alterations in the intracellular trafficking and/or metabolism of cobalamin and the increased growth autonomy of human melanoma cells.

Growth of methionine-dependent human prostate cancer (PC-3) is inhibited by ethionine combined with methionine starvation

“…Each of these cofactors might be defective in tumour cells, leading to less MET synthesis. Studies have shown that some MET-dependent cell lines lack sufficient cobalamin to assure HCY methylation (Liteplo et al, 1991;Pezacka et al, 1992). MET dependency might also be a result of the low availability of HCY that would be metabolized in the transsulphuration pathway (Figure 1), which requires cystathionine j-synthase.…”

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

“…MET-dependent tumour cells appear to synthesize MET from HCY by an active MET synthase, but at levels not adequate to both sustain growth and meet their high transmethylation requirements (Judde et al, 1989). The most likely biochemical defect leading to MET dependency is thought to be related to the synthesis and availability of methylcobalamine, which is directly involved in the transfer of methyl groups from 5-methyltetrahydrofolate to HCY (Liteplo et al, 1991;Fiskerstrand et al, 1994).…”

Pathological Metabolism of Methionine in Malignant Cells Is a Potential Target for the Antitumor Therapy

“…Однако на культуре клеток карциносаркомы Уокера показано, что 5-MTHF не влияет на метиониновую зависимость [83]. На примере клеток меланомы, лейкоза и глиомы GaMg дискутируется спорный ме-ханизм развития метиониновой зависимости через нарушение метаболизма кобаламина со снижением активности MTR [84][85][86]. …”