Reduced free‐methionine in methionine‐dependent SV40‐transformed human fibroblasts synthesizing apparently normal amounts of methionine

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Abstract
Many different types of cancer cells have been shown to be methionine-dependent. These cells, unlike normal cells, grow poorly or not at all when methionine is replaced by its immediate precursor homocysteine in the growth medium (Met−Hcy+ medium). We have previously shown that apparently normal total amounts of methionine are synthesized by methionine-dependent SV40-transformed human fibroblasts. However, methionine-dependent cells in Met−Hcy+ medium accumulate reduced amounts of S-adenosylmethionine (AdoMet) and elevated amounts of S-adenosylhomo-cysteine (AdoHcy) that together probably limit growth. In this report, we demonstrate that the amount of free methionine is low in methionine-dependent SV40-transformed human fibroblasts in Met−Hcy+ medium compared to normal human diploid fibroblasts. In contrast, in Met+Hcy− medium, the amount of free methionine is comparable in both cell types. The deficient pool of free methionine in methionine-dependent cells in Met−Hcy+ medium allows only low amounts of AdoMet to be formed. However, large amounts of the biosynthesized methionine are channeled into protein synthesis. Possible mechanisms are discussed to explain this cancer-associated metabolic defect.