Inhibition of cellular uptake of folate by blocking synthesis of the membrane folate receptor

Localized folate and vitamin B-12 deficiency in squamous cell lung cancer is associated with global DNA hypomethylation

We measured the concentrations of folate and vitamin B-12 in paired tissue samples of squamous cell cancer (SCC) and adjacent grossly normal-appearing uninvolved bronchial mucosa (from which SCC developed and also "at risk" of developing SCC) of the lung in 12 subjects to determine the involvement of these vitamins in 1) lung carcinogenesis and 2) global DNA methylation. The folate concentrations were significantly lower in SCCs than in uninvolved tissues (p = 0.03). The vitamin B-12 concentrations were also significantly lower in SCCs than in uninvolved tissues (p = 0.02). The radiolabeled methyl incorporation (inversely related to the degree of in vivo DNA methylation) was significantly higher in SCCs than in uninvolved tissues (p < 0.0001). The correlation between folate and radiolabeled methyl incorporation was inverse and statistically significant in SCCs (p = 0.03). The correlation between vitamin B-12 and radiolabeled methyl incorporation also was inverse and statistically significant in SCCs (p = 0.009). The relationship between tissue vitamin B-12 and DNA methylation was minimal in uninvolved tissues. The relationship between folate and DNA methylation, however, was inverse in uninvolved tissues. In the multiple regression models that included both vitamins, only folate was inversely associated with radiolabeled methyl incorporation in uninvolved and cancerous tissues. These results suggested that folate might be the limiting vitamin for proper DNA methylation in SCC as well as in tissues at risk of developing SCC. Several possible mechanisms of folate deficiency, including inactivation of the vitamin by exposure to carcinogens of cigarette smoke and underexpression or absence of folate receptor in SCCs and associated premalignant lesions, are discussed in light of these findings.

The folate cycle and disease in humans

The prevalence of hyperhomocysteinemia in renal disease patients, its treatment by folate administration, and its aggravation by the 677 C-->T mutation of methylene-tetrahydrofolate (methylene-THF) reductase has established the folate cycle as an important factor in the pathogenesis and management of renal disease. Proper function of the folate cycle depends on normal function of involved enzymes adequate of the vitamin and its correct disposition within the body. Vital processes in folate disposition include conversion of dietary folylpolyglutamates to monoglutamates, intestinal absorption, receptor and carrier-mediated transport across cell membranes, and cellular export. Folate coenzymes are responsible for the one-carbon unit transfer in intermediary metabolism and are required for several reactions in key metabolic processes, for example of purine, pyrimidine and methionine synthesis, and glycine and serine metabolism. Methionine synthase and its recently discovered reducing protein as well as methylene tetrahydrofolate reductase are key folate enzymes in homocysteine metabolism. Deficiencies of these enzymes are important causes of severe disease in the rare remethylation defects causing homocystinuria. Knowledge of their catalytic and molecular properties is important in understanding possible causes of moderate hyperhomocysteinemia, as for example, the well-known 677 C-->T transition of methylene tetrahydrofolate reductase.