Lipotropic factors and oncogenesis.

Abstract

The lipotropes (choline, methionine, folate, and vitamin B12) have a rich history, with many fluctuations in scientific effort and popularity, covering the past 6 decades. A thin thread of common interest in 1-carbon metabolism and a small band of dedicated individuals have kept this area of biology alive. Today, the lipotropes are enjoying a resurgence of interest and effort with promise for significant contributions to some of our most serious chronic diseases. Between 1920, when Banting and Best initiated a work that led to the discovery of insulin, and 1982-83, when investigators working in 3 laboratories announced that lipotrope deficiency alone could result in liver cancer in rodents, many have used this model to study nutritional problems and, more recently, carcinogenesis. Lipotropes are important to lipid metabolism and to synthesis and maintenance of cellular membranes. When weanling rats were fed a diet low in lipotropes, within a few days the liver accumulated lipid, first in the centrilobular zone and later throughout the entire lobule and lobe. If the diet was continued for a longer period, the liver underwent fibrosis and cirrhosis with some rats ultimately developing hepatocellular carcinoma. Although lipotrope deficiency can result in liver cancer, all hepatocarcinogens tested thus far were enhanced in their activity by diets low in lipotropes. Important changes associated with lipotrope deficiency included membrane damage, decreased serum very low density lipoprotein and drug metabolizing enzymes, decreases in S-adenosylmethionine and in methylation of cytosine, increases in cellular peroxidation products and free radicals, decreased immunocompetence, and a markedly shortened lag time for chemical induction of liver cancer in animals. The overall effect of lipotrope deficiency is an increase in the susceptibility to cancer in animals; the exact mechanisms are unclear.