Expressions of c-myc and insulin-like growth factor-1 mRNA in the liver of growing rats vary reciprocally in response to changes in dietary protein.

To investigate molecular mechanisms of growth control by diets, we examined the effects of nutrition on the expression of c-myc and insulin-like growth factor-1 (IGF-1) genes in the liver of growing rats. In the first study, rats were fed either a 24% casein, 24% zein or protein-free diet, or were starved for 3 d. The levels of the two mRNAs in the tissues were then determined by Northern blot hybridization. In the liver, levels of the two mRNAs varied in a reciprocal anner in response to changes in either quantity or quality of diet. The expression of c-myc mRNA was greatly enhanced by consumption of the protein-free diet or by starvation, whereas the IGF-1 mRNA levels were reduced markedly by consumption of the zein diet or the protein-free diet or by starvation. In another study, the casein and zein diets were fed to rats that had been adapted to a 2-h meal-feeding pattern, first with nonpurifled diet for 10 d and then with the protein-free diet for 3 d before the experiment. In rats fed casein, the level of c-myc mRNA decreased 75% within 8 h after consumption of the casein diet, whereas the IGF-1 mRNA level increased 100% during that period. Consumption of the zein diet did not affect the level of either mRNA. Because quantity of food intake did not differ between the rats fed casein and those fed zein, expression of the two genes in the liver was affected by the quality of the protein consumed. These results indicate that quality and quantity of diets changed the expression of c-myc and IGF-1 genes and thus demonstrate the possibility that nutrition not only supplies material for body components but also affects the signal transduction for growth in young growing rats.