Effects of Dietary Protein on the Induction of DNA Synthesis and Expression of Growth-Related Genes in Liver and Kidney of Growing Rats.

Abstract

SummaryTo investigate molecular mechanisms of growth control by protein nutrition, we examined whether nutritive quality of protein affects the induction of DNA synthesis in liver and kidney of growing rats in relation to expression of growth-related genes such as c-myc, c-fos, c-Ha-ras, and ornithine decarboxylase (ODC). Rats were adapted to 2-h meal feeding schedule at first with laboratory chow for 10 days and then with a protein-free diet for 3 days prior to experiments. When protein free diet was fed to the rats, the levels of c-myc, ODC and c-Ha-ras mRNAs increased in the liver within 2 days. However, substantial changes in the levels of those mRNAs were not observed in the kidney. The level of c-fos mRNA in these tissues was too low to detect by our method. Feeding of casein diet to rats that had been maintained on protein-free diet for 3 days caused a rapid decrease in the level of c-myc mRNA and induced DNA synthesis in the liver. On the other hand, zein diet, which lacks tryptophan and lysine, did not lower the c-myc mRNA level nor induced DNA synthesis in the liver. However, if zein diet was supplemented with tryptophan and lysine, a decrease in c-myc mRNA level and an induction of DNA synthesis were observed. The levels of ODC and c-Ha-ras mRNAs were not changed by feeding of casein or zein diet. Neither casein nor zein induced DNA synthesis and changed the levels of the mRNA in the kidney. The amount of food intake during the 2-h feeding period was not different among the diets. These results suggest that the liver cells are arrested in G 1 phase during the feeding of protein-free diet and good quality of protein is required to progress the cell cycle to enter S phase.

c-myc mRNA is Stabilized by Deprivation of Amino Acids in Primary Cultured Rat Hepatocytes.

“…Therefore, it seems likely that the level of c-myc mRNA in cultured hepatocytes is mainly regulated by amino acids. On the other hand, the level of ODC and c-Ha-ras mRNAs were not influenced by the medium change although they were increased in the growing rat liver by feeding of protein-free diet (7), suggesting that factors other than amino acids were involved in the regulation of the expression of these two mRNAs. In fact, we have reported that glucagon increases the level of ODC mRNA in cultured hepatocytes incubated either in the presence or absence of amino acids (12).…”

“…We recently reported that hepatic levels of several growth-related gene trans cripts, such as c-myc, ornithine decarboxylase (ODC) and c-Ha-ras mRNAs were elevated when growing rats were fed protein-free diets (6,7). To determine whether the expression of these growth-related genes were affected directly by changes in the nutritional environment, we examined the levels of these mRNAs in cultured hepatocytes in the presence and absence of amino acids.…”

“…The c-myc mRNA is extremely unstable, with a half-life of less than 1 h (21,22), which can be lengthened in response to growth factors and or translational inhibitors (19,23). We and others have reported independently that protein deprivation induces c-myc expression in liver of rats and re-feeding of protein diet to the rats causes a rapid decrease in the level of c-myc mRNA and induces DNA synthesis (6,7,(24)(25)(26). Mead et al demonstrated that the increase in hepatic c-myc mRNA level during protein deprivation is regulated by a post transcriptional mechanism (26).…”

“…demonstrated that the level of c-myc mRNA and the rate of DNA synthesis in the liver were changed depending on the nutritive quality of dietary protein (6,7). In contrast to many reports in which a transient increase in c-myc expression was observed when cell growth was stimulated (8)(9)(10)(11), the level of c-myc mRNA in the liver paradoxically increased by feeding of protein-free diet on which their growth was arrested.…”

“…From these results, we hypothesiz ed that the liver cell cycle was arrested at G1 phase during feeding of protein-free diet, because c-myc mRNA was known to increase transiently at G 1 phase when quiescent cells were stimulated by growth factors (11), and that good quality proteins were required for the cell cycle to progress to S phase. In this context, we suggested the possibility that protein nutrition not only supplies material for body components but also plays a role as a signal for the progression of the liver cell cycle in young growing rats (7).…”