We previously demonstrated that the integration of albumin mRNA into functional polysomes was regulated by the supply of branched-chain amino acids (BCAA) in the liver of galactosamine-treated rats. To study the mechanism of this regulation, we investigated interaction between rat liver proteins and albumin transcripts. When albumin transcript was incubated with ribosome salt wash (RSW) extracts prepared from liver, a specific RNA-protein complex (p65) formed. Competition experiments showed that a pyrimidine-rich sequence in the coding region of albumin mRNA was required for the formation of p65. The level of p65 was increased in the RSW extracts prepared from liver of galactosamine-treated rats infused with a standard amino acid formula, compared with a BCAA-enriched amino acid formula. The protein in p65 appears to be polypyrimidine tract-binding protein (PTB), because the formation of p65 was reduced in the RSW extracts pre-incubated with anti-PTB antibody. In cell-free translation analysis, immunodepletion of PTB from rabbit reticulocyte lysate caused an increase in albumin translation. These results suggest that binding of PTB to albumin mRNA suppresses its translation. A supply of BCAA may interfere with this binding and improve the translation efficiency of albumin mRNA in injured liver.

Summary Our recent study demonstrates that polypyrimidine tract-binding protein (PTB), which is a sequence specific RNA-binding protein, attenuates albumin synthesis in a cell-free translation system. In this study, the effects of food intake on regulation of albumin synthesis through binding of PTB to albumin messenger RNA (mRNA) were investigated. Rats were divided into 1 of 3 groups: fed; fasted for 36 h; or fasted for 36 h and then refed for 24 h. No significant differences in albumin mRNA levels were found among fed, fasted and refed rats. However, a decrease in the proportion of albumin mRNA associated with polysomes was identified in fasted rats. Furthermore, UV-cross linking analysis demonstrated that levels of albumin mRNA-PTB complex were increased in liver extracts from fasted rats. No significant differences in PTB levels in liver homogenate were found among the experimental groups. However, PTB level in the cytoplasmic fraction was higher in fasted rats than in fed rats. In refed rats, PTB level in the cytoplasmic fraction returned to a level comparable to that in fed rats, but was inhibited by treatment with rapamycin, a mammalian target of rapamycin (mTOR) inhibitor. These results suggest that localization of PTB is regulated by food intake through mTOR signaling, and alterations in level of albumin mRNA-PTB complex play a role in mediating the effects of food intake on albumin synthesis in the rat liver. Key Words albumin synthesis, food intake, polypyrimidine tract-binding protein, mammalian target of rapamycin, rats Plasma albumin has become an indispensable factor for evaluating nutritional status. In the regulation of albumin synthesis, meal stimulation is one of the most important factors. For example, albumin synthesis is activated by meal feeding in healthy subjects, whereas fibrinogen synthesis remains unchanged ( 1 ). Furthermore, albumin synthesis is reportedly influenced by varying the relative contribution of protein from animal and vegetable sources in the meal ( 2 ), and consumption of only the protein component of a meal is sufficient to stimulate albumin synthesis ( 3 ).In rats under conditions of protein malnutrition, fractional and absolute synthesis rates of total protein in liver are decreased, and the rate of hepatic synthesis of secreted proteins, particularly albumin, is greatly reduced compared with the rate of synthesis of liver domestic proteins ( 4 ). As with protein deficiency, a reduction in the rate of albumin synthesis is observed under fasting ( 5 -7 ). Level of albumin messenger RNA (mRNA) is unchanged by fasting and refeeding ( 8 ), and reduced albumin synthesis in fasted rats is associated with disaggregation of membrane-bound polysomes and a shift of albumin mRNA to free polysomes and to the post-ribosomal supernatant fraction ( 9 , 10 ). Translational regulation thus plays an important role in reduced albumin synthesis under fasting.Polypyrimidine tract-binding protein (PTB) is an abundant eukaryotic RNA-binding protein implicated in several aspects of mRNA me...