RESUMO
Livers of insulin-treated diabetic rats accumulate albumin and malic enzyme mRNAs at very different rates. We now report that in normal rats insulin directs a specific increase in malic enzyme mRNA, while albumin mRNA levels remain unaltered. These studies support the contention that insulin regulates the accumulation of hepatic mRNAs in a highly specific manner. To evaluate whether or not albumin and malic enzyme mRNA levels are determined by altered rates of transcription, in vitro transcription assays were performed. The results of these studies demonstrate that increased malic enzyme mRNA levels in insulin-treated normal rats and increased malic enzyme and albumin mRNA levels in insulin-treated diabetic rats do not involve altered rates of transcription of the genetic sequences encoding these proteins. For these two specific proteins, insulin mediates changes in mRNA levels by a post-transcriptional mechanism.
Assuntos
Diabetes Mellitus Experimental/metabolismo , Insulina/farmacologia , Fígado/metabolismo , Malato Desidrogenase/genética , Processamento Pós-Transcricional do RNA/efeitos dos fármacos , RNA Mensageiro/genética , Albumina Sérica/genética , Animais , Núcleo Celular/metabolismo , Fígado/efeitos dos fármacos , Masculino , Hibridização de Ácido Nucleico , RNA Mensageiro/efeitos dos fármacos , Ratos , Ratos Endogâmicos , Valores de Referência , Transcrição Gênica/efeitos dos fármacosRESUMO
We have compared the rate of accumulation of hepatic albumin and malic enzyme mRNAs following insulin treatment of diabetic rats to determine whether insulin coordinately increases mRNA levels or specifically induces the accumulation of individuals mRNAs. Initially, the quantities of both albumin and malic enzyme mRNAs are reduced in diabetic rats compared to normal rats as determined by RNA blot analysis using complementary DNA probes. Following insulin administration for 12 h, albumin and malic enzyme mRNA levels increase at similar rates. However, after 12 h the rate of malic enzyme mRNA accumulation increases dramatically while albumin mRNA continues to increase at its initial rate. This accelerated rate of accumulation of malic enzyme mRNA continued through 60 h of hormone treatment and was associated with the onset of hepatic lipogenesis. Thus, our results suggest that insulin regulates the accumulation of mRNAs encoding these two inducible proteins in an asynchronous manner directly related to the metabolic requirements of the animal.
Assuntos
Insulina/fisiologia , Fígado/fisiologia , Malato Desidrogenase/genética , Albumina Sérica/genética , Animais , Regulação da Expressão Gênica , Masculino , Peso Molecular , RNA Mensageiro/genética , RatosRESUMO
A comparison of the regulatory processes controlling hepatic malic enzyme activity following treatment of diabetic rats with insulin or with a high fructose diet demonstrated several important differences. Insulin treatment caused a 50-fold increase in activity, due to a 12-fold increase in enzyme quantity and a 4-fold increase in specific activity(units/nmol). Dietary fructose caused a 3-fold increase in enzyme activity, due to a 3-fold increase in enzyme quantity, with no change in the specific activity of the enzyme. Thus, while fructose initiated a minor increase in malic enzyme activity, insulin was more effective, causing a substantially greater increase in enzyme activity and activating a hormone specific alteration in the catalytic activity of each enzyme molecule.
