Effect of insulin deficiency on hepatic ribosomal aggregation

Time course of changes in albumin synthesis and mRNA in diabetic and insulin-treated diabetic rats

Albumin synthesis in rat liver in vivo decreased from 12.7 to 2.2% of total protein synthesis during the first 3 days after the induction of diabetes and then remained relatively constant at this depressed rate for another 3 days. Insulin treatment begun on the 3rd day after the induction of diabetes restored albumin synthesis to control values within 3 days. Hybridization of total polyadenylate-containing RNA with a specific albumin cDNA probe revealed a close correspondence between the relative abundance of albumin mRNA and the relative rate of albumin synthesis after induction of diabetes and in response to insulin treatment. The apparent half-life of albumin mRNA, based on the rate of change of the message from one steady-state level to another, was approximately 22 h in both diabetic and insulin-treated diabetic rats. Diabetes of 3-day duration had no effect on the average sizes of total and albumin-synthesizing polysomes or on the ribosomal half-transit time for total protein and albumin. However, the number of albumin-synthesizing polysomes decreased as a result of diabetes to approximately one-third the number found in control livers. Taken together the results indicate that albumin synthesis was regulated by the availability of albumin mRNA and not by alterations in degradation, sequestration, or translation of message.

Protein synthesis in liver and small intestine in protein deprivation and diabetes

Protein synthesis (as a percent of the protein pool synthesized per day) has been measured in liver and small intestine of young male rats from the incorporation of 100 mumol [1-14C]leucine/100 g body wt into protein over 10 min. Dietary protein deprivation for 8 days depressed protein synthesis in liver (30%), jejunal mucosa (20%), and jejunal serosa (25%). In serosa, reduced levels of RNA relative to protein could account for altered synthesis; in liver and mucosa, the amount of protein synthesized per unit RNA was reduced. In liver of streptozotocin-diabetic rats protein synthesis was depressed 45%, whereas it was maintained in jejunal mucosa and serosa. Depressed synthesis in liver was accompanied by both a loss of RNA relative to protein and a reduction in the protein synthesized per RNA.

Inhibition by somatostatin of adenosine-3',5'-monophosphate stimulated hepatic ribosomal protein synthesis

Somatostatin which inhibits the secretion of various pituitary and intestinal hormones has been suspected to exert these effects by inhibiting adenosine-3',5'-monophosphate accumulation in the respective endocrine gland. Our results, obtained by cell free protein synthesis and by sedimentation through sucrose gradients of ribosomes, prepared from rat liver after incubation with cyclic AMP and/or somatostatin also suggest this antagonism between somatostatin and cyclic AMP. In addition, they indicate that this antagonism is not restricted to endocrine tissues.

Stimulation of 3H-uridine incorporation into ribosomal ribonucleic acids by insulin

Ribosomal RNA was extracted from hepatic ribosomal subunits of rats treated with alloxan or with alloxan and insulin and additionally injected with 3H-uridine. Sedimentation of the extracted RNA through sucrose, gradients with registration of the optical density and with measurement of the radioactivity of rRNA reveals reduction of uridine incorporation after the induction of diabetes of diabetes mellitus. If alloxan-diabetic animals are substituted with insulin, this decrease in uridine incorporation is reversed to incorporation rates which even exceed uridine incorporation of control animals. With regard to the reduction of protein synthesis in diabetes mellitus this finding of reduced rRNA synthesis is suggested to be an additional factor in the reduction of ribosomal aggregation.

The effect of experimental diabetes mellitus and insulin replacement on hepatic ultrastructure and protein synthesis

The following study was conducted in order to define the specific alterations in hepatic ultrastructure responsible for the decrease in hepatic protein synthesis associated with experimental diabetes. Rats received intravenous alloxan (70 mg/kg) and 48 h later were either sacrificed or given insulin for 1, 2, 4, 6, or 24 h. Specimens for electron microscopic evaluation and morphometric analysis were taken from the same livers used to isolate ribosomes for measurement of in vitro protein synthesis. Our results show that hepatocytes from animals with untreated alloxan diabetes show varying degrees of disorganization and loss of rough endoplasmic reticulum (RER) which is directly related to the severity of the alloxan diabetes. A significant correlation existed between the severity of ultrastructural changes as judged by the loss of both membrane and polysome components of the RER and degree of inhibition of protein synthesis (P < 0.001). Abnormalities of hepatic ultrastructure and protein synthesis were reversed within 24 h of insulin administration. The data are consistent with the view that it is the relative decrease in hepatic polysomes that results from the loss of RER in alloxan diabetes that is responsible for the decrease in hepatic protein synthesis.